WIRELESS ENERGY TRANSFER, INCLUDING INTERFERENCE ENHANCEMENT

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First Claim
1. An apparatus for use in wireless energy transfer, the apparatus comprising:
 a first resonator structure configured for energy transfer with a second resonator structure, over a distance D larger than a characteristic size L_{1 }of said first resonator structure and larger than a characteristic size L_{2 }of said second resonator structure, wherein the energy transfer has a rate κ and is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, wherein said resonant field of the first resonator structure has a resonance angular frequency ω_{1}, a resonance frequencywidth Γ_{1}, and a resonance quality factor Q_{1}=ω_{1}/2Γ_{1 }at least larger than 300, and said resonant field of the second resonator structure has a resonance angular frequency ω_{2}, a resonance frequencywidth Γ_{2}, and a resonance quality factor Q_{2}=ω_{2}/2Γ_{2 }at least larger than 300, wherein the absolute value of the difference of said angular frequencies ω_{1 }and ω_{2 }is smaller than the broader of said resonant widths Γ_{1 }and Γ_{2}, and the quantity κ/√{square root over (Γ_{1}Γ_{2})} is at least larger than 20, and further comprising a power supply coupled to the first structure and configured to drive the first resonator structure or the second resonator structure at an angular frequency away from the resonance angular frequencies and shifted towards a frequency corresponding to an odd normal mode for the resonator structures to reduce radiation from the resonator structures by destructive farfield interference.
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Accused Products
Abstract
Disclosed is an apparatus for use in wireless energy transfer, which includes a first resonator structure configured for energy transfer with a second resonator structure over a distance D larger than characteristic sizes, L1 and L2, of the first and second resonator structures. A power generator is coupled to the first structure and configured to drive the first resonator structure or the second resonator structure at an angular frequency away from the resonance angular frequencies and shifted towards a frequency corresponding to an odd normal mode for the resonator structures to reduce radiation from the resonator structures by destructive farfield interference.
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Witricity Corporation

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Witricity Corporation

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Energous Corporation

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Energous Corporation

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Energous Corporation

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Energous Corporation

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Energous Corporation

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Energous Corporation

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Massachusetts Institute of Technology

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Massachusetts Institute of Technology

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Massachusetts Institute of Technology

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Witricity Corporation

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Energous Corporation

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Witricity Corporation

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Witricity Corporation

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Witricity Corporation

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Witricity Corporation

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Witricity Corporation

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Witricity Corporation

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Energous Corporation

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Energous Corporation

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Energous Corporation

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Energous Corporation

Wireless power harvesting and transmission with heterogeneous signals  
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Witricity Corporation

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Witricity Corporation

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Witricity Corporation

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Witricity Corporation

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Witricity Corporation

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Energous Corporation

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Energous Corporation

Laptop computer as a transmitter for wireless charging  
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Energous Corporation

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Energous Corporation

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Energous Corporation

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Energous Corporation

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Energous Corporation

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Energous Corporation

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Energous Corporation

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Energous Corporation

Systems and methods for device and power receiver pairing  
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Energous Corporation

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Energous Corporation

Wireless power transfer frequency adjustment  
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Witricity Corporation

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Witricity Corporation

Transmittersand methods for adjusting wireless power transmission based on information from receivers  
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Energous Corporation

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Energous Corporation

Transducer sound arrangement for pocketforming  
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Energous Corporation

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Energous Corporation

Antenna tile arrangements in electronic device enclosures  
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Energous Corporation

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Energous Corporation

Synchronous rectifier design for wireless power receiver  
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Energous Corporation

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Energous Corporation

Systems and methods for selecting which power transmitter should deliver wireless power to a receiving device in a wireless power delivery network  
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Energous Corporation

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Energous Corporation

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Energous Corporation

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Energous Corporation

Systems and methods of object detection using one or more video cameras in wireless power charging systems  
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Energous Corporation

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Energous Corporation

Hybrid charging method for wireless power transmission based on pocketforming  
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Energous Corporation

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Energous Corporation

Integrated miniature PIFA with artificial magnetic conductor metamaterials  
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Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless charging and powering of electronic devices in a vehicle  
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Current Assignee
Energous Corporation

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Energous Corporation

Systems and methods for assigning groups of antennas to transmit wireless power to different wireless power receivers  
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Energous Corporation

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Energous Corporation

System and method to control a wireless power transmission system by configuration of wireless power transmission control parameters  
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Energous Corporation

Sponsoring Entity
Energous Corporation

Boostchargerboost system for enhanced power delivery  
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Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Cluster management of transmitters in a wireless power transmission system  
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Bell Douglas, Michael Nelson, Michael Leabman

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Bell Douglas, Michael Nelson, Michael Leabman

Wireless power delivery using a base station to control operations of a plurality of wireless power transmitters  
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US 9,882,427 B2
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Energous Corporation

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Energous Corporation

Cluster management of transmitters in a wireless power transmission system  
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US 9,882,430 B1
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Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for using servers to generate charging schedules for wireless power transmission systems  
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US 9,882,394 B1
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Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for a configuration web service to provide configuration of a wireless power transmitter within a wireless power transmission system  
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US 9,887,584 B1
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Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for wireless power transmission by comparing voltage levels associated with power waves transmitted by antennas of a plurality of antennas of a transmitter to determine appropriate phase adjustments for the power waves  
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US 9,887,739 B2
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Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for a configuration web service to provide configuration of a wireless power transmitter within a wireless power transmission system  
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US 9,891,669 B2
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Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for determining optimal charging positions to maximize efficiency of power received from wirelessly delivered sound wave energy  
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US 9,893,535 B2
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Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless charging of tools using a toolbox transmitter  
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US 9,893,555 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

System and method for providing health safety in a wireless power transmission system  
Patent #
US 9,893,554 B2
Filed 12/29/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless power transfer systems with shield openings  
Patent #
US 9,892,849 B2
Filed 04/16/2015

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

Methodology for multiple pocketforming  
Patent #
US 9,893,768 B2
Filed 12/27/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods of object detection in wireless power charging systems  
Patent #
US 9,893,538 B1
Filed 02/25/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Antenna for wireless charging systems  
Patent #
US 9,899,744 B1
Filed 10/28/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for configuring operational conditions for a plurality of wireless power transmitters at a system configuration interface  
Patent #
US 9,899,844 B1
Filed 06/23/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for assigning groups of antenas of a wireless power transmitter to different wireless power receivers, and determining effective phases to use for wirelessly transmitting power using the assigned groups of antennas  
Patent #
US 9,900,057 B2
Filed 12/29/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

System and method for generating a power receiver identifier in a wireless power network  
Patent #
US 9,899,873 B2
Filed 05/23/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless charging methods and systems for game controllers, based on pocketforming  
Patent #
US 9,899,861 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods of transmitting power transmission waves based on signals received at first and second subsets of a transmitter's antenna array  
Patent #
US 9,906,065 B2
Filed 12/27/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Identifying receivers in a wireless charging transmission field  
Patent #
US 9,906,275 B2
Filed 09/15/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Receivers for wireless power transmission  
Patent #
US 9,912,199 B2
Filed 05/10/2013

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for automatically testing the communication between power transmitter and wireless receiver  
Patent #
US 9,917,477 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for wireless power transmission by modifying a number of antenna elements used to transmit power waves to a receiver  
Patent #
US 9,923,386 B1
Filed 06/23/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Phase and amplitude detection in wireless energy transfer systems  
Patent #
US 9,929,721 B2
Filed 10/12/2016

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

Wireless power transmitters that transmit at determined times based on power availability and consumption at a receiving mobile device  
Patent #
US 9,935,482 B1
Filed 12/31/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

System and method to control a wireless power transmission system by configuration of wireless power transmission control parameters  
Patent #
US 9,939,864 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Home base station for multiple room coverage with multiple transmitters  
Patent #
US 9,941,707 B1
Filed 12/29/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

System and method for manually selecting and deselecting devices to charge in a wireless power network  
Patent #
US 9,941,747 B2
Filed 07/14/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods of object detection in wireless power charging systems  
Patent #
US 9,941,752 B2
Filed 02/25/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless sound charging of clothing and smart fabrics  
Patent #
US 9,941,705 B2
Filed 05/13/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless power transmission with selective range  
Patent #
US 9,941,754 B2
Filed 12/27/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Power generation for implantable devices  
Patent #
US 9,943,697 B2
Filed 07/27/2016

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

Systems and methods for identifying sensitive objects in a wireless charging transmission field  
Patent #
US 9,948,135 B2
Filed 09/22/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless power transfer for a seatvesthelmet system  
Patent #
US 9,948,145 B2
Filed 12/31/2014

Current Assignee
Witricity Corporation

Sponsoring Entity
Dish Technologies LLC

Object detection for wireless energy transfer systems  
Patent #
US 9,952,266 B2
Filed 02/13/2015

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

System and method for selfsystem analysis for detecting a fault in a wireless power transmission Network  
Patent #
US 9,954,374 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless power transfer systems for surfaces  
Patent #
US 9,954,375 B2
Filed 06/19/2015

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

Systems and methods for assigning a power receiver to individual power transmitters based on location of the power receiver  
Patent #
US 9,965,009 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for extending battery life of portable electronic devices charged by sound  
Patent #
US 9,966,784 B2
Filed 06/03/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Multimode transmitter  
Patent #
US 9,966,765 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for using a transmitter access policy at a network service to determine whether to provide power to wireless power receivers in a wireless power network  
Patent #
US 9,967,743 B1
Filed 06/23/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless power receiver with boost converters directly coupled to a storage element  
Patent #
US 9,973,008 B1
Filed 12/29/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Receivers for wireless power transmission  
Patent #
US 9,973,021 B2
Filed 12/27/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless electric field power transfer system, method, transmitter and receiver therefor  
Patent #
US 9,979,206 B2
Filed 09/04/2015

Current Assignee
SOLACE POWER INC.

Sponsoring Entity
SOLACE POWER INC.

Antenna tile arrangements configured to operate as one functional unit  
Patent #
US 9,979,440 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

System for tracking and reporting status and usage information in a wireless power management system  
Patent #
US 9,991,741 B1
Filed 12/31/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Power transmission device, power transmission method, and power transmission system  
Patent #
US 10,003,385 B2
Filed 08/01/2016

Current Assignee
Sony Corporation

Sponsoring Entity
Sony Corporation

Battery life of portable electronic devices  
Patent #
US 10,003,211 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Modular antennas with heat sinks in wireless power transmission systems  
Patent #
US 10,008,886 B2
Filed 03/03/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless power transmission system calculating the battery charge state of the receiver based on the supply impedance of the power source and the summed impedance of the wireless transmitter, wireless receiver, medium therebetween, and battery charge circuit  
Patent #
US 10,008,888 B2
Filed 07/18/2014

Current Assignee
Electronics and Telecommunications Research Institute

Sponsoring Entity
Electronics and Telecommunications Research Institute

Wireless power transmitter configured to transmit power waves to a predicted location of a moving wireless power receiver  
Patent #
US 10,008,875 B1
Filed 09/16/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Method for automatically testing the operational status of a wireless power receiver in a wireless power transmission system  
Patent #
US 10,008,889 B2
Filed 08/21/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless power receiver having a charger system for enhanced power delivery  
Patent #
US 10,014,728 B1
Filed 06/23/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Foreign object detection in wireless energy transfer systems  
Patent #
US 10,018,744 B2
Filed 05/07/2015

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

Systems and methods for selecting antennas to generate and transmit power transmission waves  
Patent #
US 10,020,678 B1
Filed 09/22/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Proximity transmitters for wireless power charging systems  
Patent #
US 10,021,523 B2
Filed 12/24/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Foreign object detection in wireless energy transfer systems  
Patent #
US 10,027,184 B2
Filed 06/01/2016

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

Antenna for transmitting wireless power signals  
Patent #
US 10,027,159 B2
Filed 03/03/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

3D triple linear antenna that acts as heat sink  
Patent #
US 10,027,180 B1
Filed 11/02/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Near field transmitters for wireless power charging of an electronic device by leaking RF energy through an aperture  
Patent #
US 10,027,158 B2
Filed 02/17/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for generating and transmitting wireless power transmission waves using antennas having a spacing that is selected by the transmitter  
Patent #
US 10,027,168 B2
Filed 09/22/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless electric field power transmission system, transmitter and receiver therefor and method of wirelessly transferring power  
Patent #
US 10,033,225 B2
Filed 06/26/2015

Current Assignee
SOLACE POWER INC.

Sponsoring Entity
SOLACE POWER INC.

Systems and methods for determining and generating a waveform for wireless power transmission waves  
Patent #
US 10,033,222 B1
Filed 09/22/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods of wireless power charging through multiple receiving devices  
Patent #
US 10,038,332 B1
Filed 02/19/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless power supply for rescue devices  
Patent #
US 10,038,337 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Social power sharing for mobile devices based on pocketforming  
Patent #
US 10,050,462 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless power transmission device having antennas oriented in three dimensions  
Patent #
US 10,050,470 B1
Filed 09/22/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Methods and systems for maximum power point transfer in receivers  
Patent #
US 10,056,782 B1
Filed 04/10/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

POWER TRANSMISSION DEVICE, POWER TRANSMISSION METHOD, AND POWER TRANSMISSION SYSTEM  
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US 20180241437A1
Filed 04/17/2018

Current Assignee
Sony Corporation

Sponsoring Entity
Sony Corporation

Stamped threedimensional antenna  
Patent #
US 10,063,108 B1
Filed 11/02/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Foreign object detection in wireless energy transfer systems  
Patent #
US 10,063,110 B2
Filed 10/19/2016

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

System and method for a selfsystem analysis in a wireless power transmission network  
Patent #
US 10,063,106 B2
Filed 05/23/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Proximity transmitters for wireless power charging systems  
Patent #
US 10,063,105 B2
Filed 12/24/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

System and method for generating a power receiver identifier in a wireless power network  
Patent #
US 10,063,064 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

PWM capacitor control  
Patent #
US 10,063,104 B2
Filed 02/08/2017

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

Integrated miniature PIFA with artificial magnetic conductor metamaterials  
Patent #
US 10,068,703 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

External or internal wireless power receiver with spacedapart antenna elements for charging or powering mobile devices using wirelessly delivered power  
Patent #
US 10,075,017 B2
Filed 12/29/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for manually adjusting when receiving electronic devices are scheduled to receive wirelessly delivered power from a wireless power transmitter in a wireless power network  
Patent #
US 10,075,008 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Voltage source isolation in wireless power transfer systems  
Patent #
US 10,075,019 B2
Filed 11/21/2016

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

Nearfield RF charging pad with multiband antenna element with adaptive loading to efficiently charge an electronic device at any position on the pad  
Patent #
US 10,079,515 B2
Filed 12/06/2017

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless energy transfer for implantable devices  
Patent #
US 10,084,348 B2
Filed 10/31/2016

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

Wireless powered house  
Patent #
US 10,090,699 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

System and method for enabling automatic charging schedules in a wireless power network to one or more devices  
Patent #
US 10,090,886 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Minimallyinvasive tissue ablation using high contrast electric fields  
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US 10,092,351 B2
Filed 02/11/2015

Current Assignee
Elwha LLC

Sponsoring Entity
Elwha LLC

Wireless energy transfer  
Patent #
US 10,097,044 B2
Filed 06/20/2016

Current Assignee
Massachusetts Institute of Technology

Sponsoring Entity
Massachusetts Institute of Technology

Wireless energy transfer for photovoltaic panels  
Patent #
US 10,097,011 B2
Filed 04/30/2015

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

Protocols for authenticated wireless power transmission  
Patent #
US 10,103,552 B1
Filed 12/29/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Transmitters for wireless power transmission  
Patent #
US 10,103,582 B2
Filed 05/10/2013

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and method for wireless power transfer  
Patent #
US 10,115,520 B2
Filed 03/14/2013

Current Assignee
Mojo Mobility Inc.

Sponsoring Entity
Mojo Mobility Inc.

Near field transmitters with harmonic filters for wireless power charging  
Patent #
US 10,116,162 B2
Filed 02/17/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Methods and systems for maximum power point transfer in receivers  
Patent #
US 10,116,170 B1
Filed 09/02/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Integrated antenna arrays for wireless power transmission  
Patent #
US 10,116,143 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for assigning a set of antennas of a wireless power transmitter to a wireless power receiver based on a location of the wireless power receiver  
Patent #
US 10,122,415 B2
Filed 12/29/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems, methods, and devices for using a battery as a antenna for receiving wirelessly delivered power from radio frequency power waves  
Patent #
US 10,122,219 B1
Filed 04/12/2018

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless charging and powering of electronic sensors in a vehicle  
Patent #
US 10,124,754 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for identifying receiver locations using sensor technologies  
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US 10,128,686 B1
Filed 09/22/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Hybrid WiFi and power router transmitter  
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US 10,128,695 B2
Filed 06/25/2013

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

System and method for providing health safety in a wireless power transmission system  
Patent #
US 10,128,693 B2
Filed 07/14/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods of providing wireless power using receiver device sensor inputs  
Patent #
US 10,128,699 B2
Filed 03/03/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for preconfiguring transmission devices for power wave transmissions based on location data of one or more receivers  
Patent #
US 10,135,294 B1
Filed 09/22/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Offpremises alert system and method for wireless power receivers in a wireless power network  
Patent #
US 10,134,260 B1
Filed 06/13/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Near field transmitters for wireless power charging of an electronic device by leaking RF energy through an aperture offset from a patch antenna  
Patent #
US 10,135,286 B2
Filed 02/17/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

3D antenna mount  
Patent #
US 10,135,112 B1
Filed 11/02/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for nullifying energy levels for wireless power transmission waves  
Patent #
US 10,135,295 B2
Filed 09/22/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless nonradiative energy transfer  
Patent #
US 10,141,790 B2
Filed 10/25/2017

Current Assignee
Massachusetts Institute of Technology

Sponsoring Entity
Massachusetts Institute of Technology

Systems and methods for maximizing wireless power transfer efficiency by instructing a user to change a receiver device's position  
Patent #
US 10,141,768 B2
Filed 12/29/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Dynamic tuning in wireless energy transfer systems  
Patent #
US 10,141,788 B2
Filed 10/14/2016

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

Near field transmitters with contact points for wireless power charging  
Patent #
US 10,141,771 B1
Filed 02/17/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for controlling communications during wireless transmission of power using application programming interfaces  
Patent #
US 10,141,791 B2
Filed 05/07/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless power transmission with selective range  
Patent #
US 10,148,133 B2
Filed 07/20/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for using a predetermined number of communication channels of a wireless power transmitter to communicate with different wireless power receivers  
Patent #
US 10,148,097 B1
Filed 12/29/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for designating a master power transmitter in a cluster of wireless power transmitters  
Patent #
US 10,153,645 B1
Filed 03/04/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for using application programming interfaces to control communications between a transmitter and a receiver  
Patent #
US 10,153,653 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for preconfiguring sensor data for wireless charging systems  
Patent #
US 10,153,660 B1
Filed 09/22/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for identifying receivers in a transmission field by transmitting exploratory power waves towards different segments of a transmission field  
Patent #
US 10,158,259 B1
Filed 09/16/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless energy transfer for rechargeable batteries  
Patent #
US 10,158,251 B2
Filed 04/01/2016

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

System and methods for using sound waves to wirelessly deliver power to electronic devices  
Patent #
US 10,158,257 B2
Filed 01/03/2017

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Modular antenna boards in wireless power transmission systems  
Patent #
US 10,164,478 B2
Filed 03/03/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for managing and controlling a wireless power network by establishing time intervals during which receivers communicate with a transmitter  
Patent #
US 10,170,917 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Radiating metamaterial antenna for wireless charging  
Patent #
US 10,177,594 B2
Filed 12/26/2017

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for wireless power system with improved performance and/or ease of use  
Patent #
US 10,186,372 B2
Filed 12/07/2017

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

Receiver device with antennas positioned in gaps  
Patent #
US 10,186,892 B2
Filed 02/17/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Boost converter and controller for increasing voltage received from wireless power transmission waves  
Patent #
US 10,186,911 B2
Filed 10/31/2017

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Wireless power transfer systems with shield openings  
Patent #
US 10,186,373 B2
Filed 02/01/2018

Current Assignee
Witricity Corporation

Sponsoring Entity
Witricity Corporation

Systems and methods for real time or near real time wireless communications between a wireless power transmitter and a wireless power receiver  
Patent #
US 10,186,893 B2
Filed 03/04/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

System and methods for pocketforming based on constructive and destructive interferences to power one or more wireless power receivers using a wireless power transmitter including a plurality of antennas  
Patent #
US 10,186,913 B2
Filed 12/27/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Cluster management of transmitters in a wireless power transmission system  
Patent #
US 10,193,396 B1
Filed 03/04/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for wirelessly transmitting power from a transmitter to a receiver by determining refined locations of the receiver in a segmented transmission field associated with the transmitter  
Patent #
US 10,199,850 B2
Filed 09/16/2015

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Radar motion detection using stepped frequency in wireless power transmission system  
Patent #
US 10,199,835 B2
Filed 03/02/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Method for automatically testing the operational status of a wireless power receiver in a wireless power transmission system  
Patent #
US 10,199,849 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Compact PIFA antenna  
Patent #
US 10,205,239 B1
Filed 12/30/2014

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

System and methods for wireless power transmission to an electronic device in accordance with userdefined restrictions  
Patent #
US 10,206,185 B2
Filed 06/03/2013

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Systems and methods for real or near real time wireless communications between a wireless power transmitter and a wireless power receiver  
Patent #
US 10,211,685 B2
Filed 03/04/2016

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Method for 3 dimensional pocketforming  
Patent #
US 10,211,680 B2
Filed 07/19/2013

Current Assignee
Energous Corporation

Sponsoring Entity
Energous Corporation

Foreign object detection in wireless energy transfer systems  
Patent #
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US 5,437,057 A
Filed 12/03/1992

Current Assignee
Xerox Corporation

Sponsoring Entity
Xerox Corporation

Contactless battery charging system  
Patent #
US 5,341,083 A
Filed 10/20/1992

Current Assignee
Electric Power Research Institute

Sponsoring Entity
Electric Power Research Institute

Separable inductive coupler  
Patent #
US 5,216,402 A
Filed 01/22/1992

Current Assignee
General Motors Corporation

Sponsoring Entity
Hughes Aircraft Company

Magnetic induction mine arming, disarming and simulation system  
Patent #
US 5,027,709 A
Filed 11/13/1990

Current Assignee
Slagle Glenn B.

Sponsoring Entity
Slagle Glenn B.

RF beam center location method and apparatus for power transmission system  
Patent #
US 4,088,999 A
Filed 05/21/1976

Current Assignee
Fletcher James C. Administrator of the National Aeronautics and Space Administration with respect to an invention of, Dickinson Richard M.

Sponsoring Entity
Fletcher James C. Administrator of the National Aeronautics and Space Administration with respect to an invention of, Dickinson Richard M.

ENERGY TRANSLATING DEVICE  
Patent #
US 3,517,350 A
Filed 07/07/1969

Current Assignee
Beaver William D.

Sponsoring Entity
Beaver William D.

63 Claims
 1. An apparatus for use in wireless energy transfer, the apparatus comprising:
 a first resonator structure configured for energy transfer with a second resonator structure, over a distance D larger than a characteristic size L_{1 }of said first resonator structure and larger than a characteristic size L_{2 }of said second resonator structure, wherein the energy transfer has a rate κ and is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, wherein said resonant field of the first resonator structure has a resonance angular frequency ω_{1}, a resonance frequencywidth Γ_{1}, and a resonance quality factor Q_{1}=ω_{1}/2Γ_{1 }at least larger than 300, and said resonant field of the second resonator structure has a resonance angular frequency ω_{2}, a resonance frequencywidth Γ_{2}, and a resonance quality factor Q_{2}=ω_{2}/2Γ_{2 }at least larger than 300, wherein the absolute value of the difference of said angular frequencies ω_{1 }and ω_{2 }is smaller than the broader of said resonant widths Γ_{1 }and Γ_{2}, and the quantity κ/√{square root over (Γ_{1}Γ_{2})} is at least larger than 20, and further comprising a power supply coupled to the first structure and configured to drive the first resonator structure or the second resonator structure at an angular frequency away from the resonance angular frequencies and shifted towards a frequency corresponding to an odd normal mode for the resonator structures to reduce radiation from the resonator structures by destructive farfield interference.
 View Dependent Claims (2)
 3. A method for wireless energy transfer involving a first resonator structure configured for energy transfer with a second resonator structure, over a distance D larger than a characteristic size L1 of said first resonator structure and larger than a characteristic size L2 of said second resonator structure, wherein the energy transfer has a rate κ and is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, wherein said resonant field of the first resonator structure has a resonance angular frequency ω1, a resonance frequencywidth Γ1, and a resonance quality factor Q1=ω1/2Γ1 at least larger than 300, and said resonant field of the second resonator structure has a resonance angular frequency ω2, a resonance frequencywidth Γ2, and a resonance quality factor Q2=ω2/2Γ2 at least larger than 300, wherein the absolute value of the difference of said angular frequencies ω1 and ω2 is smaller than the broader of said resonant widths Γ1 and Γ2, and the quantity κ/√{square root over (Γ1Γ2)} is at least larger than 20, the method comprising:
 driving the first resonator structure or the second resonator structure at an angular frequency away from the resonance angular frequencies and shifted towards a frequency corresponding to an odd normal mode for the resonator structures to reduce radiation from the resonator structures by destructive farfield interference.
 View Dependent Claims (4)
 5. An apparatus for use in wireless energy transfer, the apparatus comprising:
 a first resonator structure configured for energy transfer with a second resonator structure, over a distance D larger than a characteristic size L_{1 }of said first resonator structure and larger than a characteristic size L_{2 }of said second resonator structure, wherein the energy transfer has a rate κ and is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, wherein said resonant field of the first resonator structure has a resonance angular frequency ω_{1}, a resonance frequencywidth Γ_{1}, and a resonance quality factor Q_{1}=ω_{1}/2Γ_{1 }at least larger than 300, and said resonant field of the second resonator structure has a resonance angular frequency ω_{2}, a resonance frequencywidth Γ_{2}, and a resonance quality factor Q_{2}=ω_{2}/2Γ_{2 }at least larger than 300, wherein the absolute value of the difference of said angular frequencies ω_{1 }and ω_{2 }is smaller than the broader of said resonant widths Γ_{1 }and Γ_{2}, and the quantity κ/√{square root over (Γ_{1}Γ_{2})} is at least larger than 20, and wherein for a desired range of the distances D, the resonance angular frequencies for the resonator structures increase transmission efficiency T by accounting for radiative interference, wherein the increase is relative to a transmission efficiency T calculated without accounting for the radiative interference.
 View Dependent Claims (6)
 7. 7 . A method for designing a wireless energy transfer apparatus, the apparatus including a first resonator structure configured for energy transfer with a second resonator structure, over a distance D larger than a characteristic size L1 of said first resonator structure and larger than a characteristic size L2 of said second resonator structure, wherein the energy transfer has a rate κ and is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, wherein said resonant field of the first resonator structure has a resonance angular frequency ω1, a resonance frequencywidth Γ1, and a resonance quality factor Q1=ω1/2Γ1 at least larger than 300, and said resonant field of the second resonator structure has a resonance angular frequency ω2, a resonance frequencywidth Γ2, and a resonance quality factor Q2=ω2/2Γ2 at least larger than 300, wherein the absolute value of the difference of said angular frequencies ω1 and ω2 is smaller than the broader of said resonant widths Γ1 and Γ2, and the quantity κ/√{square root over (Γ1Γ2)} is at least larger than 20, the method comprising:
 selecting the resonance angular frequencies for the resonator structures to substantially optimize the transmission efficiency by accounting for radiative interference between the resonator structures.
 View Dependent Claims (8)
 9. An apparatus for use in wireless energy transfer, the apparatus comprising:
 a first resonator structure configured for energy transfer with a second resonator structure over a distance D, wherein the energy transfer is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, with a coupling factor k, wherein said resonant field of the first resonator structure has a resonance angular frequency ω_{1}, a resonance frequencywidth Γ_{1}, and a resonance quality factor Q_{1}=ω_{1}/2Γ_{1}, and is radiative in the far field, with an associated radiation quality factor Q_{1,rad}≧Q_{1}, and said resonant field of the second resonator structure has a resonance angular frequency ω_{2}, a resonance frequencywidth Γ_{2}, and a resonance quality factor Q_{2}=ω_{2}/2Γ_{2 }and is radiative in the far field, with an associated radiation quality factor Q_{2,rad}≧Q_{2}, wherein an absolute value of a difference of said angular frequencies ω_{1 }and ω_{2 }is smaller than broader of said resonant widths Γ_{1 }and Γ_{2}, and an average resonant angular frequency is defined as ω_{o}=√{square root over (ω_{1}ω_{2})}, corresponding to an average resonant wavelength λ_{o}=2πc/ω_{0}, where c is the speed of light in free space, and a strongcoupling factor being defined as U=k√{square root over (Q_{1}Q_{2})}, wherein the apparatus is configured to employ interference between said radiative far fields of the resonant fields of the first and second resonator, with an interference factor V_{rad}, to reduce a total amount of radiation from the apparatus compared to an amount of radiation from the apparatus in the absence of interference, a stronginterference factor being defined as V=V_{rad}√{square root over ((Q_{1}/Q_{1,rad})(Q_{2}/Q_{2,rad}))}{square root over ((Q_{1}/Q_{1,rad})(Q_{2}/Q_{2,rad}))}.
 View Dependent Claims (10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30)
 31. An apparatus for use in wireless energy transfer, the apparatus comprising:
 a first resonator structure configured for energy transfer with a second resonator structure over a distance D, wherein the energy transfer is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, with a coupling factor k, wherein said resonant field of the first resonator structure has a resonance angular frequency ω_{1}, a resonance frequencywidth Γ_{1}, and a resonance quality factor Q_{1}=ω_{1}/2Γ_{1}, and is radiative in the far field, with an associated radiation quality factor Q_{1,rad}≧Q_{1}, and said resonant field of the second resonator structure has a resonance angular frequency ω_{2}, a resonance frequencywidth Γ_{2}, and a resonance quality factor Q_{2}=ω_{2}/2Γ_{2}, and is radiative in the far field, with an associated radiation quality factor Q_{2,rad}≧Q_{2}, wherein an absolute value of a difference of said angular frequencies ω_{1 }and ω_{2 }is smaller than the broader of said resonant widths Γ_{1 }and Γ_{2}, and an average resonant angular frequency is defined as ω_{o}=√{square root over (ω_{1}ω_{2})}, corresponding to an average resonant wavelength λ_{o}=2πc/ω_{o}, where c is the speed of light in free space, and a strongcoupling factor is defined as U=k√{square root over (Q_{1}Q_{2})}, wherein the apparatus is configured to employ interference between said radiative far fields of the resonant fields of the first and second resonator, with an interference factor V_{rad}, to increase efficiency of energy transfer for the apparatus compared to efficiency for the apparatus in the absence of interference, the stronginterference factor being defined as V=V_{rad}√{square root over ((Q_{1}/Q_{1,rad})(Q_{2}/Q_{2,rad}))}{square root over ((Q_{1}/Q_{1,rad})(Q_{2}/Q_{2,rad}))}.
 View Dependent Claims (32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54)
 55. A method for wireless energy transfer, the method comprising:
 providing a first resonator structure configured for energy transfer with a second resonator structure over a distance D, wherein the energy transfer is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, with a coupling factor k, wherein said resonant field of the first resonator structure has a resonance angular frequency ω_{1}, a resonance frequencywidth Γ_{1}, and a resonance quality factor Q_{1}=ω_{1}/2Γ_{1}, and is radiative in the far field, with an associated radiation quality factor Q_{1,rad}≧Q_{1}, and said resonant field of the second resonator structure has a resonance angular frequency ω_{2}, a resonance frequencywidth Γ_{2}, and a resonance quality factor Q_{2}=ω_{2}/2Γ_{2}, and is radiative in the far field, with an associated radiation quality factor Q_{2,rad}≧Q_{2}, wherein an absolute value of a difference of said angular frequencies ω_{1 }and ω_{2 }is smaller than broader of said resonant widths Γ_{1 }and Γ_{2}, and an average resonant angular frequency is defined as ω_{o}=√{square root over (ω_{1}ω_{2})}, corresponding to an average resonant wavelength λ_{o}=2πc/ω_{o}, where c is the speed of light in free space, and the strongcoupling factor is defined as U=k√{square root over (Q_{1}Q_{2})}, and employing interference between said radiative far fields of the resonant fields of the first and second resonator, with an interference factor V_{rad}, to reduce a total amount of radiation from the first and second resonator compared to an amount of radiation from the first and second resonator in the absence of interference, a stronginterference factor being defined as V=V_{rad}√{square root over ((Q_{1}/Q_{1,rad})(Q_{2}/Q_{2,rad}))}{square root over ((Q_{1}/Q_{1,rad})(Q_{2}/Q_{2,rad}))}.
 View Dependent Claims (56, 57, 58)
 59. A method for wireless energy transfer, the method comprising:
 providing a first resonator structure configured for energy transfer with a second resonator structure over a distance D, wherein the energy transfer is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, with a coupling factor k, wherein said resonant field of the first resonator structure has a resonance angular frequency ω_{1}, a resonance frequencywidth Γ_{1}, and a resonance quality factor Q_{1}=ω_{1}/2Γ_{1}, and is radiative in the far field, with an associated radiation quality factor Q_{1,rad}≧Q_{1}, and said resonant field of the second resonator structure has a resonance angular frequency ω_{2}, a resonance frequencywidth Γ_{2}, and a resonance quality factor Q_{2}=ω_{2}/2Γ_{2}, and is radiative in the far field, with an associated radiation quality factor Q_{2,rad}≧Q_{2}, wherein an absolute value of the difference of said angular frequencies ω_{1 }and ω_{2 }is smaller than the broader of said resonant widths Γ_{1 }and Γ_{2 }, and an average resonant angular frequency is defined as ω_{o}=√{square root over (ω_{1ω}_{2})}, corresponding to an average resonant wavelength λ_{o}=2πc/ω_{o}, where c is the speed of light in free space, and the strongcoupling factor is defined as U=k√{square root over (Q_{1}Q_{2})}, and employing interference between said radiative far fields of the resonant fields of the first and second resonator, with an interference factor V_{rad}, to increase efficiency of energy transfer between the first and second resonator compared to efficiency of energy transfer between the first and second resonator in the absence of interference, a stronginterference factor being defined as V=V_{rad}√{square root over ((Q_{1}/Q_{1,rad})(Q_{2}/Q_{2,rad}))}{square root over ((Q_{1}/Q_{1,rad})(Q_{2}/Q_{2,rad}))}.
 View Dependent Claims (60, 61, 62, 63)
1 Specification
Pursuant to U.S.C. §119(e), this application claims priority to U.S. Provisional Application Ser. No. 61/127,661, filed May 14, 2008.
This application is also related by subject matter to the following commonly owned applications: U.S. Utility patent application Ser. No. 12/055,963, filed Mar. 26, 2008; U.S. Utility patent application Ser. No. 11/481,077, filed Jul. 5, 2006; U.S. Provisional Application Ser. No. 60/698,442, filed Jul. 12, 2005; U.S. Provisional Application Ser. No. 60/908,383, filed Mar. 27, 2007; U.S. Provisional Application Ser. No. 60/908,666, filed Mar. 28, 2007; and International Application No. PCT/US2007/070892, filed Jun. 11, 2007.
The contents of the prior applications are incorporated herein by reference in their entirety.
BACKGROUNDThe disclosure relates to wireless energy transfer. Wireless energy transfer can for example, be useful in such applications as providing power to autonomous electrical or electronic devices.
Radiative modes of omnidirectional antennas (which work very well for information transfer) are not suitable for such energy transfer, because a vast majority of energy is wasted into free space. Directed radiation modes, using lasers or highlydirectional antennas, can be efficiently used for energy transfer, even for long distances (transfer distance L<sub>TRANS</sub>>>L<sub>DEV</sub>, where L<sub>DEV </sub>is the characteristic size of the device and/or the source), but require existence of an uninterruptible lineofsight and a complicated tracking system in the case of mobile objects. Some transfer schemes rely on induction, but are typically restricted to very closerange (L<sub>TRANS</sub><<L<sub>DEV</sub>) or low power (˜mW) energy transfers.
The rapid development of autonomous electronics of recent years (e.g. laptops, cellphones, household robots, that all typically rely on chemical energy storage) has led to an increased need for wireless energy transfer.
SUMMARYEfficient wireless energytransfer between two resonant objects can be achieved at midrange distances, provided these resonant objects are designed to operate in the ‘strongcoupling’ regime. We describe an implementation of a method to increase the efficiency of energytransfer or to suppress the power radiated, which can be harmful or a cause of interference to other communication systems, by utilizing destructive interference between the radiated farfields of the resonant coupled objects. ‘Strong coupling’ is a necessary condition for efficient energytransfer, in the absence of farfield interference. ‘Strong coupling’ can be demonstrated in the case of realistic systems: selfresonant conducting coils, capacitivelyloaded conducting coils, inductivelyloaded conducting rods and dielectric disks, all bearing highQ electromagnetic resonant modes. Also, an analytical model can be developed to take farfield interference into account for wireless energytransfer systems. The analytical model can be used to demonstrate the efficiency enhancement and radiation suppression, in the presence of interference. In an example implementation, we describe improved performance based on the above principles in the case of two realistic systems: capacitivelyloaded conducting coils and dielectric disks, both bearing highQ electromagnetic resonant modes and farfield interference.
In an aspect, an apparatus for use in wireless energy transfer includes a first resonator structure configured for energy transfer with a second resonator structure, over a distance D larger than a characteristic size L<sub>1 </sub>of said first resonator structure and larger than a characteristic size L<sub>2 </sub>of said second resonator structure. The energy transfer has a rate κ and is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure. The resonant field of the first resonator structure has a resonance angular frequency ω<sub>1</sub>, a resonance frequencywidth Γ<sub>1</sub>, and a resonance quality factor Q<sub>1</sub>=ω<sub>1</sub>/2Γ<sub>1 </sub>at least larger than 300, and the said resonant field of the second resonator structure has a resonance angular frequency ω<sub>2</sub>, a resonance frequencywidth Γ<sub>2</sub>, and a resonance quality factor Q<sub>2</sub>=ω<sub>2</sub>/2Γ<sub>2 </sub>at least larger than 300. The absolute value of the difference of said angular frequencies ω<sub>1 </sub>and ω<sub>2 </sub>is smaller than the broader of said resonant widths Γ<sub>1 </sub>and Γ<sub>2</sub>, and the quantity κ/√{square root over (Γ<sub>1</sub>Γ<sub>2</sub>)} is at least larger than 20. The apparatus also includes a power supply coupled to the first structure and configured to drive the first resonator structure or the second resonator structure at an angular frequency away from the resonance angular frequencies and shifted towards a frequency corresponding to an odd normal mode for the resonator structures to reduce radiation from the resonator structures by destructive farfield interference.
In some examples, the power supply is configured to drive the first resonator structure or the second resonator structure at the angular frequency away from the resonance angular frequencies and shifted towards the frequency corresponding to an odd normal mode for the resonator structures to substantially suppress radiation from the resonator structures by destructive farfield interference.
In an aspect, a method for wireless energy transfer involves a first resonator structure configured for energy transfer with a second resonator structure, over a distance D larger than a characteristic size L<sub>1 </sub>of said first resonator structure and larger than a characteristic size L<sub>2 </sub>of said second resonator structure, wherein the energy transfer has a rate κ and is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, said resonant field of the first resonator structure has a resonance angular frequency ω<sub>1</sub>, a resonance frequencywidth Γ<sub>1</sub>, and a resonance quality factor Q<sub>1</sub>=ω<sub>1</sub>/2Γ1 at least larger than 300, and said resonant field of the second resonator structure has a resonance angular frequency ω<sub>2 </sub>a resonance frequencywidth Γ<sub>2</sub>, and a resonance quality factor Q<sub>2</sub>=ω<sub>2</sub>2Γ<sub>2 </sub>at least larger than 300, the absolute value of the difference of said angular frequencies ω<sub>1 </sub>and ω<sub>2 </sub>is smaller than the broader of said resonant widths Γ<sub>1 </sub>and Γ<sub>2</sub>, and the quantity κ/√{square root over (Γ<sub>1</sub>Γ<sub>2</sub>)} is at least larger than 20. The method includes driving the first resonator structure or the second resonator structure at an angular frequency away from the resonance angular frequencies and shifted towards a frequency corresponding to an odd normal mode for the resonator structures to reduce radiation from the resonator structures by destructive farfield interference.
In some examples, the first resonator structure or the second resonator structure is driven at the angular frequency away from the resonance angular frequencies and shifted towards the frequency corresponding to an odd normal mode for the resonator structures to substantially suppress radiation from the resonator structures by destructive farfield interference.
In an aspect, an apparatus for use in wireless energy transfer includes a first resonator structure configured for energy transfer with a second resonator structure, over a distance D larger than a characteristic size L<sub>1 </sub>of said first resonator structure and larger than a characteristic size L<sub>2 </sub>of said second resonator structure. The energy transfer has a rate κ and is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure. The resonant field of the first resonator structure has a resonance angular frequency ω<sub>1</sub>, a resonance frequencywidth Γ<sub>1</sub>, and a resonance quality factor Q<sub>1</sub>=ω<sub>1</sub>/2Γ<sub>1 </sub>at least larger than 300, and the resonant field of the second resonator structure has a resonance angular frequency ω<sub>2</sub>, a resonance frequencywidth Γ<sub>2</sub>, and a resonance quality factor Q<sub>2</sub>=ω<sub>2</sub>/2Γ<sub>2 </sub>at least larger than 300. The absolute value of the difference of said angular frequencies ω<sub>1 </sub>and ω<sub>2 </sub>is smaller than the broader of said resonant widths Γ<sub>1 </sub>and Γ<sub>2</sub>, and the quantity κ/√{square root over (Γ<sub>1</sub>Γ<sub>2</sub>)} is at least larger than 20. For a desired range of the distances D, the resonance angular frequencies for the resonator structures increase transmission efficiency T by accounting for radiative interference, wherein the increase is relative to a transmission efficiency T calculated without accounting for the radiative interference.
In some examples, the resonance angular frequencies for the resonator structures are selected by optimizing the transmission efficiency T to account for both a resonance quality factor U and an interference factor V.
In an aspect, a method involves designing a wireless energy transfer apparatus, the apparatus including a first resonator structure configured for energy transfer with a second resonator structure, over a distance D larger than a characteristic size L<sub>1 </sub>of said first resonator structure and larger than a characteristic size L<sub>2 </sub>of said second resonator structure, wherein the energy transfer has a rate κ and is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, wherein said resonant field of the first resonator structure has a resonance angular frequency ω<sub>1</sub>, a resonance frequencywidth Γ<sub>1</sub>, and a resonance quality factor Q<sub>1</sub>=ω<sub>1</sub>/2Γ<sub>1 </sub>at least larger than 300, and said resonant field of the second resonator structure has a resonance angular frequency ω<sub>2</sub>, a resonance frequencywidth Γ<sub>2</sub>, and a resonance quality factor Q<sub>2</sub>=ω<sub>2</sub>/2Γ<sub>2 </sub>at least larger than 300, wherein the absolute value of the difference of said angular frequencies ω<sub>1 </sub>and ω<sub>2 </sub>is smaller than the broader of said resonant widths Γ<sub>1 </sub>and Γ<sub>2</sub>, and the quantity κ/√{square root over (Γ<sub>1</sub>Γ<sub>2</sub>)} is at least larger than 20. The method includes selecting the resonance angular frequencies for the resonator structures to substantially optimize the transmission efficiency by accounting for radiative interference between the resonator structures.
In some examples, the resonance angular frequencies for the resonator structures are selected by optimizing the transmission efficiency T to account for both a resonance quality factor U and an interference factor V.
In an aspect, an apparatus for use in wireless energy transfer includes a first resonator structure configured for energy transfer with a second resonator structure over a distance D. The energy transfer is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, with a coupling factor k. The resonant field of the first resonator structure has a resonance angular frequency ω<sub>1</sub>, a resonance frequencywidth Γ<sub>1</sub>, and a resonance quality factor Q<sub>1</sub>=ω<sub>1</sub>/2Γ<sub>1</sub>, and is radiative in the far field, with an associated radiation quality factor Q<sub>1,rad</sub>≧Q<sub>1</sub>, and the resonant field of the second resonator structure has a resonance angular frequency ω<sub>2</sub>, a resonance frequencywidth Γ<sub>2</sub>, and a resonance quality factor Q<sub>2</sub>=ω<sub>2</sub>/2Γ<sub>2</sub>, and is radiative in the far field, with an associated radiation quality factor Q<sub>2,rad</sub>≧Q<sub>2</sub>. An absolute value of a difference of said angular frequencies ω<sub>1 </sub>and ω<sub>2 </sub>is smaller than broader of said resonant widths Γ<sub>1 </sub>and Γ<sub>2</sub>, and an average resonant angular frequency is defined as ω<sub>o</sub>=√{square root over (ω<sub>1</sub>ω<sub>2</sub>)}, corresponding to an average resonant wavelength λ<sub>o</sub>=2πc/ω<sub>o</sub>, where c is the speed of light in free space, and a strongcoupling factor being defined as U=k√{square root over (Q<sub>1</sub>Q<sub>2</sub>)}. The apparatus is configured to employ interference between said radiative far fields of the resonant fields of the first and second resonator, with an interference factor V<sub>rad</sub>, to reduce a total amount of radiation from the apparatus compared to an amount of radiation from the apparatus in the absence of interference, a stronginterference factor being defined as V=V<sub>rad</sub>√{square root over ((Q<sub>1</sub>/Q<sub>1,rad</sub>)(Q<sub>2</sub>/Q<sub>2,rad</sub>))}{square root over ((Q<sub>1</sub>/Q<sub>1,rad</sub>)(Q<sub>2</sub>/Q<sub>2,rad</sub>))}.
The following are examples within the scope of this aspect.
The apparatus has Q<sub>1</sub>/Q<sub>1,rad</sub>≧0.01 and Q<sub>2</sub>/Q<sub>2,rad</sub>≧0.01. The apparatus has Q<sub>1</sub>/Q<sub>1,rad</sub>≧0.1 and Q<sub>2</sub>/Q<sub>2,rad</sub>≧0.1. The apparatus has D/λ<sub>o </sub>larger than 0.001 and the stronginterference factor V is larger than 0.01. The apparatus has D/λ<sub>o </sub>larger than 0.001 and the stronginterference factor V is larger than 0.1. The apparatus includes the second resonator structure.
During operation, a power generator is coupled to one of the first and second resonant structure, with a coupling rate κ<sub>g</sub>, and is configured to drive the resonator structure, to which it is coupled, at a driving frequency f, corresponding to a driving angular frequency ω=2πf, wherein U<sub>g </sub>is defined as κ<sub>g</sub>/Γ<sub>1</sub>, if the power generator is coupled to the first resonator structure and defined as κ<sub>g</sub>/Γ<sub>2</sub>, if the power generator is coupled to the second resonator structure. The driving frequency is different from the resonance frequencies of the first and second resonator structures and is closer to a frequency corresponding to an odd normal mode of the system of the two resonator structures, wherein the detuning of the first resonator from the driving frequency is defined as D<sub>1</sub>=(ω−ω<sub>1</sub>)/Γ<sub>1 </sub>and the detuning of the second resonator structure from the driving frequency is defined as D<sub>2</sub>=(ω−ω<sub>2</sub>)/Γ<sub>2</sub>.
D<sub>1 </sub>is approximately equal to UV<sub>rad </sub>and D<sub>2 </sub>is approximately equal to UV<sub>rad</sub>. U<sub>g </sub>is chosen to maximize the ratio of the energytransfer efficiency to the radiation efficiency. U<sub>g </sub>is approximately equal to √{square root over (1+U<sup>2</sup>−V<sub>rad</sub><sup>2</sup>U<sup>2</sup>+V<sup>2</sup>−2VV<sub>rad</sub>)}. f is at least larger than 100 kHz and smaller than 500 MHz. f is at least larger than 1 MHz and smaller than 50 MHz. The apparatus further includes the power generator. During operation, a power load is coupled to the resonant structure to which the power generator is not coupled, with a coupling rate κ<sub>1</sub>, and is configured to receive from the resonator structure, to which it is coupled, a usable power, wherein U<sub>1 </sub>is defined as κ<sub>1</sub>/Γ<sub>1</sub>, if the power load is coupled to the first resonator structure and defined as κ<sub>1</sub>/Γ<sub>2</sub>, if the power load is coupled to the second resonator structure. U<sub>1 </sub>is chosen to maximize the ratio of the energytransfer efficiency to the radiation efficiency. The driving frequency is different from the resonance frequencies of the first and second resonator structures and is closer to a frequency corresponding to an odd normal mode of the system of the two resonator structures, wherein the detuning of the first resonator from the driving frequency is defined as D<sub>1</sub>=(ω−ω<sub>1</sub>)/Γ<sub>1 </sub>and is approximately equal to UV<sub>rad</sub>, and the detuning of the second resonator structure from the driving frequency is defined as D<sub>2</sub>=(ω−ω<sub>2</sub>)/Γ<sub>2 </sub>and is approximately equal to UV<sub>rad</sub>, and U<sub>1 </sub>is approximately equal to √{square root over (1+U<sup>2</sup>−V<sub>rad</sub><sup>2</sup>+U<sup>2</sup>+V<sup>2</sup>−VV<sub>rad</sub>)}.
At least one of the first and second resonator structures comprises a capacitively loaded loop or coil of at least one of a conducting wire, a conducting Litz wire, and a conducting ribbon. The characteristic size of said loop or coil is less than 30 cm and the width of said conducting wire or Litz wire or ribbon is less than 2 cm. The characteristic size of said loop or coil is less than 1 m and the width of said conducting wire or Litz wire or ribbon is less than 2 cm.
The apparatus further includes a feedback mechanism for maintaining the resonant frequency of one or more of the resonant objects. The feedback mechanism comprises an oscillator with a fixed driving frequency and is configured to adjust the resonant frequency of the one or more resonant objects to be detuned by a fixed amount with respect to the fixed frequency.
In an aspect, an apparatus for use in wireless energy transfer includes a first resonator structure configured for energy transfer with a second resonator structure over a distance D. The energy transfer is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, with a coupling factor k. The resonant field of the first resonator structure has a resonance angular frequency ω<sub>1</sub>, a resonance frequencywidth Γ<sub>1</sub>, and a resonance quality factor Q<sub>1</sub>=ω<sub>1</sub>/2Γ<sub>1</sub>, and is radiative in the far field, with an associated radiation quality factor Q<sub>1,rad</sub>≧Q<sub>1</sub>, and the resonant field of the second resonator structure has a resonance angular frequency ω<sub>2</sub>, a resonance frequencywidth Γ<sub>2</sub>, and a resonance quality factor Q<sub>2</sub>=ω<sub>2</sub>/2Γ<sub>2</sub>, and is radiative in the far field, with an associated radiation quality factor Q<sub>2,rad</sub>≧Q<sub>2</sub>. An absolute value of a difference of said angular frequencies ω<sub>1 </sub>and ω<sub>2 </sub>is smaller than the broader of said resonant widths Γ<sub>1 </sub>and Γ<sub>2</sub>, and an average resonant angular frequency is defined as ω<sub>o</sub>=√{square root over (ω<sub>1</sub>ω<sub>2</sub>)}, corresponding to an average resonant wavelength λ<sub>o</sub>=2πc/ω<sub>o</sub>, where c is the speed of light in free space, and a strongcoupling factor is defined as U=k√{square root over (Q<sub>1</sub>Q<sub>2</sub>)}. The apparatus is configured to employ interference between said radiative far fields of the resonant fields of the first and second resonator, with an interference factor V<sub>rad</sub>, to increase efficiency of energy transfer for the apparatus compared to efficiency for the apparatus in the absence of interference, the stronginterference factor being defined as V=V<sub>rad</sub>√{square root over ((Q<sub>1</sub>/Q<sub>1,rad</sub>)(Q<sub>2</sub>/Q<sub>2,rad</sub>))}{square root over ((Q<sub>1</sub>/Q<sub>1,rad</sub>)(Q<sub>2</sub>/Q<sub>2,rad</sub>))}.
The following are examples within the scope of this aspect.
The apparatus has Q<sub>1</sub>/Q<sub>1,rad</sub>≧0.05 and Q<sub>2</sub>/Q<sub>2,rad</sub>≧0.05. The apparatus has Q<sub>1</sub>/Q<sub>1,rad</sub>≧0.5 and Q<sub>2</sub>/Q<sub>2,rad</sub>≧0.5. The apparatus has D/λ<sub>o </sub>larger than 0.01 and the stronginterference factor V is larger than 0.05. The apparatus has D/λ<sub>1 </sub>larger than 0.01 and the stronginterference factor V is larger than 0.5. The apparatus further includes the second resonator structure.
During operation, a power generator is coupled to one of the first and second resonant structure, with a coupling rate κ<sub>g</sub>, and is configured to drive the resonator structure, to which it is coupled, at a driving frequency f, corresponding to a driving angular frequency ω=2πf, wherein U<sub>g </sub>is defined as κ<sub>g</sub>/Γ<sub>1</sub>, if the power generator is coupled to the first resonator structure and defined as κ<sub>g</sub>/Γ<sub>2</sub>, if the power generator is coupled to the second resonator structure. The driving frequency is different from the resonance frequencies of the first and second resonator structures and is closer to a frequency corresponding to an odd normal mode of the system of the two resonator structures, wherein the detuning of the first resonator from the driving frequency is defined as D<sub>1</sub>=(ω−ω<sub>1</sub>)/Γ<sub>1 </sub>and the detuning of the second resonator structure from the driving frequency is defined as D<sub>2</sub>=(ω−ω<sub>2</sub>)/Γ<sub>2</sub>.
D<sub>1 </sub>is approximately equal to UV and D<sub>2 </sub>is approximately equal to UV. U<sub>g </sub>is chosen to maximize the energytransfer efficiency. U<sub>g </sub>is approximately equal to √{square root over ((1+U<sup>2</sup>)(1−V<sup>2</sup>))}{square root over ((1+U<sup>2</sup>)(1−V<sup>2</sup>))}. f is at least larger than 100 kHz and smaller than 500 MHz. f is at least larger than 1 MHz and smaller than 50 MHz. The apparatus further includes the power generator.
During operation, a power load is coupled to the resonant structure to which the power generator is not coupled, with a coupling rate κ<sub>1</sub>, and is configured to receive from the resonator structure, to which it is coupled, a usable power, wherein U<sub>1 </sub>is defined as κ<sub>1</sub>/Γ<sub>1</sub>, if the power load is coupled to the first resonator structure and defined as κ<sub>1</sub>/Γ<sub>2</sub>, if the power load is coupled to the second resonator structure. U<sub>1 </sub>is chosen to maximize the energytransfer efficiency. The driving frequency is different from the resonance frequencies of the first and second resonator structures and is closer to a frequency corresponding to an odd normal mode of the system of the two resonator structures, wherein the detuning of the first resonator from the driving frequency is defined as D<sub>1</sub>=(ω−ω<sub>1</sub>)/Γ<sub>1 </sub>and is approximately equal to UV, and the detuning of the second resonator structure from the driving frequency is defined as D<sub>2</sub>=(ω−ω<sub>2</sub>)/Γ<sub>2 </sub>and is approximately equal to UV, and U<sub>1 </sub>is approximately equal to √{square root over ((1+U<sup>2</sup>)(1−V<sup>2</sup>))}{square root over ((1+U<sup>2</sup>)(1−V<sup>2</sup>))}.
At least one of the first and second resonator structures comprises a capacitively loaded loop or coil of at least one of a conducting wire, a conducting Litz wire, and a conducting ribbon. The characteristic size of said loop or coil is less than 30 cm and the width of said conducting wire or Litz wire or ribbon is less than 2 cm. The characteristic size of said loop or coil is less than 1 m and the width of said conducting wire or Litz wire or ribbon is less than 2 cm. The apparatus includes a feedback mechanism for maintaining the resonant frequency of one or more of the resonant objects. The feedback mechanism comprises an oscillator with a fixed driving frequency and is configured to adjust the resonant frequency of the one or more resonant objects to be detuned by a fixed amount with respect to the fixed frequency. The feedback mechanism is configured to monitor an efficiency of the energy transfer, and adjust the resonant frequency of the one or more resonant objects to maximize the efficiency. The resonance angular frequencies for the resonator structures are selected to optimize the energytransfer efficiency by accounting for both the strongcoupling factor U and the stronginterference interference factor V.
In an aspect, a method for wireless energy transfer includes providing a first resonator structure configured for energy transfer with a second resonator structure over a distance D, wherein the energy transfer is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, with a coupling factor k, wherein said resonant field of the first resonator structure has a resonance angular frequency ω<sub>1</sub>, a resonance frequencywidth Γ<sub>1</sub>, and a resonance quality factor Q<sub>1</sub>=ω<sub>1</sub>/2Γ<sub>1</sub>, and is radiative in the far field, with an associated radiation quality factor Q<sub>1,rad</sub>≧Q<sub>1</sub>, and resonant field of the second resonator structure has a resonance angular frequency ω<sub>2</sub>, a resonance frequencywidth Γ<sub>2</sub>, and a resonance quality factor Q<sub>2</sub>=ω<sub>2</sub>/2Γ<sub>2</sub>, and is radiative in the far field, with an associated radiation quality factor Q<sub>2,rad</sub>≧Q<sub>2</sub>, wherein an absolute value of a difference of said angular frequencies ω<sub>1 </sub>and ω<sub>2 </sub>is smaller than broader of said resonant widths Γ<sub>1 </sub>and Γ<sub>2</sub>, and an average resonant angular frequency is defined as ω<sub>o</sub>=√{square root over (ω<sub>1</sub>ω<sub>2</sub>)}, corresponding to an average resonant wavelength λ<sub>o</sub>=2πc/ω<sub>o</sub>, where c is the speed of light in free space, and the strongcoupling factor is defined as U=k√{square root over (Q<sub>1</sub>Q<sub>2</sub>)}, and employing interference between said radiative far fields of the resonant fields of the first and second resonator, with an interference factor V<sub>rad</sub>, to reduce a total amount of radiation from the first and second resonator compared to an amount of radiation from the first and second resonator in the absence of interference, a stronginterference factor being defined as V=V<sub>rad</sub>√{square root over ((Q<sub>1</sub>/Q<sub>1,rad</sub>)(Q<sub>2</sub>/Q<sub>2,rad</sub>))}{square root over ((Q<sub>1</sub>/Q<sub>1,rad</sub>)(Q<sub>2</sub>/Q<sub>2,rad</sub>))}.
The following are examples within the scope of this aspect.
The method has Q<sub>1</sub>/Q<sub>1,rad</sub>≧0.01 and Q<sub>2</sub>/Q<sub>2,rad</sub>≧0.01. During operation, a power generator is coupled to one of the first and second resonant structure and is configured to drive the resonator structure, to which it is coupled, at a driving frequency f, corresponding to a driving, angular frequency ω=2πf, wherein the driving frequency is different from the resonance frequencies of the first and second resonator structures and is closer to a frequency corresponding to an odd normal mode of the system of the two resonator structures. During operation, a power load is coupled to the resonant structure to which the power generator is not coupled and is configured to receive from the resonator structure, to which it is coupled, a usable power. In an aspect, a method for wireless energy transfer includes providing a first resonator structure configured for energy transfer with a second resonator structure over a distance D, wherein the energy transfer is mediated by evanescenttail coupling of a resonant field of the first resonator structure and a resonant field of the second resonator structure, with a coupling factor k, wherein said resonant field of the first resonator structure has a resonance angular frequency ω<sub>1</sub>, a resonance frequencywidth Γ<sub>1</sub>, and a resonance quality factor Q<sub>1</sub>=ω<sub>1</sub>/2Γ<sub>1</sub>, and is radiative in the far field, with an associated radiation quality factor Q<sub>1,rad</sub>≧Q<sub>1</sub>, and said resonant field of the second resonator structure has a resonance angular frequency ω<sub>2</sub>, a resonance frequencywidth Γ<sub>2</sub>, and a resonance quality factor Q<sub>2</sub>=ω<sub>2</sub>/2Γ<sub>2</sub>, and is radiative in the far field, with an associated radiation quality factor Q<sub>2,rad</sub>≧Q<sub>2</sub>, wherein an absolute value of the difference of said angular frequencies ω<sub>1 </sub>and ω<sub>2 </sub>is smaller than the broader of said resonant widths Γ<sub>1 </sub>and Γ<sub>2</sub>, and an average resonant angular frequency is defined as ω<sub>o</sub>=√{square root over (ω<sub>1</sub>ω<sub>2</sub>)}, corresponding to an average resonant wavelength λ<sub>o</sub>=2πc/ω<sub>o</sub>, where c is the speed of light in free space, and the strongcoupling factor is defined as U=k√{square root over (Q<sub>1</sub>Q<sub>2</sub>)}, and employing interference between said radiative far fields of the resonant fields of the first and second resonator, with an interference factor V<sub>rad</sub>, to increase efficiency of energy transfer between the first and second resonator compared to efficiency of energy transfer between the first and second resonator in the absence of interference, a stronginterference factor being defined as V=V<sub>rad</sub>√{square root over ((Q<sub>1</sub>/Q<sub>1,rad</sub>)(Q<sub>2</sub>/Q<sub>2,rad</sub>))}{square root over ((Q<sub>1</sub>/Q<sub>1,rad</sub>)(Q<sub>2</sub>/Q<sub>2,rad</sub>))}.
The following are examples within the scope of this aspect.
The method has Q<sub>1</sub>/Q<sub>1,rad</sub>≧0.05 and Q<sub>2</sub>/Q<sub>2,rad</sub>≧0.05. During operation, a power generator is coupled to one of the first and second resonant structure and is configured to drive the resonator structure, to which it is coupled, at a driving frequency f, corresponding to a driving angular frequency ω=2πf, wherein the driving frequency is different from the resonance frequencies of the first and second resonator structures and is closer to a frequency corresponding to an odd normal mode of the system of the two resonator structures. During operation, a power load is coupled to the resonant structure to which the power generator is not coupled and is configured to receive from the resonator structure, to which it is coupled, a usable power. The resonance angular frequencies for the resonator structures are selected to optimize the energytransfer efficiency by accounting for both the strongcoupling factor U and the stronginterference interference factor V.
Various examples may include any of the above features, alone or in combination. Other features, objects, and advantages of the disclosure will be apparent from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 shows a schematic of an example wireless energy transfer scheme.
FIGS. 2(a)(b) show the efficiency of power transmission η<sub>P </sub>for (a) U=1 and (b) U=3, as a function of the frequency detuning D<sub>o </sub>and for different values of the loading rate U<sub>o</sub>.
FIG. 2(c) shows the optimal (for zero detuning and under conditions of impedance matching) efficiency for energy transfer η<sub>E</sub>* and power transmission η<sub>P</sub>*, as a function of the couplingtoloss figureofmerit U.
FIG. 3 shows an example of a selfresonant conductingwire coil.
FIG. 4 shows an example of a wireless energy transfer scheme featuring two selfresonant conductingwire coils
FIG. 5 is a schematic of an experimental system demonstrating wireless energy transfer.
FIG. 6 shows a comparison between experimental and theoretical results for the coupling rate of the system shown schematically in FIG. 5.
FIG. 7 shows a comparison between experimental and theoretical results for the strongcoupling factor of the system shown schematically in FIG. 5.
FIG. 8 shows a comparison between experimental and theoretical results for the powertransmission efficiency of the system shown schematically in FIG. 5.
FIG. 9 shows an example of a capacitively loaded conductingwire coil, and illustrates the surrounding field.
FIG. 10 shows an example wireless energy transfer scheme featuring two capacitively loaded conductingwire coils, and illustrates the surrounding field.
FIG. 11 illustrates an example circuit model for wireless energy transfer.
FIG. 12 shows the efficiency, total (loaded) device Q, and source and device currents, voltages and radiated powers (normalized to 1 Watt of output power to the load) as functions of the resonant frequency, for a particular choice of source and device loop dimensions, wp and N<sub>s </sub>and different choices of N<sub>d</sub>=1,2,3,4,5,6,10 (red, green, blue, magenta, yellow, cyan, black respectively).
FIG. 13 shows the efficiency, total (loaded) device Q, and source and device currents, voltages and radiated powers (normalized to 1 Watt of output power to the load) as functions of frequency and wp for a particular choice of source and device loop dimensions, and number of turns N<sub>s </sub>and N<sub>d</sub>.
FIG. 14 shows an example of an inductivelyloaded conductingwire coil.
FIG. 15 shows (a) an example of a resonant dielectric disk, and illustrates the surrounding field and (b) a wireless energy transfer scheme featuring two resonant dielectric disks, and illustrates the surrounding field.
FIGS. 16(a)(b) show the efficiency of power transmission η<sub>P </sub>for (a) U=1, V=0.5 and (b) U=3, V=0.5, as a function of the frequency detuning Do and for different values of the loading rate U<sub>o</sub>. (The dotted lines show, for comparison, the results when there is no interference, as shown in FIG. 2(a)(b).) and FIGS. 16(c)(d) show the optimal (for optimal detuning and under conditions of impedance matching) efficiency for energy transfer (only in (c)) and power transmission, as a function of the strongcoupling factor U and the stronginterference factor V.
FIG. 17 shows coupledmode theory (CMT) results for (a) the coupling factor k and (b) the strongcoupling factor U as a function of the relative distance D/r between two identical capacitivelyloaded conducting singleturn loops, for three different dimensions of the loops. Note that, for conducting material, copper (σ=5.998·10<sup>7 </sup>S/m) was used.
FIG. 18 shows AT results for the interference factor V<sub>rad </sub>as a function of the distance D (normalized to the wavelength λ) between two capacitivelyloaded conducting loops.
FIG. 19 shows CMT results for the strongcoupling factor U and AT results for the interference factor V<sub>rad </sub>and stronginterference factor V as a function of the resonant eigenfrequency of two identical capacitivelyloaded conducting singleturn loops with r=30 cm and a=2 cm, at a relative distance D/r=5 between them. Note that, for conducting material, copper (σ=5.998·10<sup>7 </sup>S/m ) was used.
FIG. 20 shows the powertransmission efficiency as a function of the resonant eigenfrequency of two identical capacitivelyloaded conducting singleturn loops. Results for two different loop dimensions are shown and for two relative distances between the identical loops. For each loops dimension and distance, four different cases are examined: without farfield interference (dotted), with farfield interference but no drivingfrequency detuning (dashed) and with drivingfrequency detuning to maximize either the efficiency (solid) or the ratio of efficiency over radiation (dashdotted).
FIG. 21 shows the drivingfrequency detunings required in the presence of farfield interference as a function of the resonant eigenfrequency of two identical capacitivelyloaded conducting singleturn loops of FIG. 20 to maximize either the efficiency (solid) or the ratio of efficiency over radiation (dashdotted).
FIG. 22(a) shows the resonant eigenfrequencies f<sub>U </sub>and fη, where the strongcoupling factor U and the powertransmission efficiency η peak respectively, as a function of the relative distance D/r between two identical loops with r=30 cm and a=2 cm.
FIG. 22(b) illustrates the strongcoupling factor U and the stronginterference factor V as a curve in the U−V plane, parametrized with the relative distance D/r between the two loops, for the cases with interference and eigenfrequency f<sub>η</sub> (solid), with interference and eigenfrequency f<sub>U </sub>(dashed), and without interference and eigenfrequency f<sub>U </sub>(dotted).
FIG. 22(c) shows the efficiency enhancement ratio of the solid curve in FIG. 22(b) relative to the dashed and dotted curves in FIG. 22(b).
FIG. 23 shows the radiation efficiency as a function of the resonant eigenfrequency of two identical capacitivelyloaded conducting singleturn loops. Results for two different loop dimensions are shown and for two relative distances between the identical loops. For each loops dimension and distance, four different cases are examined: without farfield interference (dotted), with farfield interference but no drivingfrequency detuning (dashed) and with drivingfrequency detuning to maximize either the efficiency (solid) or the ratio of efficiency over radiation (dashdotted).
FIG. 24 shows CMT results for (a) the coupling factor k and (b) the strongcoupling factor U, for three different m values of subwavelength resonant modes of two same dielectric disks at distance D/r=5 (and also a couple more distances for m=2), when varying their ε in the range 250≧ε≧35. Note that diskmaterial losstangent tan δ=6·10<sup>−6</sup>ε−2·10<sup>−4 </sup>was used. (c) Relative U error between CMT and numerical FEFD calculations of part (b).
FIG. 25 shows Antenna Theory (AT) results for (a) the normalized interference term 2Λ/√{square root over (ω<sub>1</sub>ω<sub>2</sub>)} and (b) magnitude of the stronginterference factor V, as a function of frequency, for the exact same parameters as in FIG. 24. (c) Relative V error between AT and numerical FEFD calculations of part (b).
FIG. 26 shows results for the overall power transmission as a function of frequency, for the same set of resonant modes and distances as in FIGS. 24 and 25, based on the predictions including interference (solid lines) and without interference, just from U (dotted lines).
FIG. 27(a) shows the frequencies f<sub>U </sub>and f<sub>η</sub>, where the strongcoupling factor U and the powertransmission efficiency η are respectively maximized, as a function of the transfer distance between the m=2 disks of FIG. 15. FIG. 27(b) shows the efficiencies achieved at the frequencies of (a) and, in inset, the enhancement ratio of the optimal (by definition) efficiency for f<sub>η</sub> versus the achievable efficiency at f<sub>U </sub>FIG. 27(c) shows the Dparametrized path of the transmission efficiency for the frequency choices of (a) on the U−V efficiency map.
FIG. 28 shows results for the radiation efficiency as a function of the transfer distance at resonant frequency f<sub>U</sub>, when the operating frequency is detuned (solid line), when it is not (dashed line), and when there is no interference whatsoever (dotted line). In the inset, we show the corresponding radiation suppression factors.
FIGS. 29(a)(b) show schematics for frequency control mechanisms.
FIGS. 30(a)(c) illustrate a wireless energy transfer scheme using two dielectric disks in the presence of various extraneous objects.
FIG. 1 shows a schematic that generally describes one example of the invention, in which energy is transferred wirelessly between two resonant objects. Referring to FIG. 1, energy is transferred, over a distance D, between a resonant source object having a characteristic size r<sub>1 </sub>and a resonant device object of characteristic size r<sub>2</sub>. Both objects are resonant objects. The wireless nonradiative energy transfer is performed using the field (e.g. the electromagnetic field or acoustic field) of the system of two resonant objects.
The characteristic size of an object can be regarded as being equal to the radius of the smallest sphere which can fit around the entire object. The characteristic thickness of an object can be regarded as being, when placed on a flat surface in any arbitrary configuration, the smallest possible height of the highest point of the object above a flat surface. The characteristic width of an object can be regarded as being the radius of the smallest possible circle that the object can pass through while traveling in a straight line. For example, the characteristic width of a cylindrical object is the radius of the cylinder.
It is to be understood that while two resonant objects are shown in the example of FIG. 1, and in many of the examples below, other examples can feature three or more resonant objects. For example, in some examples, a single source object can transfer energy to multiple device objects. In some examples, energy can be transferred from a first resonant object to a second resonant object, and then from the second resonant object to a third resonant object, and so forth.
Initially, we present a theoretical framework for understanding nonradiative wireless energy transfer. Note however that it is to be understood that the scope of the invention is not bound by theory.
Different temporal schemes can be employed, depending on the application, to transfer energy between two resonant objects. Here we will consider two particularly simple but important schemes: a onetime finiteamount energytransfer scheme and a continuous finiterate energytransfer (power) scheme.
Let the source and device objects be 1, 2 respectively and their resonance eigemodes, which we will use for the energy exchange, have angular frequencies ω<sub>1,2</sub>, frequencywidths due to intrinsic (absorption, radiation etc.) losses Γ<sub>1,2 </sub>and (generally) vector fields F<sub>1,2</sub>(r), normalized to unity energy. Once the two resonant objects are brought in proximity, they can interact and an appropriate analytical framework for modeling this resonant interaction is that of the wellknown coupledmode theory (CMT). In this picture, the field of the system of the two resonant objects 1, 2 can be approximated by F(r,t)=a<sub>1</sub>(t)F<sub>1</sub>(r)+a<sub>2</sub>(t)F<sub>2</sub>(r), where a<sub>1,2</sub>(t) are the field amplitudes, with a<sub>1,2</sub>(t)<sup>2 </sup>equal to the energy stored inside the object 1, 2 respectively, due to the normalization. Then, using e<sup>−iωt </sup>time dependence, the field amplitudes can be shown to satisfy, to lowest order:
<maths id="MATHUS00001" num="00001"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mrow><mfrac><mo></mo><mrow><mo></mo><mi>t</mi></mrow></mfrac><mo></mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo>=</mo><mstyle><mspace width="0.em" height="0.ex"/></mstyle><mo></mo><mrow><mrow><mrow><mo></mo><mrow><mi></mi><mo></mo><mrow><mo>(</mo><mrow><msub><mi>ω</mi><mn>1</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>Γ</mi><mn>1</mn></msub></mrow></mrow><mo>)</mo></mrow></mrow></mrow><mo></mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>κ</mi><mn>11</mn></msub><mo></mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>κ</mi><mn>12</mn></msub><mo></mo><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow></mrow></mrow><mo></mo><mstyle><mtext></mtext></mstyle><mo></mo><mrow><mrow><mfrac><mo></mo><mrow><mo></mo><mi>t</mi></mrow></mfrac><mo></mo><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo>=</mo><mrow><mrow><mrow><mo></mo><mrow><mi></mi><mo></mo><mrow><mo>(</mo><mrow><msub><mi>ω</mi><mn>2</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>Γ</mi><mn>2</mn></msub></mrow></mrow><mo>)</mo></mrow></mrow></mrow><mo></mo><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>κ</mi><mn>21</mn></msub><mo></mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>κ</mi><mn>22</mn></msub><mo></mo><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where κ<sub>11,22 </sub>are the shifts in each object's frequency due to the presence of the other, which are a secondorder correction and can be absorbed into the eigenfrequencies by setting ω<sub>1,2</sub>→ω<sub>1,2</sub>+κ<sub>11,22</sub>, and κ<sub>12,21 </sub>are the coupling coefficients, which from the reciprocity requirement of the system must satisfy κ<sub>21</sub>=κ<sub>12</sub>≡κ.
The normal modes of the combined system are found, by substituting [a<sub>1</sub>(t), a<sub>2</sub>(t)]=[A<sub>1</sub>, A<sub>2</sub>]e<sup>−i <o ostyle="single">ω</o>t</sup>, to have complex frequencies
<maths id="MATHUS00002" num="00002"><math overflow="scroll"><mtable><mtr><mtd><mrow><msub><mover><mi>ω</mi><mi>_</mi></mover><mo>±</mo></msub><mo>=</mo><mrow><mfrac><mrow><msub><mi>ω</mi><mn>1</mn></msub><mo>+</mo><msub><mi>ω</mi><mn>2</mn></msub></mrow><mn>2</mn></mfrac><mo></mo><mrow><mrow><mi></mi><mo></mo><mfrac><mrow><msub><mi>Γ</mi><mn>1</mn></msub><mo>+</mo><msub><mi>Γ</mi><mn>2</mn></msub></mrow><mn>2</mn></mfrac></mrow><mo>±</mo><msqrt><mrow><msup><mrow><mo>(</mo><mrow><mfrac><mrow><msub><mi>ω</mi><mn>1</mn></msub><mo></mo><msub><mi>ω</mi><mn>2</mn></msub></mrow><mn>2</mn></mfrac><mo></mo><mrow><mi></mi><mo></mo><mfrac><mrow><msub><mi>Γ</mi><mn>1</mn></msub><mo></mo><msub><mi>Γ</mi><mn>2</mn></msub></mrow><mn>2</mn></mfrac></mrow></mrow><mo>)</mo></mrow><mn>2</mn></msup><mo>+</mo><msup><mi>κ</mi><mn>2</mn></msup></mrow></msqrt></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
whose splitting we denote as δ<sub>E</sub>≡ <o ostyle="single">ω</o><sub>+</sub>− <o ostyle="single">ω</o><sub>−</sub>. Note that, at exact resonance ω<sub>1</sub>=ω<sub>2 </sub>and for Γ<sub>1</sub>=Γ<sub>2</sub>, we get δ<sub>E</sub>=2κ.
Assume now that at time t=0 the source object 1 has finite energy a<sub>1</sub>(0)<sup>2</sup>, while the device object has a<sub>2</sub>(0)<sup>2</sup>=0. Since the objects are coupled, energy will be transferred from 1 to 2. With these initial conditions, Eqs. (1) can be solved, predicting the evolution of the device fieldamplitude to be
<maths id="MATHUS00003" num="00003"><math overflow="scroll"><mtable><mtr><mtd><mrow><mfrac><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow><mrow><mo></mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mn>0</mn><mo>)</mo></mrow></mrow><mo></mo></mrow></mfrac><mo>=</mo><mrow><mfrac><mrow><mn>2</mn><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>κ</mi></mrow><msub><mi>δ</mi><mi>E</mi></msub></mfrac><mo></mo><mrow><mi>sin</mi><mo></mo><mrow><mo>(</mo><mfrac><mrow><msub><mi>δ</mi><mi>E</mi></msub><mo></mo><mi>t</mi></mrow><mn>2</mn></mfrac><mo>)</mo></mrow></mrow><mo></mo><mrow><msup><mi></mi><mrow><mo></mo><mfrac><mrow><msub><mi>Γ</mi><mn>1</mn></msub><mo>+</mo><msub><mi>Γ</mi><mn>2</mn></msub></mrow><mn>2</mn></mfrac></mrow></msup><mo>.</mo></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>3</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
The energytransfer efficiency will be η<sub>E</sub>≡a<sub>2</sub>(t)<sup>2</sup>/a<sub>1</sub>(0)<sup>2</sup>. Note that, at exact resonance ω<sub>1</sub>=ω<sub>2 </sub>and in the special case Γ<sub>1</sub>=Γ<sub>2</sub>≡Γ<sub>o</sub>, Eq. (3) can be written as
<maths id="MATHUS00004" num="00004"><math overflow="scroll"><mtable><mtr><mtd><mrow><mfrac><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><mi>T</mi><mo>)</mo></mrow></mrow><mrow><mo></mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mn>0</mn><mo>)</mo></mrow></mrow><mo></mo></mrow></mfrac><mo>=</mo><mrow><mrow><mi>sin</mi><mo></mo><mrow><mo>(</mo><mi>UT</mi><mo>)</mo></mrow></mrow><mo>·</mo><msup><mi></mi><mrow><mo></mo><mi>T</mi></mrow></msup></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>4</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where T≡Γ<sub>o</sub>t and U=κ/Γ<sub>o</sub>.
In some examples, the system designer can adjust the duration of the coupling t at will. In some examples, the duration t can be adjusted to maximize the device energy (and thus efficiency η<sub>E</sub>). Then, in the special case Γ<sub>1</sub>=Γ<sub>2</sub>=Γ<sub>o</sub>, it can be inferred from Eq. (4) that η<sub>E </sub>is maximized for
<maths id="MATHUS00005" num="00005"><math overflow="scroll"><mtable><mtr><mtd><mrow><msub><mi>T</mi><mo>*</mo></msub><mo>=</mo><mfrac><mrow><msup><mi>tan</mi><mrow><mo></mo><mn>1</mn></mrow></msup><mo></mo><mi>U</mi></mrow><mi>U</mi></mfrac></mrow></mtd><mtd><mrow><mo>(</mo><mn>5</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
resulting in an optimal energytransfer efficiency
<maths id="MATHUS00006" num="00006"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>η</mi><msup><mi>E</mi><mo>*</mo></msup></msub><mo>≡</mo><mrow><msub><mi>η</mi><mi>E</mi></msub><mo></mo><mrow><mo>(</mo><msub><mi>T</mi><mo>*</mo></msub><mo>)</mo></mrow></mrow></mrow><mo>=</mo><mrow><mfrac><msup><mi>U</mi><mn>2</mn></msup><mrow><mn>1</mn><mo>+</mo><msup><mi>U</mi><mn>2</mn></msup></mrow></mfrac><mo></mo><mrow><mi>exp</mi><mo>(</mo><mrow><mo></mo><mfrac><mrow><mn>2</mn><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msup><mi>tan</mi><mrow><mo></mo><mn>1</mn></mrow></msup><mo></mo><mi>U</mi></mrow><mi>U</mi></mfrac></mrow><mo>)</mo></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>6</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
which is only a function of the couplingtoloss ratio U=κ/Γ<sub>o </sub>and tends to unity when U>>1, as depicted in FIG. 2(c). In general, also for Γ<sub>1</sub>≠Γ<sub>2</sub>, the energy transfer is nearly perfect, when the coupling rate is much faster than all loss rates (κ/Γ<sub>1,2</sub>>>1).
In a real wireless energytransfer system, the source object can be connected to a power generator (not shown in FIG. 1), and the device object can be connected to a power consuming load (e.g. a resistor, a battery, an actual device, not shown in FIG. 1). The generator will supply the energy to the source object, the energy will be transferred wirelessly and nonradiatively from the source object to the device object, and the load will consume the energy from the device object. To incorporate such supply and consumption mechanisms into this temporal scheme, in some examples, one can imagine that the generator is very briefly but very strongly coupled to the source at time t=0 to almost instantaneously provide the energy, and the load is similarly very briefly but very strongly coupled to the device at the optimal time t=t* to almost instantaneously drain the energy. For a constant powering mechanism, at time t=t* also the generator can again be coupled to the source to feed a new amount of energy, and this process can be repeated periodically with a period t*.
Let the generator be continuously supplying energy to the source object 1 at a rate κ<sub>1 </sub>and the load continuously draining energy from the device object 2 at a rate κ<sub>2</sub>. Field amplitudes s<sub>±1,2</sub>(t) are then defined, so that s<sub>±1,2</sub>(t)<sup>2 </sup>is equal to the power ingoing to (for the + sign) or outgoing from (for the − sign) the object 1, 2 respectively, and the CMT equations are modified to
<maths id="MATHUS00007" num="00007"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mfrac><mo></mo><mrow><mo></mo><mi>t</mi></mrow></mfrac><mo></mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo>=</mo><mstyle><mspace width="0.em" height="0.ex"/></mstyle><mo></mo><mrow><mrow><mrow><mrow><mo></mo><mrow><mi></mi><mo></mo><mrow><mo>(</mo><mrow><msub><mi>ω</mi><mn>1</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>Γ</mi><mn>1</mn></msub></mrow></mrow><mo>)</mo></mrow></mrow></mrow><mo></mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>κ</mi><mn>11</mn></msub><mo></mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>κ</mi><mn>12</mn></msub><mo></mo><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo></mo><mrow><msub><mi>κ</mi><mn>1</mn></msub><mo></mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo>+</mo><mrow><msqrt><mrow><mn>2</mn><mo></mo><msub><mi>κ</mi><mn>1</mn></msub></mrow></msqrt><mo></mo><mrow><msub><mi>s</mi><mrow><mo>+</mo><mn>1</mn></mrow></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow><mo></mo><mstyle><mtext></mtext></mstyle><mo></mo><mstyle><mspace width="4.4em" height="4.4ex"/></mstyle><mo></mo><mrow><mfrac><mo></mo><mrow><mo></mo><mi>t</mi></mrow></mfrac><mo></mo><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow></mrow></mrow><mo>=</mo><mrow><mrow><mrow><mrow><mo></mo><mrow><mi></mi><mo></mo><mrow><mo>(</mo><mrow><msub><mi>ω</mi><mn>2</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>Γ</mi><mn>2</mn></msub></mrow></mrow><mo>)</mo></mrow></mrow></mrow><mo></mo><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>κ</mi><mn>21</mn></msub><mo></mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>κ</mi><mn>22</mn></msub><mo></mo><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo></mo><mrow><msub><mi>κ</mi><mn>2</mn></msub><mo></mo><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow><mo></mo><mstyle><mtext></mtext></mstyle><mo></mo><mstyle><mspace width="4.4em" height="4.4ex"/></mstyle><mo></mo><mrow><msub><mi>s</mi><mrow><mo></mo><mn>1</mn></mrow></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow></mrow><mo>=</mo><mrow><mrow><mrow><msqrt><mrow><mn>2</mn><mo></mo><msub><mi>κ</mi><mn>1</mn></msub></mrow></msqrt><mo></mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow><mo></mo><mrow><mrow><msub><mi>s</mi><mrow><mo>+</mo><mn>1</mn></mrow></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow><mo></mo><mstyle><mtext></mtext></mstyle><mo></mo><mstyle><mspace width="4.4em" height="4.4ex"/></mstyle><mo></mo><mrow><msub><mi>s</mi><mrow><mo></mo><mn>2</mn></mrow></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow></mrow><mo>=</mo><mrow><msqrt><mrow><mn>2</mn><mo></mo><msub><mi>κ</mi><mn>2</mn></msub></mrow></msqrt><mo></mo><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><mi>t</mi><mo>)</mo></mrow></mrow></mrow></mrow></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>7</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where again we can set ω<sub>1,2</sub>→ω<sub>1,2</sub>+κ<sub>11,22 </sub>and κ<sub>21</sub>=κ<sub>12</sub>≡K.
Assume now that the excitation is at a fixed frequency ω, namely has the form s<sub>+1</sub>(t)=S<sub>+1</sub>e<sup>1ωt</sup>. Then the response of the linear system will be at the same frequency, namely a<sub>1,2</sub>(t)=A<sub>1,2</sub>e<sup>−iωt </sup>and s<sub>−1,2</sub>(t)=S<sub>−1,2</sub>e<sup>−iωt</sup>. By substituting these into Eqs. (7), using δ<sub>1,2</sub>≡ω−ω<sub>1,2</sub>, and solving the system, we find the fieldamplitude transmitted to the load (S<sub>21 </sub>scatteringmatrix element)
<maths id="MATHUS00008" num="00008"><math overflow="scroll"><mrow><mo> </mo><mtable><mtr><mtd><mtable><mtr><mtd><mrow><msub><mi>S</mi><mn>21</mn></msub><mo>≡</mo><mi/><mo></mo><mfrac><msub><mi>S</mi><mrow><mo></mo><mn>2</mn></mrow></msub><msubsup><mi>S</mi><mrow><mo>+</mo><mn>1</mn></mrow><mo></mo></msubsup></mfrac></mrow></mtd></mtr><mtr><mtd><mrow><mo>=</mo><mi/><mo></mo><mfrac><mrow><mn>2</mn><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>κ</mi><mo></mo><msqrt><mrow><msub><mi>κ</mi><mn>1</mn></msub><mo></mo><msub><mi>κ</mi><mn>2</mn></msub></mrow></msqrt></mrow><mrow><mrow><mrow><mo>(</mo><mrow><msub><mi>Γ</mi><mn>1</mn></msub><mo>+</mo><msub><mi>κ</mi><mn>1</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>δ</mi><mn>1</mn></msub></mrow></mrow><mo>)</mo></mrow><mo></mo><mrow><mo>(</mo><mrow><msub><mi>Γ</mi><mn>2</mn></msub><mo>+</mo><msub><mi>κ</mi><mn>2</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>δ</mi><mn>2</mn></msub></mrow></mrow><mo>)</mo></mrow></mrow><mo>+</mo><msup><mi>κ</mi><mn>2</mn></msup></mrow></mfrac></mrow></mtd></mtr><mtr><mtd><mrow><mo>=</mo><mi/><mo></mo><mrow><mfrac><mrow><mn>2</mn><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>U</mi><mo></mo><msqrt><mrow><msub><mi>U</mi><mn>1</mn></msub><mo></mo><msub><mi>U</mi><mn>2</mn></msub></mrow></msqrt></mrow><mrow><mrow><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mn>1</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>D</mi><mn>1</mn></msub></mrow></mrow><mo>)</mo></mrow><mo></mo><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mn>2</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>D</mi><mn>2</mn></msub></mrow></mrow><mo>)</mo></mrow></mrow><mo>+</mo><msup><mi>U</mi><mn>2</mn></msup></mrow></mfrac><mo>.</mo></mrow></mrow></mtd></mtr></mtable></mtd><mtd><mrow><mo>(</mo><mn>8</mn><mo>)</mo></mrow></mtd></mtr></mtable></mrow></math></maths>
and the fieldamplitude reflected to the generator (S<sub>11 </sub>scatteringmatrix element)
<maths id="MATHUS00009" num="00009"><math overflow="scroll"><mrow><mo> </mo><mtable><mtr><mtd><mtable><mtr><mtd><mrow><msub><mi>S</mi><mn>11</mn></msub><mo>≡</mo><mi/><mo></mo><mfrac><msub><mi>S</mi><mrow><mo></mo><mn>1</mn></mrow></msub><msub><mi>S</mi><mrow><mo>+</mo><mn>1</mn></mrow></msub></mfrac></mrow></mtd></mtr><mtr><mtd><mrow><mo>=</mo><mi/><mo></mo><mfrac><mrow><mrow><mrow><mo>(</mo><mrow><msub><mi>Γ</mi><mn>1</mn></msub><mo></mo><msub><mi>κ</mi><mn>1</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>δ</mi><mn>1</mn></msub></mrow></mrow><mo>)</mo></mrow><mo></mo><mrow><mo>(</mo><mrow><msub><mi>Γ</mi><mn>2</mn></msub><mo>+</mo><msub><mi>κ</mi><mn>2</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>δ</mi><mn>2</mn></msub></mrow></mrow><mo>)</mo></mrow></mrow><mo>+</mo><msup><mi>κ</mi><mn>2</mn></msup></mrow><mrow><mrow><mrow><mo>(</mo><mrow><msub><mi>Γ</mi><mn>1</mn></msub><mo>+</mo><msub><mi>κ</mi><mn>1</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>δ</mi><mn>1</mn></msub></mrow></mrow><mo>)</mo></mrow><mo></mo><mrow><mo>(</mo><mrow><msub><mi>Γ</mi><mn>2</mn></msub><mo>+</mo><msub><mi>κ</mi><mn>2</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>δ</mi><mn>2</mn></msub></mrow></mrow><mo>)</mo></mrow></mrow><mo>+</mo><msup><mi>κ</mi><mn>2</mn></msup></mrow></mfrac></mrow></mtd></mtr><mtr><mtd><mrow><mo>=</mo><mi/><mo></mo><mfrac><mrow><mrow><mrow><mo>(</mo><mrow><mn>1</mn><mo></mo><msub><mi>U</mi><mn>1</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>D</mi><mn>1</mn></msub></mrow></mrow><mo>)</mo></mrow><mo></mo><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mn>2</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>D</mi><mn>2</mn></msub></mrow></mrow><mo>)</mo></mrow></mrow><mo>+</mo><msup><mi>U</mi><mn>2</mn></msup></mrow><mrow><mrow><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mn>1</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>D</mi><mn>1</mn></msub></mrow></mrow><mo>)</mo></mrow><mo></mo><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mn>2</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>D</mi><mn>2</mn></msub></mrow></mrow><mo>)</mo></mrow></mrow><mo>+</mo><msup><mi>U</mi><mn>2</mn></msup></mrow></mfrac></mrow></mtd></mtr></mtable></mtd><mtd><mrow><mo>(</mo><mn>9</mn><mo>)</mo></mrow></mtd></mtr></mtable></mrow></math></maths>
where D<sub>1,2</sub>≡δ<sub>1,2</sub>/Γ<sub>1,2</sub>, U<sub>1,2</sub>≡κ<sub>1,2</sub>/Γ<sub>1,2 </sub>and U≡κ/√{square root over (Γ<sub>1</sub>Γ<sub>2</sub>)}. Similarly, the scatteringmatrix elements S<sub>12</sub>, S<sub>22 </sub>are given by interchanging 1<img id="CUSTOMCHARACTER00001" he="2.12mm" wi="3.56mm" file="US20090284083A120091119P00001.TIF" imgcontent="character" imgformat="tif"/>2 in Eqs. (8), (9) and, as expected from reciprocity, S<sub>21</sub>=S<sub>12</sub>. The coefficients for power transmission (efficiency) and reflection and loss are respectively η<sub>P</sub>≡S<sub>21</sub><sup>2</sup>=S<sub>−2</sub><sup>2</sup>/S<sub>+1</sub><sup>2 </sup>and S<sub>11</sub><sup>2</sup>=S<sub>−1</sub><sup>2</sup>/S<sub>+1</sub><sup>2 </sup>and 1−S<sub>21</sub><sup>2</sup>−S<sub>11</sub><sup>2</sup>=(2 Γ<sub>1</sub>A<sub>1</sub><sup>2</sup>+2Γ<sub>2</sub>A<sub>2</sub><sup>2</sup>)S<sub>+1</sub><sup>2</sup>.
In practice, in some implementations, the parameters D<sub>1,2</sub>, U<sub>1,2 </sub>can be designed (engineered), since one can adjust the resonant frequencies ω<sub>1,2 </sub>(compared to the desired operating frequency ω) and the generator/load supply/drain rates κ<sub>1,2 </sub>Their choice can target the optimization of some system performancecharacteristic of interest:
In some examples, a goal can be to maximize the power transmission (efficiency) η<sub>P</sub>≡S<sub>21</sub><sup>2 </sup>of the system, so one would require
<FORM>η<sub>P</sub>(D<sub>1,2</sub>)=η<sub>P</sub>(U<sub>1,2</sub>)=0 (10)</FORM>
Since S<sub>21 </sub>(from Eq. (8)) is symmetric upon interchanging 1<img id="CUSTOMCHARACTER00002" he="2.12mm" wi="3.56mm" file="US20090284083A120091119P00001.TIF" imgcontent="character" imgformat="tif"/>2, the optimal values for D<sub>1,2 </sub>(determined by Eqs. (10)) will be equal, namely D<sub>1</sub>=D<sub>2</sub>≡D<sub>o</sub>, and similarly U<sub>1</sub>=U<sub>2</sub>≡U<sub>o</sub>. Then,
<maths id="MATHUS00010" num="00010"><math overflow="scroll"><mtable><mtr><mtd><mrow><msub><mi>S</mi><mn>21</mn></msub><mo>=</mo><mfrac><mrow><mn>2</mn><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>UU</mi><mi>o</mi></msub></mrow><mrow><msup><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mi>o</mi></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>D</mi><mi>o</mi></msub></mrow></mrow><mo>)</mo></mrow><mn>2</mn></msup><mo>+</mo><msup><mi>U</mi><mn>2</mn></msup></mrow></mfrac></mrow></mtd><mtd><mrow><mo>(</mo><mn>11</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
and from the condition η′<sub>P</sub>(D<sub>o</sub>)=0 we get that, for fixed values of U and U<sub>o</sub>, the efficiency can be maximized for the following values of the symmetric detuning
<maths id="MATHUS00011" num="00011"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>D</mi><mi>o</mi></msub><mo>=</mo><mtable><mtr><mtd><mrow><mrow><mo>±</mo><msqrt><mrow><msup><mi>U</mi><mn>2</mn></msup><mo></mo><msup><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mi>o</mi></msub></mrow><mo>)</mo></mrow><mn>2</mn></msup></mrow></msqrt></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mrow><mi>if</mi><mo></mo><mstyle><mspace width="0.8em" height="0.8ex"/></mstyle><mo></mo><mi>U</mi></mrow><mo>></mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mi>o</mi></msub></mrow></mrow></mtd></mtr><mtr><mtd><mrow><mn>0</mn><mo>,</mo></mrow></mtd><mtd><mrow><mrow><mi>if</mi><mo></mo><mstyle><mspace width="0.8em" height="0.8ex"/></mstyle><mo></mo><mi>U</mi></mrow><mo>≤</mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mi>o</mi></msub></mrow></mrow></mtd></mtr></mtable></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mo>(</mo><mn>12</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
which, in the case U>1+U<sub>o</sub>, can be rewritten for the two frequencies at which the efficiency peaks as
<maths id="MATHUS00012" num="00012"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>ω</mi><mo>±</mo></msub><mo>=</mo><mrow><mfrac><mrow><mrow><msub><mi>ω</mi><mn>1</mn></msub><mo></mo><msub><mi>Γ</mi><mn>2</mn></msub></mrow><mo>+</mo><mrow><msub><mi>ω</mi><mn>2</mn></msub><mo></mo><msub><mi>Γ</mi><mn>1</mn></msub></mrow></mrow><mrow><msub><mi>Γ</mi><mn>1</mn></msub><mo>+</mo><msub><mi>Γ</mi><mn>2</mn></msub></mrow></mfrac><mo>±</mo><mrow><mfrac><mrow><mn>2</mn><mo></mo><msqrt><mrow><msub><mi>Γ</mi><mn>1</mn></msub><mo></mo><msub><mi>Γ</mi><mn>2</mn></msub></mrow></msqrt></mrow><mrow><msub><mi>Γ</mi><mn>1</mn></msub><mo>+</mo><msub><mi>Γ</mi><mn>2</mn></msub></mrow></mfrac><mo></mo><msqrt><mrow><msup><mi>κ</mi><mn>2</mn></msup><mo></mo><mrow><mrow><mo>(</mo><mrow><msub><mi>Γ</mi><mn>1</mn></msub><mo>+</mo><msub><mi>κ</mi><mn>1</mn></msub></mrow><mo>)</mo></mrow><mo></mo><mrow><mo>(</mo><mrow><msub><mi>Γ</mi><mn>2</mn></msub><mo>+</mo><msub><mi>κ</mi><mn>2</mn></msub></mrow><mo>)</mo></mrow></mrow></mrow></msqrt></mrow></mrow></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mo>(</mo><mn>13</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
whose splitting we denote as δ<sub>P</sub>≡ <o ostyle="single">ω</o><sub>+</sub>− <o ostyle="single">ω</o><sub>−</sub>. Note that, at exact resonance ω<sub>1</sub>=ω<sub>2</sub>, and for Γ<sub>1</sub>=Γ<sub>2</sub>≡Γ<sub>o </sub>and κ<sub>1</sub>=κ<sub>2</sub>≡κ<sub>o</sub>, we get δ<sub>P</sub>=2√{square root over (κ<sup>2</sup>−(Γ<sub>o</sub>+κ<sub>o</sub>)<sup>2</sup>)}<δ<sub>E</sub>, namely the transmissionpeak splitting is smaller than the normalmode splitting. Then, by substituting D<sub>o </sub>into η<sub>P </sub>from Eq. (12), from the condition η′<sub>P</sub>(U<sub>o</sub>)=0 we get that, for fixed value of U, the efficiency can be maximized for
<FORM>U<sub>o</sub>*=√{square root over (1+<sup>2</sup>)} <img id="CUSTOMCHARACTER00003" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>D<sub>o</sub>*=0 (14)</FORM>
which is known as ‘critical coupling’ condition, whereas for U<sub>o</sub><U<sub>o</sub>*, the system is called ‘undercoupled’ and for U<sub>o</sub>>U<sub>o</sub>*. it is called ‘overcoupled’. The dependence of the efficiency on the frequency detuning D<sub>o </sub>for different values of U<sub>o </sub>(including the ‘criticalcoupling’ condition) are shown in FIG. 2(a,b). The overall optimal power efficiency using Eqs. (14) is
<maths id="MATHUS00013" num="00013"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mrow><msub><mi>η</mi><msup><mi>P</mi><mo>*</mo></msup></msub><mo>≡</mo><mrow><msub><mi>η</mi><mi>P</mi></msub><mo></mo><mrow><mo>(</mo><mrow><mrow><msub><mi>D</mi><mi>o</mi></msub><mo>.</mo></mrow><mo>,</mo><msub><mi>U</mi><msup><mi>o</mi><mo>*</mo></msup></msub></mrow><mo>)</mo></mrow></mrow></mrow><mo>=</mo><mrow><mfrac><mrow><msub><mi>U</mi><msup><mi>o</mi><mo>*</mo></msup></msub><mo></mo><mn>1</mn></mrow><mrow><msub><mi>U</mi><msup><mi>o</mi><mo>*</mo></msup></msub><mo></mo><mn>1</mn></mrow></mfrac><mo>=</mo><msup><mrow><mo>(</mo><mfrac><mi>U</mi><mrow><mn>1</mn><mo>+</mo><msqrt><mrow><mn>1</mn><mo>+</mo><msup><mi>U</mi><mn>2</mn></msup></mrow></msqrt></mrow></mfrac><mo>)</mo></mrow><mn>2</mn></msup></mrow></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mo>(</mo><mn>15</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
which is again only a function of the couplingtoloss ratio U=κ/√{square root over (Γ<sub>1</sub>Γ<sub>2</sub>)} and tends to unity when U>>1, as depicted in FIG. 2(c).
In some examples, a goal can be to minimize the power reflection at the side of the generator S<sub>11</sub><sup>2 </sup>and the load S<sub>22</sub><sup>2</sup>, so one would then need
<FORM>S<sub>11,22</sub>=0<img id="CUSTOMCHARACTER00004" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>(1∓U<sub>1</sub>−iD<sub>1</sub>)(1±U<sub>2</sub>−iD<sub>2</sub>)+U<sup>2</sup>=0, (16)</FORM>
The equations above present ‘impedance matching’ conditions. Again, the set of these conditions is symmetric upon interchanging 1<img id="CUSTOMCHARACTER00005" he="2.12mm" wi="3.56mm" file="US20090284083A120091119P00001.TIF" imgcontent="character" imgformat="tif"/>2, so, by substituting D<sub>1</sub>=D<sub>2</sub>≡D<sub>o </sub>and U<sub>1</sub>=U<sub>2</sub>≡U<sub>o </sub>into Eqs. (16), we get
<FORM>(1−iD<sub>o</sub>)<sup>2</sup>−U<sub>o</sub><sup>2</sup>+U<sup>2</sup>=0, (17)</FORM>
from which we easily find that the values of D<sub>o </sub>and U<sub>o </sub>that cancel all reflections are again exactly those in Eqs. (14).
It can be seen that, for this particular problem, the two goals and their associated sets of conditions (Eqs. (10) and Eqs. (16)) result in the same optimized values of the intrasource and intradevice parameters D<sub>1,2</sub>, U<sub>1,2</sub>. Note that for a lossless system this would be an immediate consequence of power conservation (Hermiticity of the scattering matrix), but this is not apparent for a lossy system.
Accordingly, for any temporal energytransfer scheme, once the parameters specific only to the source or to the device (such as their resonant frequencies and their excitation or loading rates respectively) have been optimally designed, the efficiency monotonically increases with the ratio of the sourcedevice couplingrate to their loss rates. Using the definition of a resonance quality factor Q=ω/2Γ and defining by analogy the coupling factor k≡1/Q<sub>κ</sub>≡2κ/√{square root over (ω<sub>1</sub>ω<sub>2</sub>)}, it is therefore exactly this ratio
<maths id="MATHUS00014" num="00014"><math overflow="scroll"><mtable><mtr><mtd><mrow><mi>U</mi><mo>=</mo><mrow><mfrac><mi>κ</mi><msqrt><mrow><msub><mi>Γ</mi><mn>1</mn></msub><mo></mo><msub><mi>Γ</mi><mn>2</mn></msub></mrow></msqrt></mfrac><mo>=</mo><mrow><mi>k</mi><mo></mo><msqrt><mrow><msub><mi>Q</mi><mn>1</mn></msub><mo></mo><msub><mi>Q</mi><mn>2</mn></msub></mrow></msqrt></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>18</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
that has been set as a figureofmerit for any system under consideration for wireless energytransfer, along with the distance over which this ratio can be achieved (clearly, U will be a decreasing function of distance). The desired optimal regime U>1 is called ‘strongcoupling’ regime and it is a necessary and sufficient condition for efficient energytransfer. In particular, for U>1 we get, from Eq. (15), η<sub>P</sub>*>17%, large enough for practical applications. The figureofmerit U is called the strongcoupling factor. We will further show how to design systems with a large strongcoupling factor.
To achieve a large strongcoupling factor U, in some examples, the energytransfer application preferably uses resonant modes of high quality factors Q, corresponding to low (i.e. slow) intrinsicloss rates Γ. This condition can be satisfied by designing resonant modes where all loss mechanisms, typically radiation and absorption, are sufficiently suppressed.
This suggests that the coupling be implemented using, not the lossy radiative farfield, which should rather be suppressed, but the evanescent (nonlossy) stationary nearfield. To implement an energytransfer scheme, usually more appropriate are finite objects, namely ones that are topologically surrounded everywhere by air, into where the near field extends to achieve the coupling. Objects of finite extent do not generally support electromagnetic states that are exponentially decaying in all directions in air away from the objects, since Maxwell's Equations in free space imply that k<sup>2</sup>=ω<sup>2</sup>/c<sub>2</sub>, where k is the wave vector, ω the angular frequency, and c the speed of light, because of which one can show that such finite objects cannot support states of infinite Q, rather there always is some amount of radiation. However, very longlived (socalled “highQ”) states can be found, whose tails display the needed exponential or exponentiallike decay away from the resonant object over long enough distances before they turn oscillatory (radiative). The limiting surface, where this change in the field behavior happens, is called the “radiation caustic”, and, for the wireless energytransfer scheme to be based on the near field rather than the far/radiation field, the distance between the coupled objects must be such that one lies within the radiation caustic of the other. One typical way of achieving a high radiationQ (Q<sub>rad</sub>) is to design subwavelength resonant objects. When the size of an object is much smaller than the wavelength of radiation in free space, its electromagnetic field couples to radiation very weakly. Since the extent of the nearfield into the area surrounding a finitesized resonant object is set typically by the wavelength, in some examples, resonant objects of subwavelength size have significantly longer evanescent fieldtails. In other words, the radiation caustic is pushed far away from the object, so the electromagnetic mode enters the radiative regime only with a small amplitude.
Moreover, most realistic materials exhibit some nonzero amount of absorption, which can be frequency dependent, and thus cannot support states of infinite Q, rather there always is some amount of absorption. However, very longlived (“highQ”) states can be found, where electromagnetic modal energy is only weakly dissipated. Some typical ways of achieving a high absorptionQ (Q<sub>abs</sub>) is to use materials which exhibit very small absorption at the resonant frequency and/or to shape the field to be localized more inside the least lossy materials.
Furthermore, to achieve a large strongcoupling factor U, in some examples, the energytransfer application preferably uses systems that achieve a high coupling factor k, corresponding to strong (i.e. fast) coupling rate κ, over distances larger than the characteristic sizes of the objects.
Since finitesized subwavelength resonant objects can often be accompanied with a high Q, as was discussed above and will be seen in examples later on, such an object will typically be the appropriate choice for the possiblymobile resonant deviceobject. In these cases, the electromagnetic field is, in some examples, of quasistatic nature and the distance, up to which sufficient coupling can be achieved, is dictated by the decaylaw of this quasistatic field.
Note, though, that in some examples, the resonant sourceobject will be immobile and thus less restricted in its allowed geometry and size. It can be therefore chosen large enough that the nearfield extent is not limited by the wavelength, and can thus have nearly infinite radiationQ. Some objects of nearly infinite extent, such as dielectric waveguides, can support guided modes, whose evanescent tails are decaying exponentially in the direction away from the object, slowly if tuned close to cutoff, therefore a good coupling can also be achieved over distances quite a few times larger than a characteristic size of the source and/or deviceobject.
In the following, examples of systems suitable for energy transfer of the type described above are described. We will demonstrate how to compute the CMT parameters ω<sub>1,2</sub>, Q<sub>1,2 </sub>and k described above and how to choose or design these parameters for particular examples in order to produce a desirable figureofmerit U=κ/√{square root over (Γ<sub>1</sub>Γ<sub>2</sub>)}=k√{square root over (Q<sub>1</sub>Q<sub>2</sub>)} at a desired distance D. In some examples, this figureofmerit is maximized when ω<sub>1,2 </sub>are tuned close to a particular angular frequency ω<sub>U</sub>.
In some examples, one or more of the resonant objects are selfresonant conducting coils. Referring to FIG. 3, a conducting wire of length 1 and crosssectional radius a is wound into a helical coil of radius r and height h (namely with N=√{square root over (l<sup>2</sup>−h<sup>2</sup>)}/2πr number of turns), surrounded by air. As described below, the wire has distributed inductance and distributed capacitance, and therefore it supports a resonant mode of angular frequency ω. The nature of the resonance lies in the periodic exchange of energy from the electric field within the capacitance of the coil, due to the charge distribution ρ(x) across it, to the magnetic field in free space, due to the current distribution j(x) in the wire. In particular, the charge conservation equation ∇·j=iωρ implies that: (i) this periodic exchange is accompanied by a π/2 phaseshift between the current and the charge density profiles, namely the energy W contained in the coil is at certain points in time completely due to the current and at other points in time completely due to the charge, and (ii) if ρ<sub>l</sub>(x) and I(x) are respectively the linear charge and current densities in the wire, where x runs along the wire,
<maths id="MATHUS00015" num="00015"><math overflow="scroll"><mrow><msub><mi>q</mi><mi>o</mi></msub><mo>=</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><mrow><mo>∫</mo><mrow><mrow><mo></mo><mi>x</mi></mrow><mo></mo><mrow><mo></mo><mrow><msub><mi>ρ</mi><mi>l</mi></msub><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow><mo></mo></mrow></mrow></mrow></mrow></mrow></math></maths>
is the maximum amount of positive charge accumulated in one side of the coil (where an equal amount of negative charge always also accumulates in the other side to make the system neutral) and I<sub>o</sub>=max{I(x)} is the maximum positive value of the linear current distribution, then I<sub>o</sub>=ωq<sub>o</sub>. Then, one can define an effective total inductance L and an effective total capacitance C of the coil through the amount of energy W inside its resonant mode:
<maths id="MATHUS00016" num="00016"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mrow><mrow><mi>W</mi><mo>≡</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><msubsup><mi>I</mi><mi>o</mi><mn>2</mn></msubsup><mo></mo><mi>L</mi></mrow></mrow><mo>⇒</mo><mi>L</mi></mrow><mo>=</mo><mrow><mfrac><msub><mi>μ</mi><mi>o</mi></msub><mrow><mn>4</mn><mo></mo><mi>π</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msubsup><mi>I</mi><mi>o</mi><mn>2</mn></msubsup></mrow></mfrac><mo></mo><mrow><mo>∫</mo><mrow><mo>∫</mo><mrow><mrow><mo></mo><mi>x</mi></mrow><mo></mo><mrow><mo></mo><msup><mi>x</mi><mi>′</mi></msup></mrow><mo></mo><mfrac><mrow><mrow><mi>j</mi><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow><mo>·</mo><mrow><mi>j</mi><mo></mo><mrow><mo>(</mo><msup><mi>x</mi><mi>′</mi></msup><mo>)</mo></mrow></mrow></mrow><mrow><mo></mo><mrow><mi>x</mi><mo></mo><msup><mi>x</mi><mi>′</mi></msup></mrow><mo></mo></mrow></mfrac></mrow></mrow></mrow></mrow></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mo>(</mo><mn>19</mn><mo>)</mo></mrow></mtd></mtr><mtr><mtd><mrow><mrow><mrow><mrow><mi>W</mi><mo>≡</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><msubsup><mi>q</mi><mi>o</mi><mn>2</mn></msubsup><mo></mo><mfrac><mn>1</mn><mi>C</mi></mfrac></mrow></mrow><mo>⇒</mo><mfrac><mn>1</mn><mi>C</mi></mfrac></mrow><mo>=</mo><mrow><mfrac><mn>1</mn><mrow><mn>4</mn><mo></mo><mi>π</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>ɛ</mi><mi>o</mi></msub><mo></mo><msubsup><mi>q</mi><mi>o</mi><mn>2</mn></msubsup></mrow></mfrac><mo></mo><mrow><mo>∫</mo><mrow><mo>∫</mo><mrow><mrow><mo></mo><mi>x</mi></mrow><mo></mo><mrow><mo></mo><msup><mi>x</mi><mi>′</mi></msup></mrow><mo></mo><mfrac><mrow><mrow><mi>ρ</mi><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow><mo></mo><mrow><mi>ρ</mi><mo></mo><mrow><mo>(</mo><msup><mi>x</mi><mi>′</mi></msup><mo>)</mo></mrow></mrow></mrow><mrow><mo></mo><mrow><mi>x</mi><mo></mo><msup><mi>x</mi><mi>′</mi></msup></mrow><mo></mo></mrow></mfrac></mrow></mrow></mrow></mrow></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mo>(</mo><mn>20</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where μ<sub>o </sub>and ε<sub>o </sub>are the magnetic permeability and electric permittivity of free space.
With these definitions, the resonant angular frequency and the effective impedance can be given by the formulas ω=1/√{square root over (LC)} and Z=√{square root over (L/C)} respectively.
Losses in this resonant system consist of ohmic (material absorption) loss inside the wire and radiative loss into free space. One can again define a total absorption resistance R<sub>abs </sub>from the amount of power absorbed inside the wire and a total radiation resistance R<sub>rad </sub>from the amount of power radiated due to electric and magneticdipole radiation:
<maths id="MATHUS00017" num="00017"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>P</mi><mrow><mi>abs</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle></mrow></msub><mo>≡</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><msubsup><mi>I</mi><mi>o</mi><mn>2</mn></msubsup><mo></mo><msub><mi>R</mi><mi>abs</mi></msub></mrow></mrow><mo>⇒</mo><mrow><msub><mi>R</mi><mi>abs</mi></msub><mo>≈</mo><mrow><msub><mi>ζ</mi><mi>c</mi></msub><mo></mo><mrow><mfrac><mi>l</mi><mrow><mn>2</mn><mo></mo><mi>π</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>a</mi></mrow></mfrac><mo>·</mo><mfrac><msubsup><mi>I</mi><mi>rms</mi><mn>2</mn></msubsup><msubsup><mi>I</mi><mi>o</mi><mn>2</mn></msubsup></mfrac></mrow></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>21</mn><mo>)</mo></mrow></mtd></mtr><mtr><mtd><mrow><mrow><mrow><msub><mi>P</mi><mi>rad</mi></msub><mo>≡</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><msubsup><mi>I</mi><mi>o</mi><mn>2</mn></msubsup><mo></mo><msub><mi>R</mi><mi>rad</mi></msub></mrow></mrow><mo>⇒</mo><mrow><msub><mi>R</mi><mi>rad</mi></msub><mo>≈</mo><mrow><mfrac><msub><mi>ζ</mi><mi>o</mi></msub><mrow><mn>6</mn><mo></mo><mi>π</mi></mrow></mfrac><mo>[</mo><mrow><msup><mrow><mo>(</mo><mfrac><mrow><mi>ω</mi><mo></mo><mrow><mo></mo><mi>p</mi><mo></mo></mrow></mrow><mi>c</mi></mfrac><mo>)</mo></mrow><mn>2</mn></msup><mo>+</mo><msup><mrow><mo>(</mo><mfrac><mrow><mi>ω</mi><mo></mo><msqrt><mrow><mo></mo><mi>m</mi><mo></mo></mrow></msqrt></mrow><mi>c</mi></mfrac><mo>)</mo></mrow><mn>4</mn></msup></mrow><mo>]</mo></mrow></mrow></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mo>(</mo><mn>22</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where c=1/√{square root over (μ<sub>o</sub>ε<sub>o</sub>)} and ζ<sub>o</sub>=√{square root over (μ<sub>o</sub>/ε<sub>o</sub>)} are the light velocity and light impedance in free space, the impedance ζ<sub>c </sub>is ζ<sub>c</sub>=1/σδ=√{square root over (μ<sub>o</sub>ω/2σ)} with σ the conductivity of the conductor and δ the skin depth at the frequency ω,
<maths id="MATHUS00018" num="00018"><math overflow="scroll"><mrow><mrow><msubsup><mi>I</mi><mi>rms</mi><mn>2</mn></msubsup><mo>=</mo><mrow><mfrac><mn>1</mn><mi>l</mi></mfrac><mo></mo><mrow><mo>∫</mo><mrow><mrow><mo></mo><mi>x</mi></mrow><mo></mo><msup><mrow><mo></mo><mrow><mi>I</mi><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow><mo></mo></mrow><mn>2</mn></msup></mrow></mrow></mrow></mrow><mo>,</mo><mrow><mi>p</mi><mo>=</mo><mrow><mo>∫</mo><mrow><mrow><mo></mo><mi>x</mi></mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>r</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><msub><mi>ρ</mi><mi>l</mi></msub><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow></mrow></mrow></mrow></mrow></math></maths>
is the electricdipole moment of the coil and
<maths id="MATHUS00019" num="00019"><math overflow="scroll"><mrow><mi>m</mi><mo>=</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><mrow><mo>∫</mo><mrow><mrow><mo></mo><mi>x</mi></mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>r</mi><mo>×</mo><mrow><mi>j</mi><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow></mrow></mrow></mrow></mrow></math></maths>
is the magneticdipole moment of the coil. For the radiation resistance formula Eq. (22), the assumption of operation in the quasistatic regime (h, r<<λ=2πc/ω) has been used, which is the desired regime of a subwavelength resonance. With these definitions, the absorption and radiation quality factors of the resonance are given by Q<sub>abs</sub>=Z/R<sub>abs </sub>and Q<sub>rad</sub>=Z/R<sub>rad </sub>respectively.
From Eq. (19)(22) it follows that to determine the resonance parameters one simply needs to know the current distribution j in the resonant coil. Solving Maxwell's equations to rigorously find the current distribution of the resonant electromagnetic eigenmode of a conductingwire coil is more involved than, for example, of a standard LC circuit, and we can find no exact solutions in the literature for coils of finite length, making an exact solution difficult. One could in principle write down an elaborate transmissionlinelike model, and solve it by brute force. We instead present a model that is (as described below) in good agreement (˜5%) with experiment. Observing that the finite extent of the conductor forming each coil imposes the boundary condition that the current has to be zero at the ends of the coil, since no current can leave the wire, we assume that the resonant mode of each coil is well approximated by a sinusoidal current profile along the length of the conducting wire. We shall be interested in the lowest mode, so if we denote by x the coordinate along the conductor, such that it runs from −l/2 to +l/2, then the current amplitude profile would have the form I(x)=I<sub>o </sub>cos(πx/l), where we have assumed that the current does not vary significantly along the wire circumference for a particular x, a valid assumption provided a<<r. It immediately follows from the continuity equation for charge that the linear charge density profile should be of the form ρ<sub>l</sub>(x)=ρ<sub>o </sub>sin (πx/l), and thus q<sub>o</sub>=∫<sub>0</sub><sup>l/2</sup>dxρ<sub>o</sub>sin(πx/l)=ρ<sub>o</sub>l/π. Using these sinusoidal profiles we find the socalled “selfinductance” L<sub>s </sub>and “selfcapacitance” C<sub>s </sub>of the coil by computing numerically the integrals Eq. (19) and (20); the associated frequency and effective impedance are ω<sub>s </sub>and Z<sub>s </sub>respectively. The “selfresistances” R<sub>s </sub>are given analytically by Eq. (21) and (22) using
<maths id="MATHUS00020" num="00020"><math overflow="scroll"><mrow><mrow><msubsup><mi>I</mi><mi>rms</mi><mn>2</mn></msubsup><mo>=</mo><mrow><mrow><mfrac><mn>1</mn><mi>l</mi></mfrac><mo></mo><mrow><msubsup><mo>∫</mo><mrow><mrow><mo></mo><mi>l</mi></mrow><mo>/</mo><mn>2</mn></mrow><mrow><mi>l</mi><mo>/</mo><mn>2</mn></mrow></msubsup><mo></mo><mstyle><mspace width="0.2em" height="0.2ex"/></mstyle><mo></mo><mrow><mrow><mo></mo><mi>x</mi></mrow><mo></mo><msup><mrow><mo></mo><mrow><msub><mi>I</mi><mi>o</mi></msub><mo></mo><mrow><mi>cos</mi><mo></mo><mrow><mo>(</mo><mrow><mi>π</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><mi>x</mi><mo>/</mo><mi>l</mi></mrow></mrow><mo>)</mo></mrow></mrow></mrow><mo></mo></mrow><mn>2</mn></msup></mrow></mrow></mrow><mo>=</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><msubsup><mi>I</mi><mi>o</mi><mn>2</mn></msubsup></mrow></mrow></mrow><mo>,</mo><mstyle><mtext></mtext></mstyle><mo></mo><mrow><mrow><mo></mo><mi>p</mi><mo></mo></mrow><mo>=</mo><mrow><msub><mi>q</mi><mi>o</mi></msub><mo></mo><msqrt><mrow><msup><mrow><mo>(</mo><mrow><mfrac><mn>2</mn><mi>π</mi></mfrac><mo></mo><mi>h</mi></mrow><mo>)</mo></mrow><mn>2</mn></msup><mo>+</mo><msup><mrow><mo>(</mo><mrow><mfrac><mrow><mn>4</mn><mo></mo><mi>N</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><mi>cos</mi><mo></mo><mrow><mo>(</mo><mrow><mi>π</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>N</mi></mrow><mo>)</mo></mrow></mrow></mrow><mrow><mrow><mo>(</mo><mrow><mrow><mn>4</mn><mo></mo><msup><mi>N</mi><mn>2</mn></msup></mrow><mo></mo><mn>1</mn></mrow><mo>)</mo></mrow><mo></mo><mi>π</mi></mrow></mfrac><mo></mo><mi>r</mi></mrow><mo>)</mo></mrow><mn>2</mn></msup></mrow></msqrt></mrow></mrow></mrow></math></maths>
and
<maths id="MATHUS00021" num="00021"><math overflow="scroll"><mrow><mrow><mrow><mo></mo><mi>m</mi><mo></mo></mrow><mo>=</mo><mrow><msub><mi>I</mi><mi>o</mi></msub><mo></mo><msqrt><mrow><msup><mrow><mo>(</mo><mrow><mfrac><mn>2</mn><mi>π</mi></mfrac><mo></mo><mi>N</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>π</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msup><mi>r</mi><mn>2</mn></msup></mrow><mo>)</mo></mrow><mn>2</mn></msup><mo>+</mo><msup><mrow><mo>(</mo><mrow><mfrac><mtable><mtr><mtd><mrow><mrow><mi>cos</mi><mo></mo><mrow><mo>(</mo><mrow><mi>π</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>N</mi></mrow><mo>)</mo></mrow><mo></mo><mrow><mo>(</mo><mrow><mrow><mn>12</mn><mo></mo><msup><mi>N</mi><mn>2</mn></msup></mrow><mo></mo><mn>1</mn></mrow><mo>)</mo></mrow></mrow><mo></mo></mrow></mtd></mtr><mtr><mtd><mrow><mi>sin</mi><mo></mo><mrow><mo>(</mo><mrow><mi>π</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>N</mi></mrow><mo>)</mo></mrow><mo></mo><mi>π</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><mi>N</mi><mo></mo><mrow><mo>(</mo><mrow><mrow><mn>4</mn><mo></mo><msup><mi>N</mi><mn>2</mn></msup></mrow><mo></mo><mn>1</mn></mrow><mo>)</mo></mrow></mrow></mrow></mtd></mtr></mtable><mrow><mrow><mo>(</mo><mrow><mrow><mn>16</mn><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msup><mi>N</mi><mn>4</mn></msup></mrow><mo></mo><mrow><mn>8</mn><mo></mo><msup><mi>N</mi><mn>2</mn></msup></mrow><mo>+</mo><mn>1</mn></mrow><mo>)</mo></mrow><mo></mo><mi>π</mi></mrow></mfrac><mo></mo><mi>hr</mi></mrow><mo>)</mo></mrow><mn>2</mn></msup></mrow></msqrt></mrow></mrow><mo>,</mo></mrow></math></maths>
and therefore the associated Q<sub>s </sub>factors can be calculated.
The results for two examples of resonant coils with subwavelength modes of λ<sub>s</sub>/r≧70 (i.e. those highly suitable for nearfield coupling and well within the quasistatic limit) are presented in Table 1. Numerical results are shown for the wavelength and absorption, radiation and total loss rates, for the two different cases of subwavelengthcoil resonant modes. Note that, for conducting material, copper (σ=5.998·10̂−7 S/m) was used. It can be seen that expected quality factors at microwave frequencies are Q<sub>s,abs</sub>≧1000 and Q<sub>s,rad</sub>≧5000.
<tables id="TABLEUS00001" num="00001"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="6"><colspec colname="1" colwidth="98pt" align="left"/><colspec colname="2" colwidth="21pt" align="center"/><colspec colname="3" colwidth="28pt" align="center"/><colspec colname="4" colwidth="28pt" align="center"/><colspec colname="5" colwidth="21pt" align="center"/><colspec colname="6" colwidth="21pt" align="center"/><thead><row><entry namest="1" nameend="6" rowsep="1">TABLE 1</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row><row><entry>single coil</entry><entry>λ<sub>s</sub>/r</entry><entry>f (MHz)</entry><entry>Q<sub>s,rad</sub></entry><entry>Q<sub>s,abs</sub></entry><entry>Q<sub>s</sub></entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row></thead><tbody valign="top"><row><entry/></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="6"><colspec colname="1" colwidth="98pt" align="left"/><colspec colname="2" colwidth="21pt" align="char" char="."/><colspec colname="3" colwidth="28pt" align="center"/><colspec colname="4" colwidth="28pt" align="char" char="."/><colspec colname="5" colwidth="21pt" align="center"/><colspec colname="6" colwidth="21pt" align="center"/><tbody valign="top"><row><entry>r = 30 cm, h = 20 cm, a = 1 cm,</entry><entry>74.7</entry><entry>13.39</entry><entry>4164</entry><entry>8170</entry><entry>2758</entry></row><row><entry>N = 4</entry></row><row><entry>r = 10 cm, h = 3 cm, a = 2 mm,</entry><entry>140</entry><entry>21.38</entry><entry>43919</entry><entry>3968</entry><entry>3639</entry></row><row><entry>N = 6</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row></tbody></tgroup></table></tables>
Referring to FIG. 4, in some examples, energy is transferred between two selfresonant conductingwire coils. The electric and magnetic fields are used to couple the different resonant conductingwire coils at a distance D between their centers. Usually, the electric coupling highly dominates over the magnetic coupling in the system under consideration for coils with h>>2r and, oppositely, the magnetic coupling highly dominates over the electric coupling for coils with h<<2r. Defining the charge and current distributions of two coils 1,2 respectively as ρ<sub>1,2</sub>(x) and j<sub>1,2</sub>(x), total charges and peak currents respectively as q<sub>1,2 </sub>and I<sub>1,2, </sub>and capacitances and inductances respectively as C<sub>1,2 </sub>and L<sub>1,2 </sub>which are the analogs of ρ(x), j(x), q<sub>o</sub>, I<sub>o</sub>, C and L for the singlecoil case and are therefore well defined, we can define their mutual capacitance and inductance through the total energy:
<maths id="MATHUS00022" num="00022"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mrow><mrow><mi>W</mi><mo>≡</mo><mrow><msub><mi>W</mi><mn>1</mn></msub><mo>+</mo><msub><mi>W</mi><mn>2</mn></msub><mo>+</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><mrow><mrow><mo>(</mo><mrow><mrow><msubsup><mi>q</mi><mn>1</mn><mo>*</mo></msubsup><mo></mo><msub><mi>q</mi><mn>2</mn></msub></mrow><mo>+</mo><mrow><msubsup><mi>q</mi><mn>2</mn><mo>*</mo></msubsup><mo></mo><msub><mi>q</mi><mn>1</mn></msub></mrow></mrow><mo>)</mo></mrow><mo>/</mo><msub><mi>M</mi><mi>C</mi></msub></mrow></mrow><mo>+</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><mrow><mo>(</mo><mrow><mrow><msubsup><mi>I</mi><mn>1</mn><mo>*</mo></msubsup><mo></mo><msub><mi>I</mi><mn>2</mn></msub></mrow><mo>+</mo><mrow><msubsup><mi>I</mi><mn>2</mn><mo>*</mo></msubsup><mo></mo><msub><mi>I</mi><mn>1</mn></msub></mrow></mrow><mo>)</mo></mrow><mo></mo><msub><mi>M</mi><mi>L</mi></msub></mrow></mrow></mrow><mo></mo><mstyle><mtext></mtext></mstyle><mo>⇒</mo><mrow><mn>1</mn><mo>/</mo><msub><mi>M</mi><mi>C</mi></msub></mrow></mrow><mo>=</mo><mrow><mfrac><mn>1</mn><mrow><mn>4</mn><mo></mo><msub><mi>πɛ</mi><mi>o</mi></msub><mo></mo><msub><mi>q</mi><mn>1</mn></msub><mo></mo><msub><mi>q</mi><mn>2</mn></msub></mrow></mfrac><mo></mo><mrow><mo>∫</mo><mrow><mo>∫</mo><mrow><mrow><mo></mo><mi>x</mi></mrow><mo></mo><mrow><mo></mo><msup><mi>x</mi><mi>′</mi></msup></mrow><mo></mo><mfrac><mrow><mrow><msub><mi>ρ</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow><mo></mo><mrow><msub><mi>ρ</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><msup><mi>x</mi><mi>′</mi></msup><mo>)</mo></mrow></mrow></mrow><mrow><mo></mo><mrow><mi>x</mi><mo></mo><msup><mi>x</mi><mi>′</mi></msup></mrow><mo></mo></mrow></mfrac><mo></mo><mi>u</mi></mrow></mrow></mrow></mrow></mrow><mo>,</mo><mstyle><mtext></mtext></mstyle><mo></mo><mrow><msub><mi>M</mi><mi>L</mi></msub><mo>=</mo><mrow><mfrac><msub><mi>μ</mi><mi>o</mi></msub><mrow><mn>4</mn><mo></mo><mi>π</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>I</mi><mn>1</mn></msub><mo></mo><msub><mi>I</mi><mn>2</mn></msub></mrow></mfrac><mo></mo><mrow><mo>∫</mo><mrow><mo>∫</mo><mrow><mrow><mo></mo><mi>x</mi></mrow><mo></mo><mrow><mo></mo><msup><mi>x</mi><mi>′</mi></msup></mrow><mo></mo><mfrac><mrow><mrow><msub><mi>j</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow><mo>·</mo><mrow><msub><mi>j</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><msup><mi>x</mi><mi>′</mi></msup><mo>)</mo></mrow></mrow></mrow><mrow><mo></mo><mrow><mi>x</mi><mo></mo><msup><mi>x</mi><mi>′</mi></msup></mrow><mo></mo></mrow></mfrac><mo></mo><mi>u</mi></mrow></mrow></mrow></mrow></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mo>(</mo><mn>23</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where
<maths id="MATHUS00023" num="00023"><math overflow="scroll"><mrow><mrow><msub><mi>W</mi><mn>1</mn></msub><mo>=</mo><mrow><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><mrow><msubsup><mi>q</mi><mn>1</mn><mn>2</mn></msubsup><mo>/</mo><msub><mi>C</mi><mn>1</mn></msub></mrow></mrow><mo>=</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><msubsup><mi>I</mi><mn>1</mn><mn>2</mn></msubsup><mo></mo><msub><mi>L</mi><mn>1</mn></msub></mrow></mrow></mrow><mo>,</mo><mrow><msub><mi>W</mi><mn>2</mn></msub><mo>=</mo><mrow><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><mrow><msubsup><mi>q</mi><mn>2</mn><mn>2</mn></msubsup><mo>/</mo><msub><mi>C</mi><mn>2</mn></msub></mrow></mrow><mo>=</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><msubsup><mi>I</mi><mn>2</mn><mn>2</mn></msubsup><mo></mo><msub><mi>L</mi><mn>2</mn></msub></mrow></mrow></mrow></mrow></math></maths>
and the retardation factor of u=exp(iωx−x′/c) inside the integral can been ignored in the quasistatic regime D<<λ of interest, where each coil is within the near field of the other. With this definition, the coupling factor is given by k=√{square root over (C<sub>1</sub>C<sub>2</sub>)}/M<sub>C</sub>+M<sub>L</sub>/√{square root over (L<sub>1</sub>L<sub>2</sub>)}.
Therefore, to calculate the coupling rate between two selfresonant coils, again the current profiles are needed and, by using again the assumed sinusoidal current profiles, we compute numerically from Eq. (23) the mutual capacitance M<sub>C,s </sub>and inductance M<sub>L,s </sub>between two selfresonant coils at a distance D between their centers, and thus k=1/Q<sub>κ</sub> is also determined.
<tables id="TABLEUS00002" num="00002"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="6"><colspec colname="offset" colwidth="14pt" align="left"/><colspec colname="1" colwidth="84pt" align="left"/><colspec colname="2" colwidth="14pt" align="center"/><colspec colname="3" colwidth="35pt" align="center"/><colspec colname="4" colwidth="35pt" align="center"/><colspec colname="5" colwidth="35pt" align="center"/><thead><row><entry/><entry namest="offset" nameend="5" rowsep="1">TABLE 2</entry></row><row><entry/><entry namest="offset" nameend="5" align="center" rowsep="1"/></row><row><entry/><entry>pair of coils</entry><entry>D/r</entry><entry>Q</entry><entry>Q<sub>κ </sub>= 1/k</entry><entry>U</entry></row><row><entry/><entry namest="offset" nameend="5" align="center" rowsep="1"/></row></thead><tbody valign="top"><row><entry/></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="6"><colspec colname="offset" colwidth="14pt" align="left"/><colspec colname="1" colwidth="84pt" align="left"/><colspec colname="2" colwidth="14pt" align="char" char="."/><colspec colname="3" colwidth="35pt" align="center"/><colspec colname="4" colwidth="35pt" align="char" char="."/><colspec colname="5" colwidth="35pt" align="char" char="."/><tbody valign="top"><row><entry/><entry>r = 30 cm, h = 20 cm,</entry><entry>3</entry><entry>2758</entry><entry>38.9</entry><entry>70.9</entry></row><row><entry/><entry>a = 1 cm, N = 4</entry><entry>5</entry><entry>2758</entry><entry>139.4</entry><entry>19.8</entry></row><row><entry/><entry>λ/r ≈ 75</entry><entry>7</entry><entry>2758</entry><entry>333.0</entry><entry>8.3</entry></row><row><entry/><entry>Q<sub>s</sub><sup>abs </sup>≈ 8170, Q<sub>s</sub><sup>rad </sup>≈ 4164</entry><entry>10</entry><entry>2758</entry><entry>818.9</entry><entry>3.4</entry></row><row><entry/><entry>r = 10 cm, h = 3 cm,</entry><entry>3</entry><entry>3639</entry><entry>61.4</entry><entry>59.3</entry></row><row><entry/><entry>a = 2 mm, N = 6</entry><entry>5</entry><entry>3639</entry><entry>232.5</entry><entry>15.7</entry></row><row><entry/><entry>λ/r ≈ 140</entry><entry>7</entry><entry>3639</entry><entry>587.5</entry><entry>6.2</entry></row><row><entry/><entry>Q<sub>s</sub><sup>abs </sup>≈ 3968, Q<sub>s</sub><sup>rad </sup>≈ 43919</entry><entry>10</entry><entry>3639</entry><entry>1580</entry><entry>2.3</entry></row><row><entry/><entry namest="offset" nameend="5" align="center" rowsep="1"/></row></tbody></tgroup></table></tables>
Referring to Table 2, relevant parameters are shown for exemplary examples featuring pairs or identical self resonant coils. Numerical results are presented for the average wavelength and loss rates of the two normal modes (individual values not shown), and also the coupling rate and figureofmerit as a function of the coupling distance D, for the two cases of modes presented in Table 1. It can be seen that for medium distances D/r=10−3 the expected couplingtoloss ratios are in the range U˜270.
An experimental realization of an example of the above described system for wireless energy transfer consists of two selfresonant coils of the type described above, one of which (the source coil) is coupled inductively to an oscillating circuit, and the second (the device coil) is coupled inductively to a resistive load, as shown schematically in FIG. 5. Referring to FIG. 5, A is a single copper loop of radius 25 cm that is part of the driving circuit, which outputs a sine wave with frequency 9.9 MHz. s and d are respectively the source and devicecoils referred to in the text. B is a loop of wire attached to the load (“lightbulb”). The various κ's represent direct couplings between the objects. The angle between coil d and the loop A is adjusted so that their direct coupling is zero, while coils s and d are aligned coaxially. The direct coupling between B and A and between B and s is negligible.
The parameters for the two identical helical coils built for the experimental validation of the power transfer scheme were h=20 cm, a=3 mm, r=30 cm and N=5.25. Both coils are made of copper. Due to imperfections in the construction, the spacing between loops of the helix is not uniform, and we have encapsulated the uncertainty about their uniformity by attributing a 10% (2 cm) uncertainty to h. The expected resonant frequency given these dimensions is f<sub>0</sub>=10.56±0.3 MHz, which is about 5% off from the measured resonance at around 9.90 MHz.
The theoretical Q for the loops is estimated to be ˜2500 (assuming perfect copper of resistivity ρ=1/σ=1.7×10<sup>−8 </sup>Ωm) but the measured value is 950±50. We believe the discrepancy is mostly due to the effect of the layer of poorly conducting copper oxide on the surface of the copper wire, to which the current is confined by the short skin depth (˜20 μm) at this frequency. We have therefore used the experimentally observed Q (and Γ<sub>1</sub>=Γ<sub>2</sub>=Γ=ω/(2Q) derived from it) in all subsequent computations.
The coupling coefficient κ can be found experimentally by placing the two selfresonant coils (finetuned, by slightly adjusting h, to the same resonant frequency when isolated) a distance D apart and measuring the splitting in the frequencies of the two resonant modes in the transmission spectrum. According to Eq. (13) derived by coupledmode theory, the splitting in the transmission spectrum should be δ<sub>P</sub>=2√{square root over (κ<sup>2</sup>−Γ<sup>2</sup>)}, when κ<sub>A,B </sub>are kept very small by keeping A and B at a relatively large distance. The comparison between experimental and theoretical results as a function of distance when the two the coils are aligned coaxially is shown in FIG. 6.
FIG. 7 shows a comparison of experimental and theoretical values for the strongcoupling factor U=κ/Γ as a function of the separation between the two coils. The theory values are obtained by using the theoretically obtained κ and the experimentally measured Γ. The shaded area represents the spread in the theoretical U due to the ˜5% uncertainty in Q. As noted above, the maximum theoretical efficiency depends only on the parameter U, which is plotted as a function of distance in FIG. 7. U is greater than 1 even for D=2.4 m (eight times the radius of the coils), thus the system is in the stronglycoupled regime throughout the entire range of distances probed.
The powergenerator circuit was a standard Colpitts oscillator coupled inductively to the source coil by means of a single loop of copper wire 25 cm in radius (see FIG. 5). The load consisted of a previously calibrated lightbulb, and was attached to its own loop of insulated wire, which was in turn placed in proximity of the device coil and inductively coupled to it. Thus, by varying the distance between the lightbulb and the device coil, the parameter U<sub>B</sub>=κ<sub>B</sub>/Γ was adjusted so that it matched its optimal value, given theoretically by Eq. (14) as U<sub>B</sub>*=√{square root over (1+U<sup>2</sup>)}. Because of its inductive nature, the loop connected to the lightbulb added a small reactive component to κ<sub>B </sub>which was compensated for by slightly retuning the coil. The work extracted was determined by adjusting the power going into the Colpitts oscillator until the lightbulb at the load was at its full nominal brightness.
In order to isolate the efficiency of the transfer taking place specifically between the source coil and the load, we measured the current at the midpoint of each of the selfresonant coils with a currentprobe (which was not found to lower the Q of the coils noticeably.) This gave a measurement of the current parameters I<sub>1 </sub>and I<sub>2 </sub>defined above. The power dissipated in each coil was then computed from P<sub>1,2</sub>=ΓLI<sub>1,2</sub><sup>2</sup>, and the efficiency was directly obtained from η=P<sub>B</sub>/(P<sub>1</sub>+P<sub>2</sub>+P<sub>B</sub>). To ensure that the experimental setup was well described by a twoobject coupledmode theory model, we positioned the device coil such that its direct coupling to the copper loop attached to the Colpitts oscillator was zero. The experimental results are shown in FIG. 8, along with the theoretical prediction for maximum efficiency, given by Eq. (15).
Using this example, we were able to transmit significant amounts of power using this setup from the source coil to the device coil, fully lighting up a 60 W lightbulb from distances more than 2 m away, for example. As an additional test, we also measured the total power going into the driving circuit. The efficiency of the wireless powertransmission itself was hard to estimate in this way, however, as the efficiency of the Colpitts oscillator itself is not precisely known, although it is expected to be far from 100%. Nevertheless, this gave an overly conservative lower bound on the efficiency. When transmitting 60 W to the load over a distance of 2 m, for example, the power flowing into the driving circuit was 400 W. This yields an overall walltoload efficiency of ˜15%, which is reasonable given the expected ˜40% efficiency for the wireless power transmission at that distance and the low efficiency of the driving circuit.
From the theoretical treatment above, we see that in typical examples it is important that the coils be on resonance for the power transmission to be practical. We found experimentally that the power transmitted to the load dropped sharply as one of the coils was detuned from resonance. For a fractional detuning Δf/f<sub>0 </sub>of a few times the inverse loaded Q, the induced current in the device coil was indistinguishable from noise.
The power transmission was not found to be visibly affected as humans and various everyday objects, such as metallic and wooden furniture, as well as electronic devices large and small, were placed between the two coils, even when they drastically obstructed the line of sight between source and device. External objects were found to have an effect only when they were closer than 10 cm from either one of the coils. While some materials (such as aluminum foil, styrofoam and humans) mostly just shifted the resonant frequency, which could in principle be easily corrected with a feedback circuit of the type described earlier, others (cardboard, wood, and PVC) lowered Q when placed closer than a few centimeters from the coil, thereby lowering the efficiency of the transfer.
This method of power transmission is believed safe for humans. When transmitting 60 W (more than enough to power a laptop computer) across 2 m, we estimated that the magnitude of the magnetic field generated is much weaker than the Earth's magnetic field for all distances except for less than about 1 cm away from the wires in the coil, an indication of the safety of the scheme even after longterm use. The power radiated for these parameters was ˜5 W, which is roughly an order of magnitude higher than cell phones but could be drastically reduced, as discussed below.
Although the two coils are currently of identical dimensions, it is possible to make the device coil small enough to fit into portable devices without decreasing the efficiency. One could, for instance, maintain the product of the characteristic sizes of the source and device coils constant.
These experiments demonstrated experimentally a system for power transmission over medium range distances, and found that the experimental results match theory well in multiple independent and mutually consistent tests.
The efficiency of the scheme and the distances covered can be appreciably improved by silverplating the coils, which should increase their Q, or by working with more elaborate geometries for the resonant objects. Nevertheless, the performance characteristics of the system presented here are already at levels where they could be useful in practical applications.
In some examples, one or more of the resonant objects are capacitivelyloaded conducting loops or coils. Referring to FIG. 9 a helical coil with N turns of conducting wire, as described above, is connected to a pair of conducting parallel plates of area A spaced by distance d via a dielectric material of relative permittivity ε, and everything is surrounded by air (as shown, N=1 and h=0). The plates have a capacitance C<sub>p</sub>=ε<sub>o</sub>εA/d, which is added to the distributed capacitance of the coil and thus modifies its resonance. Note however, that the presence of the loading capacitor modifies significantly the current distribution inside the wire and therefore the total effective inductance L and total effective capacitance C of the coil are different respectively from L<sub>s </sub>and C<sub>s</sub>, which are calculated for a selfresonant coil of the same geometry using a sinusoidal current profile. Since some charge is accumulated at the plates of the external loading capacitor, the charge distribution ρ inside the wire is reduced, so C<C<sub>s</sub>, and thus, from the charge conservation equation, the current distribution j flattens out, so L>L<sub>s</sub>. The resonant frequency for this system is ω=1/√{square root over (L(C+C<sub>p</sub>))}<ω<sub>s</sub>=1/√{square root over (L<sub>s</sub>C<sub>s</sub>)}, and I(x)→I<sub>o </sub>cos(πx/l)<img id="CUSTOMCHARACTER00006" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>C→C<sub>s</sub><img id="CUSTOMCHARACTER00007" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>ω→ω<sub>s</sub>, as C<sub>p</sub>→0.
In general, the desired CMT parameters can be found for this system, but again a very complicated solution of Maxwell's Equations is required. Instead, we will analyze only a special case, where a reasonable guess for the current distribution can be made. When C<sub>p</sub>>>C<sub>s</sub>>C, then ω≈1/√{square root over (LC<sub>p</sub>)}<<ω<sub>s </sub>and Z≈√{square root over (L/C<sub>p</sub>)}<<Z<sub>s</sub>, while all the charge is on the plates of the loading capacitor and thus the current distribution is constant along the wire. This allows us now to compute numerically L from Eq. (19). In the case h=0 and N integer, the integral in Eq. (19) can actually be computed analytically, giving the formula L=μ<sub>o</sub>r[ln(8r/a)−2]N<sup>2</sup>. Explicit analytical formulas are again available for R from Eq. (21) and (22), since I<sub>rms</sub>=I<sub>o</sub>, p≈0 and m=I<sub>o</sub>Nπr<sup>2 </sup>(namely only the magneticdipole term is contributing to radiation), so we can determine also Q<sub>abs</sub>=ωL/R<sub>abs </sub>and Q<sub>rad</sub>=ωL/R<sub>rad</sub>. At the end of the calculations, the validity of the assumption of constant current profile is confirmed by checking that indeed the condition C<sub>p</sub>>>C<sub>s</sub><img id="CUSTOMCHARACTER00008" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>ω<<ω<sub>s </sub>is satisfied. To satisfy this condition, one could use a large external capacitance, however, this would usually shift the operational frequency lower than the optimal frequency, which we will determine shortly; instead, in typical examples, one often prefers coils with very small selfcapacitance C<sub>s </sub>to begin with, which usually holds, for the types of coils under consideration, when N=1, so that the selfcapacitance comes from the charge distribution across the single turn, which is almost always very small, or when N>1 and h>>2Na, so that the dominant selfcapacitance comes from the charge distribution across adjacent turns, which is small if the separation between adjacent turns is large.
The external loading capacitance C<sub>p </sub>provides the freedom to tune the resonant frequency (for example by tuning A or d). Then, for the particular simple case h=0, for which we have analytical formulas, the total Q=ωL/(R<sub>abs</sub>+R<sub>rad</sub>) becomes highest at the optimal frequency
<maths id="MATHUS00024" num="00024"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>ω</mi><mi>Q</mi></msub><mo>=</mo><msup><mrow><mo>[</mo><mrow><mfrac><msup><mi>c</mi><mn>4</mn></msup><mi>π</mi></mfrac><mo></mo><mrow><msqrt><mfrac><msub><mi>ɛ</mi><mi>o</mi></msub><mrow><mn>2</mn><mo></mo><mi>σ</mi></mrow></mfrac></msqrt><mo>·</mo><mfrac><mn>1</mn><msup><mi>aNr</mi><mn>3</mn></msup></mfrac></mrow></mrow><mo>]</mo></mrow><mrow><mn>2</mn><mo>/</mo><mn>7</mn></mrow></msup></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mo>(</mo><mn>24</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
reaching the value
<maths id="MATHUS00025" num="00025"><math overflow="scroll"><mtable><mtr><mtd><mrow><msub><mi>Q</mi><mi>max</mi></msub><mo>=</mo><mrow><mfrac><mn>6</mn><mrow><mn>7</mn><mo></mo><mi>π</mi></mrow></mfrac><mo></mo><mrow><msup><mrow><mo>(</mo><mrow><mn>2</mn><mo></mo><msup><mi>π</mi><mn>2</mn></msup><mo></mo><msub><mi>η</mi><mi>o</mi></msub><mo></mo><mfrac><mrow><mi>σ</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msup><mi>a</mi><mn>2</mn></msup><mo></mo><msup><mi>N</mi><mn>2</mn></msup></mrow><mi>r</mi></mfrac></mrow><mo>)</mo></mrow><mrow><mn>3</mn><mo>/</mo><mn>7</mn></mrow></msup><mo>·</mo><mrow><mrow><mo>[</mo><mrow><mrow><mi>ln</mi><mo></mo><mrow><mo>(</mo><mfrac><mrow><mn>8</mn><mo></mo><mi>r</mi></mrow><mi>a</mi></mfrac><mo>)</mo></mrow></mrow><mo></mo><mn>2</mn></mrow><mo>]</mo></mrow><mo>.</mo></mrow></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>25</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
At lower frequencies it is dominated by ohmic loss and at higher frequencies by radiation. Note, however, that the formulas above are accurate as long as ω<sub>Q</sub><<ω<sub>s </sub>and, as explained above, this holds almost always when N=1, and is usually less accurate when N>1, since h=0 usually implies a large selfcapacitance. A coil with large h can be used, if the selfcapacitance needs to be reduced compared to the external capacitance, but then the formulas for L and ω<sub>Q</sub>, Q<sub>max </sub>are again less accurate. Similar qualitative behavior is expected, but a more complicated theoretical model is needed for making quantitative predictions in that case.
The results of the above analysis for two examples of subwavelength modes of λ/r≧70 (namely highly suitable for nearfield coupling and well within the quasistatic limit) of coils with N=1 and h=0 at the optimal frequency Eq. (24) are presented in Table 3. To confirm the validity of constantcurrent assumption and the resulting analytical formulas, modesolving calculations were also performed using another completely independent method: computational 3D finiteelement frequencydomain (FEFD) simulations (which solve Maxwell's Equations in frequency domain exactly apart for spatial discretization) were conducted, in which the boundaries of the conductor were modeled using a complex impedance ζ<sub>c</sub>=√{square root over (μ<sub>o</sub>ω/2σ)} boundary condition, valid as long as ζ<sub>c</sub>/ζ<sub>o</sub><<1 (<10<sup>−5 </sup>for copper in the microwave). Table 3 shows Numerical FEFD (and in parentheses analytical) results for the wavelength and absorption, radiation and total loss rates, for two different cases of subwavelengthloop resonant modes. Note that for conducting material copper (σ=5.998·10<sup>7 </sup>S/m) was used. Specific parameters of the plot in FIG. 4 are highlighted in bold in the table. The two methods (analytical and computational) are in good agreement and show that, in some examples, the optimal frequency is in the lowMHz microwave range and the expected quality factors are Q<sub>abs</sub>≧1000 and Q<sub>rad</sub>≧10000.
<tables id="TABLEUS00003" num="00003"><table frame="none" colsep="0" rowsep="0" pgwide="1"><tgroup align="left" colsep="0" rowsep="0" cols="6"><colspec colname="1" colwidth="70pt" align="left"/><colspec colname="2" colwidth="49pt" align="center"/><colspec colname="3" colwidth="49pt" align="center"/><colspec colname="4" colwidth="49pt" align="center"/><colspec colname="5" colwidth="42pt" align="center"/><colspec colname="6" colwidth="42pt" align="center"/><thead><row><entry namest="1" nameend="6" rowsep="1">TABLE 3</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row><row><entry>single coil</entry><entry>λ/r</entry><entry>f</entry><entry>Q<sub>rad</sub></entry><entry>Q<sub>abs</sub></entry><entry>Q</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row></thead><tbody valign="top"><row><entry>r = 30 cm, a = 2 cm</entry><entry>111.4 (112.4)</entry><entry>8.976 (8.897)</entry><entry>29546 (30512)</entry><entry>4886 (5117)</entry><entry>4193 (4381)</entry></row><row><entry>ε = 10, A = 138 cm<sup>2</sup>,</entry></row><row><entry>d = 4 mm</entry></row><row><entry>r = 10 cm, a = 2 mm</entry><entry>69.7 (70.4)</entry><entry>43.04 (42.61)</entry><entry>10702 (10727)</entry><entry>1545 (1604)</entry><entry>1350 (1395)</entry></row><row><entry>ε = 10, A = 3.14 cm<sup>2</sup>,</entry></row><row><entry>d = 1 mm</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row></tbody></tgroup></table></tables>
Referring to FIG. 10, in some examples, energy is transferred between two capacitivelyloaded coils. For the rate of energy transfer between two capacitivelyloaded coils 1 and 2 at distance D between their centers, the mutual inductance M<sub>L </sub>can be evaluated numerically from Eq. (23) by using constant current distributions in the case ω<<ω<sub>s</sub>. In the case h=0, the coupling is only magnetic and again we have an analytical formula, which, in the quasistatic limit r<<D<<λ and for the relative orientation shown in FIG. 10, is M<sub>L</sub>≈πμ<sub>o</sub>/2·(r<sub>1</sub>r<sub>2</sub>)<sup>2</sup>N<sub>1</sub>N<sub>2</sub>/D<sup>3</sup>, which means that k ∝(√{square root over (r<sub>1</sub>r<sub>2</sub>)}/D)<sup>3 </sup>is independent of the frequency ω and the number of turns N<sub>1</sub>, N<sub>2</sub>, Consequently, the resultant coupling figureofmerit of interest is
<maths id="MATHUS00026" num="00026"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mi>U</mi><mo>=</mo><mrow><mrow><mi>k</mi><mo></mo><msqrt><mrow><msub><mi>Q</mi><mn>1</mn></msub><mo></mo><msub><mi>Q</mi><mn>2</mn></msub></mrow></msqrt></mrow><mo>≈</mo><mrow><msup><mrow><mo>(</mo><mfrac><msqrt><mrow><msub><mi>r</mi><mn>1</mn></msub><mo></mo><msub><mi>r</mi><mn>2</mn></msub></mrow></msqrt><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>D</mi></mrow></mfrac><mo>)</mo></mrow><mn>3</mn></msup><mo>·</mo><mfrac><mrow><mrow><msup><mi>π</mi><mn>2</mn></msup><mo></mo><msub><mi>η</mi><mi>o</mi></msub><mo></mo><mfrac><msqrt><mrow><msub><mi>r</mi><mn>1</mn></msub><mo></mo><msub><mi>r</mi><mn>2</mn></msub></mrow></msqrt><mi>λ</mi></mfrac></mrow><mo></mo><mrow><mrow><mo>·</mo><msub><mi>N</mi><mn>1</mn></msub></mrow><mo></mo><msub><mi>N</mi><mn>2</mn></msub></mrow></mrow><mrow><munder><mo>∏</mo><mrow><mrow><mi>j</mi><mo>=</mo><mn>1</mn></mrow><mo>,</mo><mn>2</mn></mrow></munder><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msup><mrow><mo>(</mo><mtable><mtr><mtd><mrow><mrow><mrow><msqrt><mfrac><mrow><mi>π</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>η</mi><mi>o</mi></msub></mrow><mrow><mi>λ</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>σ</mi></mrow></mfrac></msqrt><mo>·</mo><mfrac><msub><mi>r</mi><mi>j</mi></msub><msub><mi>a</mi><mi>j</mi></msub></mfrac></mrow><mo></mo><msub><mi>N</mi><mi>j</mi></msub></mrow><mo>+</mo></mrow></mtd></mtr><mtr><mtd><mrow><mfrac><mn>8</mn><mn>3</mn></mfrac><mo></mo><msup><mi>π</mi><mn>5</mn></msup><mo></mo><msup><mrow><msub><mi>η</mi><mi>o</mi></msub><mo></mo><mrow><mo>(</mo><mfrac><msub><mi>r</mi><mi>j</mi></msub><mi>λ</mi></mfrac><mo>)</mo></mrow></mrow><mn>4</mn></msup><mo></mo><msubsup><mi>N</mi><mi>j</mi><mn>2</mn></msubsup></mrow></mtd></mtr></mtable><mo>)</mo></mrow><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msup></mrow></mfrac></mrow></mrow></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mo>(</mo><mn>26</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
which again is more accurate for N<sub>1</sub>=N<sub>2</sub>=1.
From Eq. (26) it can be seen that the optimal frequency ω<sub>U</sub>, where the figureofmerit is maximized to the value U<sub>max</sub>, is close to the frequency ω<sub>Q</sub><sub><sub2>1</sub2></sub><sub>Q</sub><sub><sub2>2 </sub2></sub>at which Q<sub>1</sub>Q<sub>2 </sub>is maximized, since k does not depend much on frequency (at least for the distances D<<λ of interest for which the quasistatic approximation is still valid). Therefore, the optimal frequency ω<sub>U</sub>≈ω<sub>Q</sub><sub><sub2>1</sub2></sub><sub>Q</sub><sub><sub2>2 </sub2></sub>is mostly independent of the distance D between the two coils and lies between the two frequencies ω<sub>Q</sub><sub><sub2>1 </sub2></sub>and ω<sub>Q</sub><sub><sub2>2 </sub2></sub>at which the singlecoil Q<sub>1 </sub>and Q<sub>2 </sub>respectively peak. For same coils, this optimal frequency is given by Eq. (24) and then the strongcoupling factor from Eq. (26) becomes
<maths id="MATHUS00027" num="00027"><math overflow="scroll"><mtable><mtr><mtd><mrow><msub><mi>U</mi><mi>max</mi></msub><mo>=</mo><mrow><msub><mi>kQ</mi><mi>max</mi></msub><mo>≈</mo><mrow><mrow><msup><mrow><mo>(</mo><mfrac><mi>r</mi><mi>D</mi></mfrac><mo>)</mo></mrow><mn>3</mn></msup><mo>·</mo><mfrac><mn>3</mn><mn>7</mn></mfrac></mrow><mo></mo><mrow><msup><mrow><mo>(</mo><mrow><mn>2</mn><mo></mo><msup><mi>π</mi><mn>2</mn></msup><mo></mo><msub><mi>η</mi><mi>o</mi></msub><mo></mo><mfrac><mrow><mi>σ</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msup><mi>a</mi><mn>2</mn></msup><mo></mo><msup><mi>N</mi><mn>2</mn></msup></mrow><mi>r</mi></mfrac></mrow><mo>)</mo></mrow><mrow><mn>3</mn><mo>/</mo><mn>7</mn></mrow></msup><mo>.</mo></mrow></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>27</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
In some examples, one can tune the capacitivelyloaded conducting loops or coils, so that their angular eigenfrequencies are close to ω<sub>U </sub>within Γ<sub>U</sub>, which is half the angular frequency width for which U>U<sub>max</sub>/2.
Referring to Table 4, numerical FEFD and, in parentheses, analytical results based on the above are shown for two systems each composed of a matched pair of the loaded coils described in Table 3. The average wavelength and loss rates are shown along with the coupling rate and coupling to loss ratio figureofmerit U=κ/Γ as a function of the coupling distance D, for the two cases. Note that the average numerical Γ<sub>rad </sub>shown are slightly different from the singleloop value of FIG. 3, analytical results for Γ<sub>rad </sub>are not shown but the singleloop value is used. (The specific parameters corresponding to the plot in FIG. 10 are highlighted with bold in the table.) Again we chose N=1 to make the constantcurrent assumption a good one and computed M<sub>L </sub>numerically from Eq. (23). Indeed the accuracy can be confirmed by their agreement with the computational FEFD modesolver simulations, which give κ through the frequency splitting of the two normal modes of the combined system (δ<sub>E</sub>=2κ from Eq. (4)). The results show that for medium distances D/r=10−3 the expected couplingtoloss ratios are in the range U˜0.550.
<tables id="TABLEUS00004" num="00004"><table frame="none" colsep="0" rowsep="0" pgwide="1"><tgroup align="left" colsep="0" rowsep="0" cols="6"><colspec colname="1" colwidth="98pt" align="left"/><colspec colname="2" colwidth="14pt" align="center"/><colspec colname="3" colwidth="28pt" align="center"/><colspec colname="4" colwidth="35pt" align="center"/><colspec colname="5" colwidth="42pt" align="center"/><colspec colname="6" colwidth="42pt" align="center"/><thead><row><entry namest="1" nameend="6" rowsep="1">TABLE 4</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row><row><entry>pair of coils</entry><entry>D/r</entry><entry>Q<sup>rad</sup></entry><entry>Q = ω/2Γ</entry><entry>Q<sub>κ </sub>= ω/2κ</entry><entry>κ/Γ</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row></thead><tbody valign="top"><row><entry/></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="6"><colspec colname="1" colwidth="98pt" align="left"/><colspec colname="2" colwidth="14pt" align="char" char="."/><colspec colname="3" colwidth="28pt" align="center"/><colspec colname="4" colwidth="35pt" align="center"/><colspec colname="5" colwidth="42pt" align="center"/><colspec colname="6" colwidth="42pt" align="center"/><tbody valign="top"><row><entry>r = 30 cm, a = 2 cm</entry><entry>3</entry><entry>30729</entry><entry>4216</entry><entry>62.6 (63.7)</entry><entry>67.4 (68.7)</entry></row><row><entry>ε = 10, A = 138 cm<sup>2</sup>, d = 4 mm</entry><entry>5</entry><entry>29577</entry><entry>4194</entry><entry>235 (248)</entry><entry>17.8 (17.6)</entry></row><row><entry>λ/r ≈ 112</entry><entry>7</entry><entry>29128</entry><entry>4185</entry><entry>589 (646)</entry><entry>7.1 (6.8)</entry></row><row><entry>Q<sup>abs </sup>≈ 4886</entry><entry>10</entry><entry>28833</entry><entry>4177</entry><entry>1539 (1828)</entry><entry>2.7 (2.4)</entry></row><row><entry>r = 10 cm, a = 2 mm</entry><entry>3</entry><entry>10955</entry><entry>1355</entry><entry>85.4 (91.3)</entry><entry>15.9 (15.3)</entry></row><row><entry>ε = 10, A = 3.14 cm<sup>2</sup>, d = 1 mm</entry><entry>5</entry><entry>10740</entry><entry>1351</entry><entry>313 (356)</entry><entry>4.32 (3.92)</entry></row><row><entry>λ/r ≈ 70</entry><entry>7</entry><entry>10759</entry><entry>1351</entry><entry>754 (925)</entry><entry>1.79 (1.51)</entry></row><row><entry>Q<sup>abs </sup>≈ 1546</entry><entry>10</entry><entry>10756</entry><entry>1351</entry><entry>1895 (2617)</entry><entry>0.71 (0.53)</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row></tbody></tgroup></table></tables>
Referring to FIG. 11, to rederive and express Eq. (15) in terms of the parameters which are more directly accessible from particular resonant objects, such as the capacitivelyloaded conducting loops, one can consider the following circuitmodel of the system, where the inductances L<sub>s</sub>, L<sub>d </sub>represent the source and device loops respectively, R<sub>s</sub>, R<sub>d </sub>their respective losses, and C<sub>s</sub>, C<sub>d </sub>are the required corresponding capacitances to achieve for both resonance at frequency ω. A voltage generator V<sub>g </sub>is considered to be connected to the source and a load resistance R<sub>l </sub>to the device. The mutual inductance is denoted by M.
Then from the source circuit at resonance (ωL<sub>s</sub>=1/ωC<sub>s</sub>):
<maths id="MATHUS00028" num="00028"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>V</mi><mi>g</mi></msub><mo>=</mo><mrow><mrow><mrow><mrow><msub><mi>I</mi><mi>s</mi></msub><mo></mo><msub><mi>R</mi><mi>s</mi></msub></mrow><mo></mo><mrow><mi>jω</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>MI</mi><mi>d</mi></msub></mrow></mrow><mo>⇒</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><msubsup><mi>V</mi><mi>g</mi><mo>*</mo></msubsup><mo></mo><msub><mi>I</mi><mi>s</mi></msub></mrow></mrow><mo>=</mo><mrow><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><msup><mrow><mo></mo><msub><mi>I</mi><mi>s</mi></msub><mo></mo></mrow><mn>2</mn></msup><mo></mo><msub><mi>R</mi><mi>s</mi></msub></mrow><mo>+</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><mi>jω</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msubsup><mi>MI</mi><mi>d</mi><mo>*</mo></msubsup><mo></mo><msub><mi>I</mi><mi>s</mi></msub></mrow></mrow></mrow></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mo>(</mo><mn>28</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
and from the device circuit at resonance (ωL<sub>d</sub>=1/ωC<sub>d</sub>):
<FORM>0=I<sub>d</sub>(R<sub>d</sub>+R<sub>l</sub>)−jωMI<sub>s</sub><img id="CUSTOMCHARACTER00009" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>jωMI<sub>s</sub>=I<sub>d</sub>(R<sub>d</sub>+R<sub>l</sub>) (29)</FORM>
So by substituting Eq. (29) to Eq. (28) and taking the real part (for timeaveraged power) we get:
<maths id="MATHUS00029" num="00029"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>P</mi><mi>g</mi></msub><mo>=</mo><mrow><mrow><mi>Re</mi><mo></mo><mrow><mo>{</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><msubsup><mi>V</mi><mi>g</mi><mo>*</mo></msubsup><mo></mo><msub><mi>I</mi><mi>s</mi></msub></mrow><mo>}</mo></mrow></mrow><mo>=</mo><mrow><mrow><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><msup><mrow><mo></mo><msub><mi>I</mi><mi>s</mi></msub><mo></mo></mrow><mn>2</mn></msup><mo></mo><msub><mi>R</mi><mi>s</mi></msub></mrow><mo>+</mo><mrow><mfrac><mn>1</mn><mn>2</mn></mfrac><mo></mo><msup><mrow><mo></mo><msub><mi>I</mi><mi>d</mi></msub><mo></mo></mrow><mn>2</mn></msup><mo></mo><mrow><mo>(</mo><mrow><msub><mi>R</mi><mi>d</mi></msub><mo>+</mo><msub><mi>R</mi><mi>l</mi></msub></mrow><mo>)</mo></mrow></mrow></mrow><mo>=</mo><mrow><msub><mi>P</mi><mi>s</mi></msub><mo>+</mo><msub><mi>P</mi><mi>d</mi></msub><mo>+</mo><msub><mi>P</mi><mi>l</mi></msub></mrow></mrow></mrow></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mo>(</mo><mn>30</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where we identified the power delivered by the generator P<sub>g</sub>=Re{V<sub>g</sub>*I<sub>s</sub>/2}, the power lost inside the source P<sub>s</sub>=I<sub>s</sub><sup>2</sup>R<sub>s</sub>/2, the power lost inside the device P<sub>d</sub>=I<sub>d</sub><sup>2</sup>R<sub>d</sub>/2 and the power delivered to the load P<sub>l</sub>=I<sub>d</sub><sup>2</sup>R<sub>l</sub>/2. Then, the power transmission efficiency is:
<maths id="MATHUS00030" num="00030"><math overflow="scroll"><mtable><mtr><mtd><mtable><mtr><mtd><mrow><msub><mi>η</mi><mi>P</mi></msub><mo>≡</mo><mi/><mo></mo><mfrac><msub><mi>P</mi><mi>l</mi></msub><msub><mi>P</mi><mi>g</mi></msub></mfrac></mrow></mtd></mtr><mtr><mtd><mrow><mo>=</mo><mi/><mo></mo><mfrac><msub><mi>R</mi><mi>l</mi></msub><mrow><mrow><msup><mrow><mo></mo><mfrac><msub><mi>I</mi><mi>s</mi></msub><msub><mi>I</mi><mi>d</mi></msub></mfrac><mo></mo></mrow><mn>2</mn></msup><mo></mo><msub><mi>R</mi><mi>s</mi></msub></mrow><mo>+</mo><mrow><mo>(</mo><mrow><msub><mi>R</mi><mi>d</mi></msub><mo>+</mo><msub><mi>R</mi><mi>l</mi></msub></mrow><mo>)</mo></mrow></mrow></mfrac></mrow></mtd></mtr><mtr><mtd><mrow><mover><mo>=</mo><mrow><mo>(</mo><mn>29</mn><mo>)</mo></mrow></mover><mo></mo><mi/><mo></mo><mrow><mfrac><msub><mi>R</mi><mi>l</mi></msub><mrow><mrow><mfrac><msup><mrow><mo>(</mo><mrow><msub><mi>R</mi><mi>d</mi></msub><mo>+</mo><msub><mi>R</mi><mi>l</mi></msub></mrow><mo>)</mo></mrow><mn>2</mn></msup><msup><mrow><mo>(</mo><mrow><mi>ω</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>M</mi></mrow><mo>)</mo></mrow><mn>2</mn></msup></mfrac><mo></mo><msub><mi>R</mi><mi>s</mi></msub></mrow><mo>+</mo><mrow><mo>(</mo><mrow><msub><mi>R</mi><mi>d</mi></msub><mo>+</mo><msub><mi>R</mi><mi>l</mi></msub></mrow><mo>)</mo></mrow></mrow></mfrac><mo>.</mo></mrow></mrow></mtd></mtr></mtable></mtd><mtd><mrow><mo>(</mo><mn>31</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
If we now choose the load impedance R<sub>l </sub>to optimize the efficiency by η<sub>P</sub>(R<sub>l</sub>)=0, we get the optimal load impedance
<maths id="MATHUS00031" num="00031"><math overflow="scroll"><mtable><mtr><mtd><mrow><mfrac><msub><mi>R</mi><msup><mi>l</mi><mo>*</mo></msup></msub><msub><mi>R</mi><mi>d</mi></msub></mfrac><mo>=</mo><msqrt><mrow><mn>1</mn><mo>+</mo><mfrac><msup><mrow><mo>(</mo><mrow><mi>ω</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>M</mi></mrow><mo>)</mo></mrow><mn>2</mn></msup><mrow><msub><mi>R</mi><mi>s</mi></msub><mo></mo><msub><mi>R</mi><mi>d</mi></msub></mrow></mfrac></mrow></msqrt></mrow></mtd><mtd><mrow><mo>(</mo><mn>32</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
and the maximum possible efficiency
<maths id="MATHUS00032" num="00032"><math overflow="scroll"><mtable><mtr><mtd><mrow><msub><mi>η</mi><msup><mi>P</mi><mo>*</mo></msup></msub><mo>=</mo><mrow><mfrac><mrow><mrow><msub><mi>R</mi><msup><mi>l</mi><mo>*</mo></msup></msub><mo>/</mo><msub><mi>R</mi><mi>d</mi></msub></mrow><mo></mo><mn>1</mn></mrow><mrow><mrow><msub><mi>R</mi><msup><mi>l</mi><mo>*</mo></msup></msub><mo>/</mo><msub><mi>R</mi><mi>d</mi></msub></mrow><mo>+</mo><mn>1</mn></mrow></mfrac><mo>=</mo><mrow><msup><mrow><mo>[</mo><mfrac><mrow><mi>ω</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><mi>M</mi><mo>/</mo><msqrt><mrow><msub><mi>R</mi><mi>s</mi></msub><mo></mo><msub><mi>R</mi><mi>d</mi></msub></mrow></msqrt></mrow></mrow><mrow><mn>1</mn><mo>+</mo><msqrt><mrow><mn>1</mn><mo>+</mo><msup><mrow><mo>(</mo><mrow><mi>ω</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><mi>M</mi><mo>/</mo><msqrt><mrow><msub><mi>R</mi><mi>s</mi></msub><mo></mo><msub><mi>R</mi><mi>d</mi></msub></mrow></msqrt></mrow></mrow><mo>)</mo></mrow><mn>2</mn></msup></mrow></msqrt></mrow></mfrac><mo>]</mo></mrow><mn>2</mn></msup><mo>.</mo></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>33</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
To check now the correspondence with the CMT model, note that κ<sub>l</sub>=R<sub>l</sub>/2L<sub>d</sub>, Γ<sub>d</sub>=R<sub>d</sub>/2L<sub>d</sub>, Γ<sub>s</sub>=R<sub>s</sub>/2L<sub>s</sub>, and κ=ωM/2√{square root over (L<sub>s</sub>L<sub>d</sub>)}, so then U<sub>l</sub>=κ<sub>l</sub>/Γ<sub>d</sub>=R<sub>l</sub>/R<sub>d </sub>and U=κ/√{square root over (Γ<sub>s</sub>Γ<sub>d</sub>)}=ωM/√{square root over (R<sub>s</sub>R<sub>d</sub>)}. Therefore, the condition Eq. (32) is identical to the condition Eq. (14) and the optimal efficiency Eq. (33) is identical to the general Eq. (15). Indeed, as the CMT analysis predicted, to get a large efficiency, we need to design a system that has a large strongcoupling factor U.
The results above can be used to increase or optimize the performance of a wireless energy transfer system, which employs capacitivelyloaded coils. For example, from the scaling of Eq. (27) with the different system parameters, one sees that to maximize the system figureofmerit U, in some examples, one can:
 Decrease the resistivity of the conducting material. This can be achieved, for example, by using good conductors (such as copper or silver) and/or lowering the temperature. At very low temperatures one could use also superconducting materials to achieve extremely good performance.
 Increase the wire radius a. In typical examples, this action can be limited by physical size considerations. The purpose of this action is mainly to reduce the resistive losses in the wire by increasing the crosssectional area through which the electric current is flowing, so one could alternatively use also a Litz wire or a ribbon instead of a circular wire.
 For fixed desired distance D of energy transfer, increase the radius of the loop r. In typical examples, this action can be limited by physical size considerations, typically especially for the device.
 For fixed desired distance vs. loopsize ratio D/r, decrease the radius of the loop r. In typical examples, this action can be limited by physical size considerations.
 Increase the number of turns N. (Even though Eq. (27) is expected to be less accurate for N>1, qualitatively it still provides a good indication that we expect an improvement in the couplingtoloss ratio with increased N.) In typical examples, this action can be limited by physical size and possible voltage considerations, as will be discussed in following paragraphs.
 Adjust the alignment and orientation between the two coils. The figureofmerit is optimized when both cylindrical coils have exactly the same axis of cylindrical symmetry (namely they are “facing” each other). In some examples, particular mutual coil angles and orientations that lead to zero mutual inductance (such as the orientation where the axes of the two coils are perpendicular and the centers of the two coils are on one of the two axes) should be avoided.
 Finally, note that the height of the coil h is another available design parameter, which can have an impact to the performance similar to that of its radius r, and thus the design rules can be similar.
The above analysis technique can be used to design systems with desired parameters. For example, as listed below, the above described techniques can be used to determine the cross sectional radius a of the wire which one should use when designing as system two same singleturn loops with a given radius in order to achieve a specific performance in terms of U=κ/Γ at a given D/r between them, when the material is copper (σ=5.998·10<sup>7 </sup>S/m):
<FORM>D/r=5, U≧10, r=30 cm<img id="CUSTOMCHARACTER00010" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>a≧9 mm</FORM>
<FORM>D/r=5, U≧10, r=5 cm<img id="CUSTOMCHARACTER00011" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>a≧3.7 mm</FORM>
<FORM>D/r=5, U≧20, r=30 cm<img id="CUSTOMCHARACTER00012" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>a≧20 mm</FORM>
<FORM>D/r=5, U≧20, r=5 cm<img id="CUSTOMCHARACTER00013" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>a≧8.3 mm</FORM>
<FORM>D/r=10, U≧1, r=30 cm<img id="CUSTOMCHARACTER00014" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>a≧7 mm</FORM>
<FORM>D/r=10, U≧1, r=5 cm<img id="CUSTOMCHARACTER00015" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>a≧2.8 mm</FORM>
<FORM>D/r=10, U≧3, r=30 cm<img id="CUSTOMCHARACTER00016" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>a≧25 mm</FORM>
<FORM>D/r=10, U≧3, r=5 cm<img id="CUSTOMCHARACTER00017" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>a≧10 mm</FORM>
Similar analysis can be done for the case of two dissimilar loops. For example, in some examples, the device under consideration is very specific (e.g. a laptop or a cell phone), so the dimensions of the device object (r<sub>d</sub>, h<sub>d</sub>, a<sub>d</sub>, N<sub>d</sub>) are very restricted. However, in some such examples, the restrictions on the source object (r<sub>s</sub>, h<sub>s</sub>, a<sub>s</sub>, N<sub>s</sub>) are much less, since the source can, for example, be placed under the floor or on the ceiling. In such cases, the desired distance is often well defined, based on the application (e.g. D˜1 m for charging a laptop on a table wirelessly from the floor). Listed below are examples (simplified to the case N<sub>s</sub>=N<sub>d</sub>=1 and h<sub>s</sub>=h<sub>d</sub>=0) of how one can vary the dimensions of the source object to achieve the desired system performance in terms of U<sub>sd</sub>=κ/√{square root over (Γ<sub>s</sub>Γ<sub>d</sub>)}, when the material is again copper (σ=5.998·10<sup>7 </sup>S/m):
<FORM>D=1.5 m, U<sub>sd</sub>≧15, r<sub>d</sub>=30 cm, a<sub>d</sub>=6 mm<img id="CUSTOMCHARACTER00018" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>r<sub>s</sub>=1.158 m, a<sub>s</sub>≧5 mm</FORM>
<FORM>D=1.5 m, U<sub>sd</sub>≧30, r<sub>d</sub>=30 cm, a<sub>d</sub>=6 mm<img id="CUSTOMCHARACTER00019" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>r<sub>s</sub>=1.15 m, a<sub>s</sub>≧33 mm</FORM>
<FORM>D=1.5 m, U<sub>sd</sub>≧1, r<sub>d</sub>=5 cm, a<sub>d</sub>=4 mm<img id="CUSTOMCHARACTER00020" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>r<sub>s</sub>=1.119 m, a<sub>s</sub>≧7 mm</FORM>
<FORM>D=1.5 m, U<sub>sd</sub>≧2, r<sub>d</sub>=5 cm, a<sub>d</sub>=4 mm<img id="CUSTOMCHARACTER00021" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>r<sub>s</sub>=1.119 m, a<sub>s</sub>≧52 mm</FORM>
<FORM>D=2 m, U<sub>sd</sub>≧10, r<sub>d</sub>=30 cm, a<sub>d</sub>=6 mm<img id="CUSTOMCHARACTER00022" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>r<sub>s</sub>=1.518 m, a<sub>s</sub>≧7 mm</FORM>
<FORM>D=2 m, U<sub>sd</sub>≧20, r<sub>d</sub>=30 cm, a<sub>d</sub>=6 mm<img id="CUSTOMCHARACTER00023" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>r<sub>s</sub>=1.514 m, a<sub>s</sub>≧50 mm</FORM>
<FORM>D=2 m, U<sub>sd</sub>≧0.5, r<sub>d</sub>=5 cm, a<sub>d</sub>=4 mm<img id="CUSTOMCHARACTER00024" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>r<sub>s</sub>=1.491 m, a<sub>s</sub>≧5 mm</FORM>
<FORM>D=2 m, U<sub>sd</sub>≧1, r<sub>d</sub>=5 cm, a<sub>d</sub>=4 mm<img id="CUSTOMCHARACTER00025" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/>r<sub>s</sub>=1.491 m, a<sub>s</sub>≧36 mm</FORM>
As described below, in some examples, the quality factor Q of the resonant objects is limited from external perturbations and thus varying the coil parameters cannot lead to improvement in Q. In such cases, one can opt to increase the strongcoupling factor U by increasing the coupling factor k. The coupling does not depend on the frequency and the number of turns. Therefore, in some examples, one can:
 Increase the wire radii a<sub>1 </sub>and a<sub>2</sub>. In typical examples, this action can be limited by physical size considerations.
 For fixed desired distance D of energy transfer, increase the radii of the coils r<sub>1 </sub>and r<sub>2</sub>. In typical examples, this action can be limited by physical size considerations, typically especially for the device.
 For fixed desired distance vs. coilsizes ratio D/√{square root over (r<sub>1</sub>r<sub>2</sub>)}, only the weak (logarithmic) dependence of the inductance remains, which suggests that one should decrease the radii of the coils r<sub>1 </sub>and r<sub>2</sub>. In typical examples, this action can be limited by physical size considerations.
 Adjust the alignment and orientation between the two coils. In typical examples, the coupling is optimized when both cylindrical coils have exactly the same axis of cylindrical symmetry (namely they are “facing” each other). Particular mutual coil angles and orientations that lead to zero mutual inductance (such as the orientation where the axes of the two coils are perpendicular and the centers of the two coils are on one of the two axes) should obviously be avoided.
 Finally, note that the heights of the coils h<sub>1 </sub>and h<sub>2 </sub>are other available design parameters, which can have an impact to the coupling similar to that of their radii r<sub>1 </sub>and r<sub>2</sub>, and thus the design rules can be similar.
Further practical considerations apart from efficiency, e.g. physical size limitations, will be discussed in detail below.
In many cases, the dimensions of the resonant objects will be set by the particular application at hand. For example, when this application is powering a laptop or a cellphone, the device resonant object cannot have dimensions larger than those of the laptop or cellphone respectively. In particular, for a system of two loops of specified dimensions, in terms of loop radii r<sub>s,d </sub>and wire radii a<sub>s,d</sub>, the independent parameters left to adjust for the system optimization are: the number of turns N<sub>s,d</sub>, the frequency f, the powerload consumption rate κ<sub>l</sub>=R<sub>l</sub>/2L<sub>d </sub>and the powergenerator feeding rate κ<sub>g</sub>=R<sub>g</sub>/2L<sub>s</sub>, where R<sub>g </sub>is the internal (characteristic) impedance of the generator.
In general, in various examples, the primary dependent variable that one wants to increase or optimize is the overall efficiency η. However, other important variables need to be taken into consideration upon system design. For example, in examples featuring capacitivelyloaded coils, the design can be constrained by, for example, the currents flowing inside the wires I<sub>s,d </sub>and the voltages across the capacitors V<sub>s,d</sub>. These limitations can be important because for ˜Watt power applications the values for these parameters can be too large for the wires or the capacitors respectively to handle. Furthermore, the total loaded (by the load) quality factor of the device Q<sub>d[l]</sub>=ω/2(Γ<sub>d</sub>+Γ<sub>l</sub>)=ωL<sub>d</sub>/(R<sub>d</sub>+R<sub>d</sub>) and the total loaded (by the generator) quality factor of the source Q<sub>s[g]</sub>=ω/2(Γ<sub>s</sub>+Γ<sub>g</sub>)=ωL<sub>s</sub>/(R<sub>s</sub>+R<sub>g</sub>) are quantities that should be preferably small, because to match the source and device resonant frequencies to within their Q's, when those are very large, can be challenging experimentally and more sensitive to slight variations. Lastly, the radiated powers P<sub>s,rad </sub>and P<sub>d,rad </sub>should be minimized for concerns about farfield interference and safety, even though, in general, for a magnetic, nonradiative scheme they are already typically small. In the following, we examine then the effects of each one of the independent variables on the dependent ones.
We define a new variable wp to express the powerload consumption rate for some particular value of U through U<sub>l</sub>=κ<sub>l</sub>/Γ<sub>d</sub>=√{square root over (1+wp·U)}. Then, in some examples, values which impact the choice of this rate are: U<sub>l</sub>=1<img id="CUSTOMCHARACTER00026" he="2.46mm" wi="3.13mm" file="US20090284083A120091119P00003.TIF" imgcontent="character" imgformat="tif"/>wp=0 to minimize the required energy stored in the source (and therefore I<sub>s </sub>and V<sub>s</sub>), U<sub>l</sub>=√{square root over (1+U<sup>2</sup>)}>1<img id="CUSTOMCHARACTER00027" he="2.46mm" wi="3.13mm" file="US20090284083A120091119P00003.TIF" imgcontent="character" imgformat="tif"/>wp=1 to maximize the efficiency, as seen earlier, or U<sub>l</sub>>>1<img id="CUSTOMCHARACTER00028" he="2.46mm" wi="3.13mm" file="US20090284083A120091119P00003.TIF" imgcontent="character" imgformat="tif"/>wp>>1 to decrease the required energy stored in the device (and therefore I<sub>d </sub>and V<sub>d</sub>) and to decrease or minimize Q<sub>d[l]</sub>. Similar is the impact of the choice of the powergenerator feeding rate U<sub>g</sub>=κ<sub>g</sub>/Γ<sub>s</sub>, with the roles of the source/device and generator/load reversed.
In some examples, increasing N<sub>s </sub>and N<sub>d </sub>increases Q<sub>s </sub>and Q<sub>d</sub>, and thus U and the efficiency significantly, as seen before. It also decreases the currents I<sub>s </sub>and I<sub>d</sub>, because the inductance of the loops increases, and thus the energy W<sub>s,d</sub>=L<sub>s,d</sub>I<sub>s,d</sub><sup>2</sup>/2 required for given output power P<sub>l </sub>can be achieved with smaller currents. However, in some examples, increasing N<sub>d </sub>and thus Q<sub>d </sub>can increase Q<sub>d[l]</sub>, P<sub>d,rad </sub>and the voltage across the device capacitance V<sub>d</sub>. Similar can be the impact of increasing N<sub>s </sub>on Q<sub>s[g]</sub>, P<sub>s,rad </sub>and V<sub>s</sub>. As a conclusion, in some examples, the number of turns N<sub>s </sub>and N<sub>d </sub>should be chosen large enough (for high efficiency) but such that they allow for reasonable voltages, loaded Q's and/or powers radiated.
With respect to the resonant frequency, again, there is an optimal one for efficiency. Close to that optimal frequency Q<sub>d[l]</sub> and/or Q<sub>s[g]</sub> can be approximately maximum. In some examples, for lower frequencies the currents typically get larger but the voltages and radiated powers get smaller. In some examples, one should pick either the frequency that maximizes the efficiency or somewhat lower.
One way to decide on an operating regime for the system is based on a graphical method. Consider two loops of r<sub>s</sub>=25 cm, r<sub>d</sub>=15 cm, h<sub>s</sub>=h<sub>d</sub>=0, a<sub>s</sub>=a<sub>d</sub>=3 mm and distance D=2 m between them. In FIG. 12, we plot some of the above dependent variables (currents, voltages and radiated powers normalized to 1 Watt of output power) in terms of frequency f and N<sub>d</sub>, given some choice for wp and N<sub>s</sub>. FIG. 12 depicts the trend of system performance explained above. In FIG. 13, we make a contour plot of the dependent variables as functions of both frequency and wp but for both N<sub>s </sub>and N<sub>d </sub>fixed. For example, a reasonable choice of parameters for the system of two loops with the dimensions given above are: N<sub>s</sub>=2, N<sub>d</sub>=6, f=10 MHz and wp=10, which gives the following performance characteristics: η=20.6%, Q<sub>d[l]</sub>=1264, I<sub>s</sub>=7.2 A, I<sub>d</sub>=1.4 A, V<sub>s</sub>=2.55 kV, V<sub>d</sub>=2.30 kV, P<sub>s,rad</sub>=0.15 W, P<sub>d,rad</sub>=0.006 W. Note that the results in FIGS. 12, 13 and the just above calculated performance characteristics are made using the analytical formulas provided above, so they are expected to be less accurate for large values of N<sub>s</sub>, N<sub>d</sub>, but still they give a good estimate of the scalings and the orders of magnitude.
Finally, one could additionally optimize for the source dimensions, since usually only the device dimensions are limited, as discussed earlier. Namely, one can add r<sub>s </sub>and a<sub>s </sub>in the set of independent variables and optimize with respect to these too for all the dependent variables of the problem (we saw how to do this only for efficiency earlier). Such an optimization would lead to improved results.
In this description, we propose that, if one ensures operation in the stronglycoupled regime at midrange distances, at least mediumpower transmission (˜W) at midrange distances with high efficiency is possible.
A straight conducting rod of length 2h and crosssectional radius a has distributed capacitance and distributed inductance, and therefore it supports a resonant mode of angular frequency ω. Using the same procedure as in the case of selfresonant coils, one can define an effective total inductance L and an effective total capacitance C of the rod through formulas Eqs. (19) and (20). With these definitions, the resonant angular frequency and the effective impedance are given again by the common formulas ω=1/√{square root over (LC)} and Z=√{square root over (L/C)} respectively. To calculate the total inductance and capacitance, one can assume again a sinusoidal current profile along the length of the conducting wire. When interested in the lowest mode, if we denote by x the coordinate along the conductor, such that it runs from −h to +h, then the current amplitude profile would have the form I(x)=I<sub>o </sub>cos(πx/2h), since it has to be zero at the open ends of the rod. This is the wellknown halfwavelength electric dipole resonant mode.
In some examples, one or more of the resonant objects are inductivelyloaded conducting rods. Referring to FIG. 14, a straight conducting rod of length 2h and crosssectional radius a, as in the previous paragraph, is cut into two equal pieces of length h, which are connected via a coil wrapped around a magnetic material of relative permeability μ, and everything is surrounded by air. The coil has an inductance L<sub>c</sub>, which is added to the distributed inductance of the rod and thus modifies its resonance. Note however, that the presence of the centerloading inductor modifies significantly the current distribution inside the wire and therefore the total effective inductance L and total effective capacitance C of the rod are different respectively from L<sub>s </sub>and C<sub>s</sub>, which are calculated for a selfresonant rod of the same total length using a sinusoidal current profile, as in the previous paragraph. Since some current is running inside the coil of the external loading inductor, the current distribution j inside the rod is reduced, so L<L<sub>s</sub>, and thus, from the charge conservation equation, the linear charge distribution ρ<sub>l </sub>flattens out towards the center (being positive in one side of the rod and negative in the other side of the rod, changing abruptly through the inductor), so C>C<sub>s</sub>. The resonant frequency for this system is ω=1/√{square root over ((L+L<sub>c</sub>)C)}<ω<sub>s</sub>=1/√{square root over (L<sub>s</sub>C<sub>s</sub>)}, and I(x)→I<sub>o </sub>cos(πx/2h) <img id="CUSTOMCHARACTER00029" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/> L→L<sub>s</sub><img id="CUSTOMCHARACTER00030" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/> ω→ω<sub>s</sub>, as L<sub>c</sub>→0.
In general, the desired CMT parameters can be found for this system, but again a very complicated solution of Maxwell's Equations is generally required. In a special case, a reasonable estimate for the current distribution can be made. When L<sub>c</sub>>>L<sub>s</sub>>L, then ω≈1/√{square root over (L<sub>c</sub>C)}<<ω<sub>s </sub>and Z≈√{square root over (L<sub>c</sub>C)}>>Z<sub>s</sub>, while the current distribution is triangular along the rod (with maximum at the centerloading inductor and zero at the ends) and thus the charge distribution is positive constant on one half of the rod and equally negative constant on the other side of the rod. This allows us to compute numerically C from Eq. (20). In this case, the integral in Eq. (20) can actually be computed analytically, giving the formula 1/C=1/(πε<sub>o</sub>h)[ln(h/a)−1]. Explicit analytical formulas are again available for R from Eq. (21) and (22), since I<sub>rms</sub>=I<sub>o</sub>, p=q<sub>o</sub>h and m=0 (namely only the electricdipole term is contributing to radiation), so we can determine also Q<sub>abs</sub>=1/ωCR<sub>abs </sub>and Q<sub>rad</sub>=1/ωCR<sub>rad</sub>. At the end of the calculations, the validity of the assumption of triangular current profile is confirmed by checking that indeed the condition L<sub>c</sub>>>L<sub>s</sub><img id="CUSTOMCHARACTER00031" he="2.46mm" wi="3.13mm" file="US20090284083A120091119P00003.TIF" imgcontent="character" imgformat="tif"/>ω<<ω<sub>s </sub>is satisfied. This condition is relatively easily satisfied, since typically a conducting rod has very small selfinductance L<sub>s </sub>to begin with.
Another important loss factor in this case is the resistive loss inside the coil of the external loading inductor L<sub>c </sub>and it depends on the particular design of the inductor. In some examples, the inductor is made of a Brooks coil, which is the coil geometry which, for fixed wire length, demonstrates the highest inductance and thus quality factor. The Brooks coil geometry has N<sub>Bc </sub>turns of conducting wire of crosssectional radius a<sub>Bc </sub>wrapped around a cylindrically symmetric coil former, which forms a coil with a square crosssection of side r<sub>Bc</sub>, where the inner side of the square is also at radius r<sub>Bc </sub>(and thus the outer side of the square is at radius 2r<sub>Bc</sub>), therefore N<sub>Bc</sub>≈(r<sub>Bc</sub>/2a<sub>Bc</sub>)<sup>2</sup>. The inductance of the coil is then L<sub>c</sub>=2.0285 μ<sub>o</sub>r<sub>Bc</sub>N<sub>Bc</sub><sup>2</sup>≈2.0285 μ<sub>o</sub>r<sub>Bc</sub><sup>5</sup>/8a<sub>Bc</sub><sup>4 </sup>and its resistance
<maths id="MATHUS00033" num="00033"><math overflow="scroll"><mrow><mrow><msub><mi>R</mi><mi>C</mi></msub><mo>≈</mo><mrow><mfrac><mn>1</mn><mi>σ</mi></mfrac><mo></mo><mfrac><msub><mi>l</mi><mi>Bc</mi></msub><mrow><mi>π</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msubsup><mi>a</mi><mi>Bc</mi><mn>2</mn></msubsup></mrow></mfrac><mo></mo><msqrt><mrow><mn>1</mn><mo>+</mo><mrow><mfrac><mrow><msub><mi>μ</mi><mi>o</mi></msub><mo></mo><mi>ωσ</mi></mrow><mn>2</mn></mfrac><mo></mo><msup><mrow><mo>(</mo><mfrac><msub><mi>a</mi><mi>Bc</mi></msub><mn>2</mn></mfrac><mo>)</mo></mrow><mn>2</mn></msup></mrow></mrow></msqrt></mrow></mrow><mo>,</mo></mrow></math></maths>
where the total wire length is l<sub>Bc</sub>≈2π(3r<sub>Bc</sub>/2)N<sub>Bc</sub>≈3πr<sub>Bc</sub><sup>3</sup>/4a<sub>Bc</sub><sup>2 </sup>and we have used an approximate squareroot law for the transition of the resistance from the dc to the ac limit as the skin depth varies with frequency.
The external loading inductance L<sub>c </sub>provides the freedom to tune the resonant frequency. For example, for a Brooks coil with a fixed size r<sub>Bc</sub>, the resonant frequency can be reduced by increasing the number of turns N<sub>Bc </sub>by decreasing the wire crosssectional radius a<sub>Bc</sub>. Then the desired resonant angular frequency ω=1/√{square root over (L<sub>c</sub>C)} is achieved for a<sub>Bc</sub>≈(2.0285 μ<sub>o</sub>r<sub>Bc</sub><sup>5</sup>ω<sup>2</sup>C)<sup>1/4 </sup>and the resulting coil quality factor is Q<sub>c</sub>≈0.169 μ<sub>o</sub>σr<sub>Bc</sub><sup>2</sup>ω/√{square root over (1+ω<sup>2</sup>μ<sub>o</sub>σ√{square root over (2.0285μ<sub>o</sub>(r<sub>Bc</sub>/4)<sup>5</sup>C)})}. Then, for the particular simple case L<sub>c</sub>>>L<sub>s</sub>, for which we have analytical formulas, the total Q=1/ωC(R<sub>c</sub>+R<sub>abs</sub>+R<sub>rad</sub>) becomes highest at some optimal frequency ω<sub>Q</sub>, reaching the value Q<sub>max</sub>, both determined by the loadinginductor specific design. For example, for the Brookscoil procedure described above, at the optimal frequency Q<sub>max</sub>≈Q<sub>c</sub>≈0.8(μ<sub>o</sub>σ<sup>2</sup>r<sub>Bc</sub><sup>3</sup>/C)<sup>1/4</sup>. At lower frequencies it is dominated by ohmic loss inside the inductor coil and at higher frequencies by radiation. Note, again, that the above formulas are accurate as long as ω<sub>Q</sub><<ω<sub>s </sub>and, as explained above, this is easy to design for by using a large inductance.
The results of the above analysis for two examples, using Brooks coils, of subwavelength modes of λ/h≧200 (namely highly suitable for nearfield coupling and well within the quasistatic limit) at the optimal frequency ω<sub>Q </sub>are presented in Table 5.
Table 5 shows in parentheses (for similarity to previous tables) analytical results for the wavelength and absorption, radiation and total loss rates, for two different cases of subwavelengthloop resonant modes. Note that for conducting material copper (σ=5.998·10<sup>7 </sup>S/m) was used. The results show that, in some examples, the optimal frequency is in the lowMHz microwave range and the expected quality factors are Q<sub>abs</sub>≧1000 and Q<sub>rad</sub>≧100000.
<tables id="TABLEUS00005" num="00005"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="6"><colspec colname="1" colwidth="49pt" align="left"/><colspec colname="2" colwidth="35pt" align="center"/><colspec colname="3" colwidth="35pt" align="center"/><colspec colname="4" colwidth="42pt" align="center"/><colspec colname="5" colwidth="28pt" align="center"/><colspec colname="6" colwidth="28pt" align="center"/><thead><row><entry namest="1" nameend="6" rowsep="1">TABLE 5</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row><row><entry>single rod</entry><entry>λ/h</entry><entry>f (MHz)</entry><entry>Q<sub>rad</sub></entry><entry>Q<sub>abs</sub></entry><entry>Q</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row></thead><tbody valign="top"><row><entry/></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="6"><colspec colname="1" colwidth="49pt" align="left"/><colspec colname="2" colwidth="35pt" align="center"/><colspec colname="3" colwidth="35pt" align="char" char="."/><colspec colname="4" colwidth="42pt" align="center"/><colspec colname="5" colwidth="28pt" align="center"/><colspec colname="6" colwidth="28pt" align="center"/><tbody valign="top"><row><entry>h = 30 cm,</entry><entry>(403.8)</entry><entry>(2.477)</entry><entry>(2.72 * 10<sup>6</sup>)</entry><entry>(7400)</entry><entry>(7380)</entry></row><row><entry>a = 2 cm</entry></row><row><entry>μ = 1,</entry></row><row><entry>r<sub>Bc </sub>= 2 cm,</entry></row><row><entry>a<sub>Bc </sub>= 0.88 mm,</entry></row><row><entry>N<sub>Bc </sub>= 129</entry></row><row><entry>h = 10 cm,</entry><entry>(214.2)</entry><entry>(14.010)</entry><entry>(6.92 * 10<sup>5</sup>)</entry><entry>(3908)</entry><entry>(3886)</entry></row><row><entry>a = 2 mm</entry></row><row><entry>μ = 1,</entry></row><row><entry>r<sub>Bc </sub>= 5 mm,</entry></row><row><entry>a<sub>Bc </sub>= 0.25 mm,</entry></row><row><entry>N<sub>Bc </sub>= 103</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row></tbody></tgroup></table></tables>
In some examples, energy is transferred between two inductivelyloaded rods. For the rate of energy transfer between two inductivelyloaded rods 1 and 2 at distance D between their centers, the mutual capacitance M<sub>C </sub>can be evaluated numerically from Eq. (23) by using triangular current distributions in the case ω<<ω<sub>s</sub>. In this case, the coupling is only electric and again we have an analytical formula, which, in the quasistatic limit h<<D<<λ and for the relative orientation such that the two rods are aligned on the same axis, is 1/M<sub>C</sub>≈1/2πε<sub>o</sub>·(h<sub>1</sub>h<sub>2</sub>)<sup>2</sup>/D<sup>3 </sup>which means that k ∝(√{square root over (h<sub>1</sub>h<sub>2</sub>)}/D)<sup>3 </sup>is independent of the frequency ω. One can then get the resultant strongcoupling factor U.
It can be seen that the optimal frequency ω<sub>U</sub>, where the figureofmerit is maximized to the value U<sub>max</sub>, is close to the frequency ω<sub>Q</sub><sub><sub2>1</sub2></sub><sub>Q</sub><sub><sub2>2</sub2></sub>, where Q<sub>1</sub>Q<sub>2 </sub>is maximized, since k does not depend much on frequency (at least for the distances D<<λ of interest for which the quasistatic approximation is still valid). Therefore, the optimal frequency ω<sub>U</sub>≈ω<sub>Q</sub><sub><sub2>1</sub2></sub><sub>Q</sub><sub><sub2>2 </sub2></sub>is mostly independent of the distance D between the two rods and lies between the two frequencies ω<sub>Q</sub><sub><sub2>1 </sub2></sub>and ω<sub>Q</sub><sub><sub2>2 </sub2></sub>at which the singlerod Q<sub>1 </sub>and Q<sub>2 </sub>respectively peak. In some typical examples, one can tune the inductivelyloaded conducting rods, so that their angular eigenfrequencies are close to ω<sub>U </sub>within Γ<sub>U</sub>, which is half the angular frequency width for which U>U<sub>max</sub>/2.
Referring to Table 6, in parentheses (for similarity to previous tables) analytical results based on the above are shown for two systems each composed of a matched pair of the loaded rods described in Table 5. The average wavelength and loss rates are shown along with the coupling rate and coupling to loss ratio figureofmerit U=κ/Γ as a function of the coupling distance D, for the two cases. Note that for Γ<sub>rad </sub>the singlerod value is used. Again we chose L<sub>c</sub>>>L<sub>s </sub>to make the triangularcurrent assumption a good one and computed M<sub>C </sub>numerically from Eq. (23). The results show that for medium distances D/h=10−3 the expected couplingtoloss ratios are in the range U˜0.5100.
<tables id="TABLEUS00006" num="00006"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="4"><colspec colname="1" colwidth="105pt" align="left"/><colspec colname="2" colwidth="28pt" align="center"/><colspec colname="3" colwidth="42pt" align="center"/><colspec colname="4" colwidth="42pt" align="center"/><thead><row><entry namest="1" nameend="4" rowsep="1">TABLE 6</entry></row><row><entry namest="1" nameend="4" align="center" rowsep="1"/></row><row><entry>pair of rods</entry><entry>D/h</entry><entry>Q<sub>κ </sub>= 1/k</entry><entry>U</entry></row><row><entry namest="1" nameend="4" align="center" rowsep="1"/></row></thead><tbody valign="top"><row><entry/></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="4"><colspec colname="1" colwidth="105pt" align="left"/><colspec colname="2" colwidth="28pt" align="char" char="."/><colspec colname="3" colwidth="42pt" align="center"/><colspec colname="4" colwidth="42pt" align="char" char="."/><tbody valign="top"><row><entry>h = 30 cm, a = 2 cm</entry><entry>3</entry><entry> (70.3)</entry><entry>(105.0)</entry></row><row><entry>μ = 1, r<sub>Bc </sub>= 2 cm, a<sub>Bc </sub>= 0.88 mm,</entry><entry>5</entry><entry> (389)</entry><entry>(19.0)</entry></row><row><entry>N<sub>Bc </sub>= 129</entry></row><row><entry>λ/h ≈ 404</entry><entry>7</entry><entry>(1115)</entry><entry>(6.62)</entry></row><row><entry>Q ≈ 7380</entry><entry>10</entry><entry>(3321)</entry><entry>(2.22)</entry></row><row><entry>h = 10 cm, a = 2 mm</entry><entry>3</entry><entry> (120)</entry><entry>(32.4)</entry></row><row><entry>μ = 1, r<sub>Bc </sub>= 5 mm, a<sub>Bc </sub>= 0.25 mm,</entry><entry>5</entry><entry> (664)</entry><entry>(5.85)</entry></row><row><entry>N<sub>Bc </sub>= 103</entry></row><row><entry>λ/h ≈ 214</entry><entry>7</entry><entry>(1900)</entry><entry>(2.05)</entry></row><row><entry>Q ≈ 3886</entry><entry>10</entry><entry>(5656)</entry><entry>(0.69)</entry></row><row><entry namest="1" nameend="4" align="center" rowsep="1"/></row></tbody></tgroup></table></tables>
In some examples, one or more of the resonant objects are dielectric objects, such as disks. Consider a two dimensional dielectric disk object, as shown in FIG. 15(a), of radius r and relative permittivity c surrounded by air that supports highQ “whisperinggallery” resonant modes. The loss mechanisms for the energy stored inside such a resonant system are radiation into free space and absorption inside the disk material. HighQ<sub>rad </sub>and longtailed subwavelength resonances can be achieved when the dielectric permittivity ε is large and the azimuthal field variations are slow (namely of small principal number m). Material absorption is related to the material loss tangent: Q<sub>abs</sub>˜Re{ε}/Im {ε}. Modesolving calculations for this type of disk resonances were performed using two independent methods: numerically, 2D finitedifference frequencydomain (FDFD) simulations (which solve Maxwell's Equations in frequency domain exactly apart for spatial discretization) were conducted with a resolution of 30 pts/r; analytically, standard separation of variables (SV) in polar coordinates was used.
<tables id="TABLEUS00007" num="00007"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="5"><colspec colname="1" colwidth="35pt" align="left"/><colspec colname="2" colwidth="49pt" align="left"/><colspec colname="3" colwidth="49pt" align="left"/><colspec colname="4" colwidth="42pt" align="left"/><colspec colname="5" colwidth="42pt" align="left"/><thead><row><entry namest="1" nameend="5" rowsep="1">TABLE 7</entry></row><row><entry namest="1" nameend="5" align="center" rowsep="1"/></row><row><entry>single disk</entry><entry>λ/r</entry><entry>Q<sup>abs</sup></entry><entry>Q<sup>rad</sup></entry><entry>Q</entry></row><row><entry namest="1" nameend="5" align="center" rowsep="1"/></row></thead><tbody valign="top"><row><entry>Re{ε} =</entry><entry>20.01 (20.00)</entry><entry>10103 (10075)</entry><entry>1988 (1992)</entry><entry>1661 (1663)</entry></row><row><entry>147.7,</entry></row><row><entry>m = 2</entry></row><row><entry>Re{ε} =</entry><entry>9.952 (9.950)</entry><entry>10098 (10087)</entry><entry>9078 (9168)</entry><entry>4780 (4802)</entry></row><row><entry>65.6,</entry></row><row><entry>m = 3</entry></row><row><entry namest="1" nameend="5" align="center" rowsep="1"/></row></tbody></tgroup></table></tables>
The results for two TEpolarized dielectricdisk subwavelength modes of λ/r≧10 are presented in Table 7. Table 7 shows numerical FDFD (and in parentheses analytical SV) results for the wavelength and absorption, radiation and total loss rates, for two different cases of subwavelengthdisk resonant modes. Note that diskmaterial losstangent Im{ε}/Re{ε}=10<sup>−4 </sup>was used. (The specific parameters corresponding to the plot in FIG. 15(a) are highlighted with bold in the table.) The two methods have excellent agreement and imply that for a properly designed resonant lowlossdielectric object values of Q<sub>rad</sub>≧2000 and Q<sub>abs</sub>˜10000 are achievable. Note that for the 3D case the computational complexity would be immensely increased, while the physics would not be significantly different. For example, a spherical object of ε=147. 7 has a whispering gallery mode with m=2, Q<sub>rad</sub>=13962, and λ/r=17.
The required values of ε, shown in Table 7, might at first seem unrealistically large. However, not only are there in the microwave regime (appropriate for approximately meterrange coupling applications) many materials that have both reasonably high enough dielectric constants and low losses (e.g. Titania, Barium tetratitanate, Lithium tantalite etc.), but also E could signify instead the effective index of other known subwavelength surfacewave systems, such as surface modes on surfaces of metallic materials or plasmonic (metallike, negativeε) materialsor metallodielectric photonic crystals or plasmonodielectric photonic crystals.
To calculate now the achievable rate of energy transfer between two disks 1 and 2, as shown in FIG. 15(b) we place them at distance D between their centers. Numerically, the FDFD modesolver simulations give κ through the frequency splitting of the normal modes of the combined system (δ<sub>E</sub>=2κ from Eq. (4)), which are even and odd superpositions of the initial singledisk modes; analytically, using the expressions for the separationofvariables eigenfields E<sub>1,2</sub>(r) CMT gives κ through
<FORM>κ=ω<sub>1</sub>/2·∫d<sup>3</sup>rε<sub>2</sub>(r)E<sub>2</sub>*(r)E<sub>1</sub>(r)/∫d<sup>3</sup>rε(r)E<sub>1</sub>(r)<sup>2</sup>,</FORM>
where ε<sub>j </sub>(r) and ε(r) are the dielectric functions that describe only the disk j (minus the constant ε<sub>o </sub>background) and the whole space respectively. Then, for medium distances D/r=10−3 and for nonradiative coupling such that D<2r<sub>c</sub>, where r<sub>c</sub>=mλ/2π is the radius of the radiation caustic, the two methods agree very well, and we finally find, as shown in Table 8, strongcoupling factors in the range U˜150. Thus, for the analyzed examples, the achieved figureofmerit values are large enough to be useful for typical applications, as discussed below.
<tables id="TABLEUS00008" num="00008"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="6"><colspec colname="1" colwidth="49pt" align="left"/><colspec colname="2" colwidth="21pt" align="center"/><colspec colname="3" colwidth="21pt" align="center"/><colspec colname="4" colwidth="35pt" align="center"/><colspec colname="5" colwidth="49pt" align="center"/><colspec colname="6" colwidth="42pt" align="center"/><thead><row><entry namest="1" nameend="6" rowsep="1">TABLE 8</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row><row><entry>two disks</entry><entry>D/r</entry><entry>Q<sup>rad</sup></entry><entry>Q = ω/2Γ</entry><entry>ω/2κ</entry><entry>κ/Γ</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row></thead><tbody valign="top"><row><entry/></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="6"><colspec colname="1" colwidth="49pt" align="left"/><colspec colname="2" colwidth="21pt" align="char" char="."/><colspec colname="3" colwidth="21pt" align="center"/><colspec colname="4" colwidth="35pt" align="center"/><colspec colname="5" colwidth="49pt" align="center"/><colspec colname="6" colwidth="42pt" align="center"/><tbody valign="top"><row><entry>Re{ε} = 147.7,</entry><entry>3</entry><entry>2478</entry><entry>1989</entry><entry>46.9 (47.5)</entry><entry>42.4 (35.0)</entry></row><row><entry>m = 2</entry><entry>5</entry><entry>2411</entry><entry>1946</entry><entry>298.0 (298.0)</entry><entry>6.5 (5.6)</entry></row><row><entry>λ/r ≈ 20</entry><entry>7</entry><entry>2196</entry><entry>1804</entry><entry>769.7 (770.2)</entry><entry>2.3 (2.2)</entry></row><row><entry>Q<sup>abs </sup>≈ 10093</entry><entry>10</entry><entry>2017</entry><entry>1681</entry><entry>1714 (1601)</entry><entry>0.98 (1.04)</entry></row><row><entry>Re{ε} = 65.6,</entry><entry>3</entry><entry>7972</entry><entry>4455</entry><entry>144 (140)</entry><entry>30.9 (34.3)</entry></row><row><entry>m = 3</entry><entry>5</entry><entry>9240</entry><entry>4824</entry><entry>2242 (2083)</entry><entry>2.2 (2.3)</entry></row><row><entry>λ/r ≈ 10</entry><entry>7</entry><entry>9187</entry><entry>4810</entry><entry>7485 (7417)</entry><entry>0.64 (0.65)</entry></row><row><entry>Q<sup>abs </sup>≈ 10096</entry></row><row><entry namest="1" nameend="6" align="center" rowsep="1"/></row></tbody></tgroup></table></tables>
Note that even though particular examples are presented and analyzed above as examples of systems that use resonant electromagnetic coupling for wireless energy transfer, those of selfresonant conducting coils, capacitivelyloaded resonant conducting coils, inductivelyloaded resonant conducting rods and resonant dielectric disks, any system that supports an electromagnetic mode with its electromagnetic energy extending much further than its size can be used for transferring energy. For example, there can be many abstract geometries with distributed capacitances and inductances that support the desired kind of resonances. In some examples, the resonant structure can be a dielectric sphere. In any one of these geometries, one can choose certain parameters to increase and/or optimize U or, if the Q's are limited by external factors, to increase and/or optimize for k or, if other system performance parameters are of importance, to optimize those.
The two objects in an energytransfer system generate radiation, which can sometimes be a significant part of the intrinsic losses, and can interfere in the far field. In the previous Sections, we analyzed systems, where this interference phenomenon was not in effect. In this description, we will repeat the analysis, including the interference effects and will show how it can be used to further enhance the power transmission efficiency and/or the radiated power.
The coupledmode equations of Eqs. (1) fail to predict such an interference phenomenon. In fact, the inability to predict interference phenomena has often been considered inherent to coupledmode theory (CMT). However, we show here that making only a simple extension to this model, it can actually very successfully predict such interference. The root of the problem stems from the fact that the coupling coefficients were tacitly assumed to be real. This is usually the case when dealing with proper (real) eigenmodes of a Hermitian (lossless) operator. However, this assumption fails when losses are included, as is for example the current case dealing with generally nonproper (leaky, radiative) eigenmodes of a nonHermitian (lossy) operator. In this case, the couplingmatrix elements will generally be complex and their imaginary parts will be shown to be directly related to farfield radiation interference.
Imagine a system of many resonators in proximity to each other. When their resonances have close enough frequencies compared to their coupling rates, the CMT assumption is that the totalsystem field ψ is approximately determined only by these resonances as the superposition ψ(t)=Σ<sub>n</sub>a<sub>n</sub>(t)ψ<sub>n</sub>, where ψ<sub>n </sub>is the eigenfield of the resonance n normalized to unity energy, and a<sub>n </sub>is the field amplitude inside it, which corresponds, due to the normalization, to a<sub>n</sub><sup>2 </sup>stored energy. The fundamental CoupledMode Equations (CME) of CMT are then those of the evolution of the vector a={a<sub>n</sub>}
<maths id="MATHUS00034" num="00034"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mfrac><mo></mo><mrow><mo></mo><mi>t</mi></mrow></mfrac><mo></mo><mi>a</mi></mrow><mo>=</mo><mrow><mrow><mrow><mo></mo><mi></mi></mrow><mo></mo><mrow><mover><mi>Ω</mi><mi>_</mi></mover><mo>·</mo><mi>a</mi></mrow></mrow><mo>+</mo><mrow><mi></mi><mo></mo><mrow><mover><mi>K</mi><mi>_</mi></mover><mo>·</mo><mi>a</mi></mrow></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>34</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where the frequency matrix <o ostyle="single">Ω</o> and the coupling matrix <o ostyle="single">K</o> are found usually using a Perturbation Theory (PT) approach.
We restate here one of the many perturbative formulations of CMT in a system of ElectroMagnetic (EM) resonators: Let μ=μ<sub>o </sub>and ε=ε<sub>o</sub>+Σ<sub>n</sub>ε<sub>n </sub>be the magneticpermeability and dielectricpermittivity functions of space that describe the whole system, where ε<sub>n </sub>is the permittivity of only the dielectric, reciprocal and generally anisotropic object n of volume V<sub>n</sub>, in excess to the constant μ<sub>o</sub>, ε<sub>o </sub>background space. Each resonator n, when alone in the background space, supports a resonant eigenmode of complex frequency Ω<sub>n</sub>=ω<sub>n</sub>−iΓ<sub>n </sub>and field profiles ψ<sub>n</sub>=[E<sub>n</sub>, H<sub>n</sub>] normalized to unity energy, satisfying the equations ∇×E<sub>n</sub>=iΩ<sub>n</sub>μ<sub>o</sub>H<sub>n </sub>and ∇×H<sub>n</sub>=−iΩ<sub>n</sub>(ε<sub>o</sub>+ε<sub>n</sub>)E<sub>n</sub>, and the boundary condition {circumflex over (n)}×E<sub>n</sub>=0 on the potential metallic surface S<sub>n </sub>of object n. The whole system fields ψ=[E, H] satisfy the equations
<maths id="MATHUS00035" num="00035"><math overflow="scroll"><mrow><mrow><mrow><mo>∇</mo><mrow><mo>×</mo><mi>E</mi></mrow></mrow><mo>=</mo><mrow><mrow><mrow><mo></mo><mi>μ</mi></mrow><mo></mo><mfrac><mo>∂</mo><mrow><mo>∂</mo><mi>t</mi></mrow></mfrac><mo></mo><mi>H</mi><mo></mo><mstyle><mspace width="0.8em" height="0.8ex"/></mstyle><mo></mo><mi>and</mi><mo></mo><mstyle><mspace width="0.8em" height="0.8ex"/></mstyle><mo></mo><mrow><mo>∇</mo><mrow><mo>×</mo><mi>H</mi></mrow></mrow></mrow><mo>=</mo><mrow><mi>ε</mi><mo></mo><mfrac><mo>∂</mo><mrow><mo>∂</mo><mi>t</mi></mrow></mfrac><mo></mo><mi>E</mi></mrow></mrow></mrow><mo>,</mo></mrow></math></maths>
and the boundary condition {circumflex over (n)}×E=0 on S=Σ<sub>n</sub>S<sub>n</sub>. Then, start by expanding ∇·(E×H<sub>n</sub><sup>−</sup>−E<sub>n</sub><sup>−</sup>×H) and integrating over all space, apply the CMT superposition assumption, and finally use the PT argument that, when the couplingrates between the resonators are small compared to their frequencies, within a sum only terms of lowest order in this small perturbation need to be kept. The result is the CME of Eq. (34), with <o ostyle="single">Ω</o>= <o ostyle="single">W</o><sup>−1</sup>· <o ostyle="single">Ω</o><sub>o</sub>· <o ostyle="single">W</o>, <o ostyle="single">K</o>= <o ostyle="single">W</o><sup>−1</sup>·K<sub>o</sub>, where <o ostyle="single">Ω</o><sub>o</sub>=diag{Ω<sub>n</sub>},
<maths id="MATHUS00036" num="00036"><math overflow="scroll"><mtable><mtr><mtd><mrow><msub><mi>K</mi><mrow><mi>o</mi><mo>,</mo><mi>nm</mi></mrow></msub><mo>=</mo><mrow><mrow><mfrac><msub><mi>Ω</mi><mi>n</mi></msub><mn>4</mn></mfrac><mo></mo><mrow><msub><mo>∫</mo><msub><mi>V</mi><mi>m</mi></msub></msub><mo></mo><mrow><mo></mo><mrow><mi>v</mi><mo></mo><mrow><mo>(</mo><mrow><msubsup><mi>E</mi><mi>n</mi><mo></mo></msubsup><mo>·</mo><msub><mi>ɛ</mi><mi>m</mi></msub><mo>·</mo><msub><mi>E</mi><mi>m</mi></msub></mrow><mo>)</mo></mrow></mrow></mrow></mrow></mrow><mo>+</mo><mrow><mfrac><mi></mi><mn>4</mn></mfrac><mo></mo><mrow><munder><mo>∮</mo><msub><mi>S</mi><mi>m</mi></msub></munder><mo></mo><mrow><mrow><mo></mo><mi>a</mi></mrow><mo></mo><mrow><mover><mi>n</mi><mo>^</mo></mover><mo>·</mo><mrow><mo>(</mo><mrow><msubsup><mi>E</mi><mi>n</mi><mo></mo></msubsup><mo>×</mo><msub><mi>H</mi><mi>m</mi></msub></mrow><mo>)</mo></mrow></mrow></mrow></mrow></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>35</mn><mo>)</mo></mrow></mtd></mtr><mtr><mtd><mrow><msub><mi>W</mi><mi>nm</mi></msub><mo>=</mo><mrow><mfrac><mn>1</mn><mn>4</mn></mfrac><mo></mo><mrow><msub><mo>∫</mo><msub><mi>V</mi><mi>m</mi></msub></msub><mo></mo><mrow><mo></mo><mrow><mi>v</mi><mo></mo><mrow><mo>(</mo><mrow><mrow><msubsup><mi>E</mi><mi>n</mi><mo></mo></msubsup><mo>·</mo><mi>ɛ</mi><mo>·</mo><msub><mi>E</mi><mi>m</mi></msub></mrow><mo>+</mo><mrow><msubsup><mi>H</mi><mi>n</mi><mo></mo></msubsup><mo>·</mo><mi>μ</mi><mo>·</mo><msub><mi>H</mi><mi>m</mi></msub></mrow></mrow><mo>)</mo></mrow></mrow></mrow></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>36</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
and where ψ<sup>n</sup>=[E<sub>n</sub><sup>−</sup>, H<sub>n</sub><sup>−</sup>] satisfy the timereversed equations (where Ω<sub>n</sub>→−Ωn). The choice of these fields in the analysis rather than ψ<sub>n</sub>*=[E<sub>n</sub>*, H<sub>n</sub>*] allows to treat also lossy (due to absorption and/or radiation) but reciprocal systems (so <o ostyle="single">K</o> is complex symmetric but nonHermitian). In the limit, though, of weak loss (highQ resonances), these two sets of fields can be approximately equal. Therefore, again to lowest order, <o ostyle="single">W</o>≈Ī, due to the unityenergy normalization, so <o ostyle="single">Ω</o>≈ <o ostyle="single">Ω</o><sub>o </sub>and for <o ostyle="single">K</o> the offdiagonal terms
<maths id="MATHUS00037" num="00037"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>K</mi><mi>nm</mi></msub><mo>≈</mo><msub><mi>K</mi><mrow><mi>o</mi><mo>,</mo><mi>mn</mi></mrow></msub><mo>≈</mo><mrow><mfrac><mi></mi><mn>4</mn></mfrac><mo></mo><mrow><msub><mo>∫</mo><msub><mi>V</mi><mi>m</mi></msub></msub><mo></mo><mrow><mo></mo><mrow><mi>v</mi><mo></mo><mrow><mo>(</mo><mrow><msubsup><mi>E</mi><mi>n</mi><mo>*</mo></msubsup><mo>·</mo><msub><mi>J</mi><mi>m</mi></msub></mrow><mo>)</mo></mrow></mrow></mrow></mrow></mrow></mrow><mo>;</mo><mrow><mi>n</mi><mo>≠</mo><mi>m</mi></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>37</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where J<sub>m </sub>includes both the volumepolarization currents J<sub>p,m</sub>=−iΩ<sub>m</sub>ε<sub>m</sub>E<sub>m </sub>in V<sub>m </sub>and the surface electric currents J<sub>s,m</sub>={circumflex over (n)}×H<sub>m </sub>on S<sub>m</sub>, while the diagonal terms K<sub>nn </sub>are higherorder small and can often lead to anomalous couplinginduced frequency shifts. The term of Eq. (37) can generally be complex K<sub>nm</sub>=κ<sub>nm</sub>+iΛ<sub>nm </sub>and, even though the physical interpretation of its real part is well understood, as describing the coupling between the resonators, it is not so the case for its imaginary part
<maths id="MATHUS00038" num="00038"><math overflow="scroll"><mtable><mtr><mtd><mtable><mtr><mtd><mrow><msub><mi>Λ</mi><mi>nm</mi></msub><mo>=</mo><mi/><mo></mo><mrow><mfrac><mn>1</mn><mn>4</mn></mfrac><mo></mo><mi>Re</mi><mo></mo><mrow><mo>{</mo><mrow><msub><mo>∫</mo><msub><mi>V</mi><mi>m</mi></msub></msub><mo></mo><mrow><mrow><mo></mo><msup><mrow><mi>v</mi><mo></mo><mrow><mo>[</mo><mrow><mrow><mi>ω</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>A</mi><mi>n</mi></msub></mrow><mo></mo><mrow><mo>∇</mo><msub><mi>φ</mi><mi>n</mi></msub></mrow></mrow><mo>]</mo></mrow></mrow><mo>*</mo></msup></mrow><mo>·</mo><msub><mi>J</mi><mi>m</mi></msub></mrow></mrow><mo>}</mo></mrow></mrow></mrow></mtd></mtr><mtr><mtd><mrow><mo>=</mo><mi/><mo></mo><mrow><mfrac><mn>1</mn><mn>4</mn></mfrac><mo></mo><mi>Re</mi><mo></mo><mrow><mo>{</mo><mrow><msub><mo>∫</mo><msub><mi>V</mi><mi>m</mi></msub></msub><mo></mo><mrow><mrow><mo></mo><msup><mrow><mi>v</mi><mo></mo><mrow><mo>[</mo><mrow><msub><mo>∫</mo><msub><mi>V</mi><mi>n</mi></msub></msub><mo></mo><mrow><mrow><mo></mo><mi>v</mi></mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mfrac><msup><mi></mi><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>k</mi><mo></mo><mrow><mo></mo><mrow><msub><mi>r</mi><mi>m</mi></msub><mo></mo><msub><mi>r</mi><mi>n</mi></msub></mrow><mo></mo></mrow></mrow></msup><mrow><mn>4</mn><mo></mo><mi>π</mi><mo></mo><mrow><mo></mo><mrow><msub><mi>r</mi><mi>m</mi></msub><mo></mo><msub><mi>r</mi><mi>n</mi></msub></mrow><mo></mo></mrow></mrow></mfrac><mo></mo><mrow><mo>(</mo><mrow><mrow><msub><mi>ωμ</mi><mn>0</mn></msub><mo></mo><msub><mi>J</mi><mi>n</mi></msub></mrow><mo>+</mo><mrow><mfrac><msub><mi>ρ</mi><mi>n</mi></msub><msub><mi>ɛ</mi><mn>0</mn></msub></mfrac><mo>∇</mo></mrow></mrow><mo>)</mo></mrow></mrow></mrow><mo>]</mo></mrow></mrow><mo>*</mo></msup></mrow><mo>·</mo><msub><mi>J</mi><mi>m</mi></msub></mrow></mrow></mrow></mrow></mrow></mtd></mtr><mtr><mtd><mrow><mo>=</mo><mi/><mo></mo><mrow><mfrac><mi>ω</mi><mrow><mn>16</mn><mo></mo><mi>π</mi></mrow></mfrac><mo></mo><mrow><msub><mo>∫</mo><msub><mi>V</mi><mi>m</mi></msub></msub><mo></mo><mrow><mrow><mo></mo><mi>v</mi></mrow><mo></mo><mrow><msub><mo>∫</mo><msub><mi>V</mi><mi>n</mi></msub></msub><mo></mo><mrow><mrow><mo></mo><mi>v</mi></mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>Re</mi><mo></mo><mrow><mo>{</mo><mrow><mrow><mo>(</mo><mrow><mfrac><mrow><msubsup><mi>ρ</mi><mi>n</mi><mo>*</mo></msubsup><mo></mo><msub><mi>ρ</mi><mi>m</mi></msub></mrow><msub><mi>ɛ</mi><mn>0</mn></msub></mfrac><mo></mo><mrow><msub><mi>μ</mi><mn>0</mn></msub><mo></mo><mrow><msubsup><mi>J</mi><mi>n</mi><mo>*</mo></msubsup><mo>·</mo><msub><mi>J</mi><mi>m</mi></msub></mrow></mrow></mrow><mo>)</mo></mrow><mo></mo><mfrac><msup><mi></mi><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>k</mi><mo></mo><mrow><mo></mo><mrow><msub><mi>r</mi><mi>m</mi></msub><mo></mo><msub><mi>r</mi><mi>n</mi></msub></mrow><mo></mo></mrow></mrow></msup><mrow><mn>4</mn><mo></mo><mi>π</mi><mo></mo><mrow><mo></mo><mrow><msub><mi>r</mi><mi>m</mi></msub><mo></mo><msub><mi>r</mi><mi>n</mi></msub></mrow><mo></mo></mrow></mrow></mfrac></mrow><mo>}</mo></mrow></mrow></mrow></mrow></mrow></mrow></mrow></mtd></mtr></mtable></mtd><mtd><mrow><mo>(</mo><mn>38</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where integration by parts was used for the ∇φ<sub>n </sub>term and the continuity equation ∇·J=iωρ, with ρ being the volume charge density.
Towards understanding this term, let us consider two resonators 1, 2 and evaluate from Eqs. (34) the total power lost from the system
<maths id="MATHUS00039" num="00039"><math overflow="scroll"><mtable><mtr><mtd><mtable><mtr><mtd><mrow><msub><mi>P</mi><mi>loss</mi></msub><mo>=</mo><mi/><mo></mo><mrow><mrow><mo></mo><mfrac><mo></mo><mrow><mo></mo><mi>t</mi></mrow></mfrac></mrow><mo></mo><mrow><mo>(</mo><mrow><msup><mrow><mo></mo><msub><mi>a</mi><mn>1</mn></msub><mo></mo></mrow><mn>2</mn></msup><mo>+</mo><msup><mrow><mo></mo><msub><mi>a</mi><mn>2</mn></msub><mo></mo></mrow><mn>2</mn></msup></mrow><mo>)</mo></mrow></mrow></mrow></mtd></mtr><mtr><mtd><mrow><mo>=</mo><mi/><mo></mo><mrow><mrow><mn>2</mn><mo></mo><msub><mi>Γ</mi><mn>1</mn></msub><mo></mo><msup><mrow><mo></mo><msub><mi>a</mi><mn>1</mn></msub><mo></mo></mrow><mn>2</mn></msup></mrow><mo>+</mo><mrow><mn>2</mn><mo></mo><msub><mi>Γ</mi><mn>2</mn></msub><mo></mo><msup><mrow><mo></mo><msub><mi>a</mi><mn>2</mn></msub><mo></mo></mrow><mn>2</mn></msup></mrow><mo>+</mo><mrow><mn>4</mn><mo></mo><msub><mi>Λ</mi><mn>12</mn></msub><mo></mo><mi>Re</mi><mo></mo><mrow><mo>{</mo><mrow><msubsup><mi>a</mi><mn>1</mn><mo>*</mo></msubsup><mo></mo><msub><mi>a</mi><mn>2</mn></msub></mrow><mo>}</mo></mrow></mrow></mrow></mrow></mtd></mtr></mtable></mtd><mtd><mrow><mo>(</mo><mn>39</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
Clearly, the term involving an interaction between the two objects should not relate to material absorption, since this is a very localized process inside each object. We therefore split this lost power into absorbed and radiated in the following way
<FORM>P<sub>abs</sub>=2Γ<sub>1,abs</sub>a<sub>1</sub><sup>2</sup>+2Γ<sub>2,abs</sub>a<sub>2</sub><sup>2 </sup> (40)</FORM>
<FORM>P<sub>rad</sub>=2Γ<sub>1,rad</sub>a<sub>1</sub><sup>2</sup>+2Γ<sub>2,rad</sub>a<sub>2</sub><sup>2</sup>+4Λ<sub>12</sub>Re{a<sub>1</sub>*a<sub>2</sub>} (41)</FORM>
so Λ<sub>12 </sub>is associated with the radiation from the twoobject system. However, we have a tool to compute this radiated power separately: Antenna Theory (AT).
Let ζ<sub>o</sub>=√{square root over (μ<sub>o</sub>/ε<sub>o</sub>)} and c<sub>o</sub>=1/√{square root over (μ<sub>o</sub>ε<sub>o</sub>)} be the background impedance and lightvelocity, and f=(g,f)=∫<sub>V </sub>dv′J<sup>v</sup>(r′)e<sup>−ik·r′</sup> the moment of the currentdistribution 4vector J<sup>v</sup>=(c<sub>o</sub>ρ,J) of an electromagnetic resonator, where unityenergy normalization is again assumed for J<sup>v </sup>and g={circumflex over (k)}·f, as can be shown using the continuity equation and integration by parts. The power radiated from one EM resonator is:
<maths id="MATHUS00040" num="00040"><math overflow="scroll"><mtable><mtr><mtd><mrow><msub><mi>P</mi><mi>rad</mi></msub><mo>=</mo><mrow><mrow><mn>2</mn><mo></mo><msub><mi>Γ</mi><mi>rad</mi></msub><mo></mo><msup><mrow><mo></mo><mi>a</mi><mo></mo></mrow><mn>2</mn></msup></mrow><mo>=</mo><mrow><mfrac><mrow><msub><mi>ς</mi><mi>o</mi></msub><mo></mo><msup><mi>k</mi><mn>2</mn></msup></mrow><mrow><mn>32</mn><mo></mo><msup><mi>π</mi><mn>2</mn></msup></mrow></mfrac><mo></mo><mrow><mo>(</mo><mrow><mo>∮</mo><mrow><mrow><mo></mo><mi>Ω</mi></mrow><mo></mo><msup><mrow><mo></mo><mi>f</mi><mo></mo></mrow><mn>2</mn></msup></mrow></mrow><mo>)</mo></mrow><mo></mo><msup><mrow><mo></mo><mi>a</mi><mo></mo></mrow><mn>2</mn></msup></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>42</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where f<sup>2</sup>=f*·f≡f<sup>2</sup>−g<sup>2</sup>. The power radiated from an ‘array’ of two resonators 1 and 2, at vectordistance D between their centers, is given by:
<maths id="MATHUS00041" num="00041"><math overflow="scroll"><mtable><mtr><mtd><mtable><mtr><mtd><mrow><msub><mi>P</mi><mi>rad</mi></msub><mo>=</mo><mi/><mo></mo><mrow><mfrac><mrow><msub><mi>ς</mi><mi>o</mi></msub><mo></mo><msup><mi>k</mi><mn>2</mn></msup></mrow><mrow><mn>32</mn><mo></mo><msup><mi>π</mi><mn>2</mn></msup></mrow></mfrac><mo></mo><mrow><mo>∮</mo><mrow><mrow><mo></mo><mi>Ω</mi></mrow><mo></mo><msup><mrow><mo></mo><mrow><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><msub><mi>f</mi><mn>1</mn></msub></mrow><mo>+</mo><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><msub><mi>f</mi><mn>2</mn></msub><mo></mo><msup><mi></mi><mrow><mrow><mo></mo><mi></mi></mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><mi>k</mi><mo>·</mo><mi>D</mi></mrow></mrow></msup></mrow></mrow><mo></mo></mrow><mn>2</mn></msup></mrow></mrow></mrow></mrow></mtd></mtr><mtr><mtd><mrow><mo>=</mo><mi/><mo></mo><mrow><mfrac><mrow><msub><mi>ς</mi><mi>o</mi></msub><mo></mo><msup><mi>k</mi><mn>2</mn></msup></mrow><mrow><mn>32</mn><mo></mo><msup><mi>π</mi><mn>2</mn></msup></mrow></mfrac><mo></mo><mrow><mo>[</mo><mtable><mtr><mtd><mrow><mrow><mrow><mo>(</mo><mrow><mo>∮</mo><mrow><mrow><mo></mo><mi>Ω</mi></mrow><mo></mo><msup><mrow><mo></mo><msub><mi>f</mi><mn>1</mn></msub><mo></mo></mrow><mn>2</mn></msup></mrow></mrow><mo>)</mo></mrow><mo></mo><msup><mrow><mo></mo><msub><mi>a</mi><mn>1</mn></msub><mo></mo></mrow><mn>2</mn></msup></mrow><mo>+</mo><mrow><mrow><mo>(</mo><mrow><mo>∮</mo><mrow><mrow><mo></mo><mi>Ω</mi></mrow><mo></mo><msup><mrow><mo></mo><msub><mi>f</mi><mn>2</mn></msub><mo></mo></mrow><mn>2</mn></msup></mrow></mrow><mo>)</mo></mrow><mo></mo><msup><mrow><mo></mo><msub><mi>a</mi><mn>2</mn></msub><mo></mo></mrow><mn>2</mn></msup></mrow><mo>+</mo></mrow></mtd></mtr><mtr><mtd><mrow><mn>2</mn><mo></mo><mi>Re</mi><mo></mo><mrow><mo>{</mo><mrow><mo>∮</mo><mrow><mrow><mo></mo><mi>Ω</mi></mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><msubsup><mi>f</mi><mn>1</mn><mo>*</mo></msubsup><mo>·</mo><msub><mi>f</mi><mn>2</mn></msub></mrow><mo></mo><msup><mi></mi><mrow><mrow><mo></mo><mi></mi></mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><mi>k</mi><mo>·</mo><mi>D</mi></mrow></mrow></msup><mo></mo><msubsup><mi>a</mi><mn>1</mn><mo>*</mo></msubsup><mo></mo><msub><mi>a</mi><mn>2</mn></msub></mrow></mrow><mo>}</mo></mrow></mrow></mtd></mtr></mtable><mo>]</mo></mrow></mrow></mrow></mtd></mtr></mtable></mtd><mtd><mrow><mo>(</mo><mn>43</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where f<sub>1</sub>*·f<sub>2</sub>≡f<sub>1</sub>*·f<sub>2</sub>−g<sub>1</sub>*·g<sub>2</sub>. Thus, by comparing Eqs. (41) and (43), using Eq. (42),
<maths id="MATHUS00042" num="00042"><math overflow="scroll"><mtable><mtr><mtd><mrow><msub><mi>Λ</mi><mn>12</mn></msub><mo>=</mo><mrow><mfrac><mrow><msub><mi>ϛ</mi><mi>o</mi></msub><mo></mo><msup><mi>k</mi><mn>2</mn></msup></mrow><mrow><mn>64</mn><mo></mo><msup><mi>π</mi><mn>2</mn></msup></mrow></mfrac><mo></mo><mrow><mfrac><mrow><mi>Re</mi><mo></mo><mrow><mo>{</mo><mrow><mo>∮</mo><mrow><mrow><mo></mo><mi>Ω</mi></mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><msubsup><mi>f</mi><mn>1</mn><mo>*</mo></msubsup><mo>·</mo><msub><mi>f</mi><mn>2</mn></msub></mrow><mo></mo><msup><mi></mi><mrow><mrow><mo></mo><mi></mi></mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><mi>k</mi><mo>·</mo><mi>D</mi></mrow></mrow></msup><mo></mo><msubsup><mi>a</mi><mn>1</mn><mo>*</mo></msubsup><mo></mo><msub><mi>a</mi><mn>2</mn></msub></mrow></mrow><mo>}</mo></mrow></mrow><mrow><mi>Re</mi><mo></mo><mrow><mo>{</mo><mrow><msubsup><mi>a</mi><mn>1</mn><mo>*</mo></msubsup><mo></mo><msub><mi>a</mi><mn>2</mn></msub></mrow><mo>}</mo></mrow></mrow></mfrac><mo>.</mo></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>44</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
namely Λ<sub>12 </sub>is exactly the interference term in AT. By substituting for the 4vector currentmoments and making the change of variables r<sub>1</sub>=r<sub>1</sub>′, r<sub>2</sub>=r<sub>2</sub>′+D,
<maths id="MATHUS00043" num="00043"><math overflow="scroll"><mtable><mtr><mtd><mtable><mtr><mtd><mrow><msub><mi>Λ</mi><mn>12</mn></msub><mo>=</mo><mi/><mo></mo><mrow><mfrac><mrow><msub><mi>ϛ</mi><mi>o</mi></msub><mo></mo><msup><mi>k</mi><mn>2</mn></msup></mrow><mrow><mn>64</mn><mo></mo><msup><mi>π</mi><mn>2</mn></msup></mrow></mfrac><mo></mo><mfrac><mrow><mi>Re</mi><mo></mo><mrow><mo>{</mo><mrow><msubsup><mo>∫</mo><msub><mi>V</mi><mn>1</mn></msub><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle></msubsup><mo></mo><mrow><mrow><mo></mo><mi>v</mi></mrow><mo></mo><mrow><msubsup><mo>∫</mo><msub><mi>V</mi><mn>2</mn></msub><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle></msubsup><mo></mo><mrow><mrow><mrow><mo></mo><msubsup><mi>vJ</mi><mn>1</mn><mo>*</mo></msubsup></mrow><mo>·</mo><msub><mi>J</mi><mn>2</mn></msub></mrow><mo></mo><mrow><mo>∮</mo><mrow><mrow><mo></mo><mi>Ω</mi></mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msup><mi></mi><mrow><mrow><mo></mo><mi></mi></mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><mi>k</mi><mo>·</mo><mrow><mo>(</mo><mrow><msub><mi>r</mi><mn>2</mn></msub><mo></mo><msub><mi>r</mi><mn>1</mn></msub></mrow><mo>)</mo></mrow></mrow></mrow></msup><mo></mo><msubsup><mi>a</mi><mn>1</mn><mo>*</mo></msubsup><mo></mo><msub><mi>a</mi><mn>2</mn></msub></mrow></mrow></mrow></mrow></mrow></mrow><mo>}</mo></mrow></mrow><mrow><mi>Re</mi><mo></mo><mrow><mo>{</mo><mrow><msubsup><mi>a</mi><mn>1</mn><mo>*</mo></msubsup><mo></mo><msub><mi>a</mi><mn>2</mn></msub></mrow><mo>}</mo></mrow></mrow></mfrac></mrow></mrow></mtd></mtr><mtr><mtd><mrow><mo>=</mo><mi/><mo></mo><mrow><mfrac><mrow><msub><mi>ϛ</mi><mi>o</mi></msub><mo></mo><msup><mi>k</mi><mn>2</mn></msup></mrow><mrow><mn>16</mn><mo></mo><msup><mi>π</mi><mn>2</mn></msup></mrow></mfrac><mo></mo><mfrac><mrow><mi>Re</mi><mo></mo><mrow><mo>{</mo><mrow><msubsup><mo>∫</mo><msub><mi>V</mi><mn>1</mn></msub><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle></msubsup><mo></mo><mrow><mrow><mo></mo><mi>v</mi></mrow><mo></mo><mrow><msubsup><mo>∫</mo><msub><mi>V</mi><mn>2</mn></msub><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle></msubsup><mo></mo><mrow><mrow><mrow><mo></mo><msubsup><mi>vJ</mi><mn>1</mn><mo>*</mo></msubsup></mrow><mo>·</mo><msub><mi>J</mi><mn>2</mn></msub></mrow><mo></mo><mfrac><mrow><mi>sin</mi><mo></mo><mrow><mo>(</mo><mrow><mi>k</mi><mo></mo><mrow><mo></mo><mrow><msub><mi>r</mi><mn>2</mn></msub><mo></mo><msub><mi>r</mi><mn>1</mn></msub></mrow><mo></mo></mrow></mrow><mo>)</mo></mrow></mrow><mrow><mo></mo><mrow><msub><mi>r</mi><mn>2</mn></msub><mo></mo><msub><mi>r</mi><mn>1</mn></msub></mrow><mo></mo></mrow></mfrac><mo></mo><msubsup><mi>a</mi><mn>1</mn><mo>*</mo></msubsup><mo></mo><msub><mi>a</mi><mn>2</mn></msub></mrow></mrow></mrow></mrow><mo>}</mo></mrow></mrow><mrow><mi>Re</mi><mo></mo><mrow><mo>{</mo><mrow><msubsup><mi>a</mi><mn>1</mn><mo>*</mo></msubsup><mo></mo><msub><mi>a</mi><mn>2</mn></msub></mrow><mo>}</mo></mrow></mrow></mfrac></mrow></mrow></mtd></mtr></mtable></mtd><mtd><mrow><mo>(</mo><mn>45</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where we evaluated the integral over all angles of k with r<sub>2</sub>−r<sub>1</sub>.
Note now that Eqs. (38) and (45) will become identical, if we can take the currents J<sub>1,2</sub><sup>v </sup>to be real. This is indeed the case for eigenmodes, where the field solution in bounded regions (such as those where the currents are flowing) is always stationary (in contrast to the leaky part of the eigenmode, which is radiative) and for high enough Q it can be chosen so that it is approximately real in the entire bounded region. Therefore, from either Eq. (38) or (45) we can write
<maths id="MATHUS00044" num="00044"><math overflow="scroll"><mtable><mtr><mtd><mrow><msub><mi>Λ</mi><mn>12</mn></msub><mo>=</mo><mrow><mfrac><mrow><msub><mi>ζ</mi><mi>o</mi></msub><mo></mo><msup><mi>k</mi><mn>2</mn></msup></mrow><mrow><mn>16</mn><mo></mo><msup><mi>π</mi><mn>2</mn></msup></mrow></mfrac><mo></mo><mrow><msubsup><mo>∫</mo><msub><mi>V</mi><mn>1</mn></msub><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle></msubsup><mo></mo><mrow><mrow><mo></mo><mi>v</mi></mrow><mo></mo><mrow><msubsup><mo>∫</mo><msub><mi>V</mi><mn>2</mn></msub><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle></msubsup><mo></mo><mrow><mrow><mrow><mo></mo><msubsup><mi>vJ</mi><mn>1</mn><mo>*</mo></msubsup></mrow><mo>·</mo><msub><mi>J</mi><mn>2</mn></msub></mrow><mo></mo><mfrac><mrow><mi>sin</mi><mo></mo><mrow><mo>(</mo><mrow><mi>k</mi><mo></mo><mrow><mo></mo><mrow><msub><mi>r</mi><mn>2</mn></msub><mo></mo><msub><mi>r</mi><mn>1</mn></msub></mrow><mo></mo></mrow></mrow><mo>)</mo></mrow></mrow><mrow><mo></mo><mrow><msub><mi>r</mi><mn>2</mn></msub><mo></mo><msub><mi>r</mi><mn>1</mn></msub></mrow><mo></mo></mrow></mfrac></mrow></mrow></mrow></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>46</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
and from Eq. (44), using Eq. (42), we can define the interference factor
<maths id="MATHUS00045" num="00045"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>V</mi><mrow><mi>rad</mi><mo>,</mo><mn>12</mn></mrow></msub><mo>≡</mo><mfrac><msub><mi>Λ</mi><mn>12</mn></msub><msqrt><mrow><msub><mi>Γ</mi><mrow><mn>1</mn><mo>,</mo><mi>rad</mi></mrow></msub><mo></mo><msub><mi>Γ</mi><mrow><mn>2</mn><mo>,</mo><mi>rad</mi></mrow></msub></mrow></msqrt></mfrac></mrow><mo>=</mo><mfrac><mrow><mo>∮</mo><mrow><mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>Ω</mi></mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><msubsup><mi>f</mi><mn>1</mn><mo>*</mo></msubsup><mo>·</mo><msub><mi>f</mi><mn>2</mn></msub></mrow><mo></mo><msup><mi></mi><mrow><mrow><mo></mo><mi></mi></mrow><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mrow><mi>k</mi><mo>·</mo><mi>D</mi></mrow></mrow></msup></mrow></mrow><msqrt><mrow><mo>∮</mo><mrow><mrow><mo></mo><mi>Ω</mi></mrow><mo></mo><msup><mrow><mo></mo><msub><mi>f</mi><mn>1</mn></msub><mo></mo></mrow><mn>2</mn></msup><mo></mo><mrow><mo>∮</mo><mrow><mrow><mo></mo><mi>Ω</mi></mrow><mo></mo><msup><mrow><mo></mo><msub><mi>f</mi><mn>2</mn></msub><mo></mo></mrow><mn>2</mn></msup></mrow></mrow></mrow></mrow></msqrt></mfrac></mrow></mtd><mtd><mrow><mo>(</mo><mn>47</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
We have shown that, in the highQ limit, both PT and AT give the same expression for the imaginary part Λ<sub>nm </sub>of the coupling coefficient, which thus physically describes within CMT the effects of farfield radiation interference. Again, this phenomenon was so far not considered to be predictable from CMT.
Physically, one can expect that farfield radiation interference can in principle be engineered to be destructive, resulting in reduced overall radiation losses for the twoobject system and thus in enhanced system efficiency. In this section, we show that, indeed, in the presence of farfield interference, energy transfer can be more efficient and with less radiated power than what our previous model predicts.
Once more, we will treat the same temporal energytransfer schemes as before (finiteamount and finiterate), so that a direct comparison can be made.
Considering again the source and device objects 1,2 to include the interference effects, the same CMT equations as in Eq. (1) can be used, but with the substitutions κ<sub>nm</sub>→K<sub>nm</sub>=κ<sub>nm</sub>+iΛ<sub>nm</sub>; n, m=1,2. The real parts κ<sub>11,22 </sub>can describe, as before, the shift in each object's resonance frequency due to the presence of the other; the imaginary parts Λ<sub>11,22 </sub>can describe the change in the losses of each object due to the presence of the other (due to absorption in it or scattering from it, in which latter case losses could be either increased or decreased); both of these are secondorder effects and, for the purposes of our mathematical analysis, can again be absorbed into the complex eigenfrequencies by setting ω<sub>1,2</sub>→ω<sub>1,2</sub>+κ<sub>11,22 </sub>and Γ<sub>1,2</sub>→Γ<sub>1,2</sub>−Λ<sub>11,22</sub>. The real parts κ<sub>12,21 </sub>can denote, as before, the coupling coefficients; the imaginary parts Λ<sub>12,21 </sub>can describe the farfield interference, as was shown in Section 3; again, from reciprocity K<sub>12</sub>=K<sub>21</sub>≡K≡κ+iΛ (note that for a Hermitian problem, the additional requirement K<sub>12</sub>=K<sub>21 </sub>* would impose K to be real, which makes sense, since without losses there cannot be any radiation interference).
Substituting κ→κ+iΛ into Eq. (2), we can find the normal modes of the system including interference effects. Note that, when the two objects are at exact resonance ω<sub>1</sub>=ω<sub>2</sub>≡ω<sub>o </sub>and Γ<sub>1</sub>=Γ<sub>2</sub>≡Γ<sub>o</sub>, the normal modes are found to be
<FORM>Ω<sub>+</sub>=(ω<sub>0</sub>+κ)−i(Γ<sub>0</sub>−Λ) and Ω<sub>−</sub>=(ω<sub>0</sub>−κ)−i(Γ<sub>0</sub>+Λ), (48)</FORM>
which is exactly the typical case for respectively the odd and even normal modes of a system of two coupled objects, where for the even mode the objects' fieldamplitudes have the same sign and thus the frequency is lowered and the radiative farfields interfere constructively so loss is increased, while for the odd mode the situation is the opposite. This is another confirmation for the fact that the coefficient Λ can describe the farfield interference phenomenon under examination.
To treat now again the problem of energy transfer to object 2 from 1, but in the presence of radiative interference, again simply substitute κ→κ+iΛ into Eq. (3). Note that, at exact resonance ω<sub>1</sub>=ω<sub>2 </sub>and, in the special case Γ<sub>1</sub>=Γ<sub>2</sub>≡Γ<sub>o</sub>, we can just substitute into Eq. (4) U→U+iV, where U≡κ/Γ<sub>o </sub>and V≡Λ/Γ<sub>o</sub>, and then, with T≡θ<sub>o</sub>t, the evolution of the device fieldamplitude becomes
<maths id="MATHUS00046" num="00046"><math overflow="scroll"><mtable><mtr><mtd><mrow><mfrac><mrow><msub><mi>a</mi><mn>2</mn></msub><mo></mo><mrow><mo>(</mo><mi>T</mi><mo>)</mo></mrow></mrow><mrow><mo></mo><mrow><msub><mi>a</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mn>0</mn><mo>)</mo></mrow></mrow><mo></mo></mrow></mfrac><mo>=</mo><mrow><mrow><mi>sin</mi><mo></mo><mrow><mo>[</mo><mrow><mrow><mo>(</mo><mrow><mi>U</mi><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>V</mi></mrow></mrow><mo>)</mo></mrow><mo></mo><mi>T</mi></mrow><mo>]</mo></mrow></mrow><mo>·</mo><msup><mi></mi><mrow><mo></mo><mi>T</mi></mrow></msup></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>49</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
Now the efficiency η<sub>E</sub>≡a<sub>2</sub>(t)<sup>2</sup>/a<sub>1</sub>(0)<sup>2 </sup>can be optimized for the normalized time T* which is the solution of the transcendental equation
<FORM>Re{(U+iV)·cot[(U+iV)T*]}=1 (50)</FORM>
and the resulting optimal energytransfer efficiency depends only on U, V and is depicted in FIG. 16(c), evidently increasing with V for a fixed U.
Similarly, to treat the problem of continuous powering of object 2 by 1, in the presence of radiative interference, simply substitute U→U+iV into the equations of Section 1.2, where V≡Λ/√{square root over (Γ<sub>1</sub>Γ<sub>2</sub>)} we call the stronginterference factor and quantifies the degree of farfield interference that the system experiences compared to loss. In practice, the parameters D<sub>1,2</sub>, U<sub>1,2 </sub>can be designed (engineered), since one can adjust the resonant frequencies ω<sub>1,2 </sub>(compared to the desired operating frequency ω) and the generator/load supply/drain rates κ<sub>1,2</sub>. Their choice can target the optimization of some system performancecharacteristic of interest.
In some examples, a goal can be to maximize the power transmission (efficiency) η<sub>P</sub>≡S<sub>21</sub><sup>2 </sup>of the system. The symmetry upon interchanging 1<img id="CUSTOMCHARACTER00032" he="2.12mm" wi="3.56mm" file="US20090284083A120091119P00001.TIF" imgcontent="character" imgformat="tif"/>2 is then preserved and, using Eq. (11), the fieldamplitude transmission coefficient becomes
<maths id="MATHUS00047" num="00047"><math overflow="scroll"><mtable><mtr><mtd><mrow><msub><mi>S</mi><mn>21</mn></msub><mo>=</mo><mrow><mfrac><mrow><mn>2</mn><mo></mo><mrow><mi></mi><mo></mo><mrow><mo>(</mo><mrow><mi>U</mi><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>V</mi></mrow></mrow><mo>)</mo></mrow></mrow><mo></mo><msub><mi>U</mi><mn>0</mn></msub></mrow><mrow><msup><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mn>0</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>D</mi><mn>0</mn></msub></mrow></mrow><mo>)</mo></mrow><mn>2</mn></msup><mo>+</mo><msup><mrow><mo>(</mo><mrow><mi>U</mi><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>V</mi></mrow></mrow><mo>)</mo></mrow><mn>2</mn></msup></mrow></mfrac><mo>.</mo></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>51</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
and from η′<sub>P</sub>(D<sub>o</sub>)=0 we get that, for fixed U, V and U<sub>o</sub>, the efficiency can be maximized for the symmetric detuning
<maths id="MATHUS00048" num="00048"><math overflow="scroll"><mtable><mtr><mtd><mrow><msub><mi>D</mi><mn>0</mn></msub><mo>=</mo><mstyle><mspace width="0.em" height="0.ex"/></mstyle><mo></mo><mtable><mtr><mtd><mrow><mrow><mrow><mn>2</mn><mo></mo><msqrt><mi>a</mi></msqrt><mo></mo><mrow><mi>cos</mi><mo>(</mo><mfrac><mrow><mi>θ</mi><mo>+</mo><mrow><mn>2</mn><mo></mo><mi>v</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>π</mi></mrow></mrow><mn>3</mn></mfrac><mo>)</mo></mrow></mrow><mo>;</mo><mrow><mi>v</mi><mo>=</mo><mn>0</mn></mrow></mrow><mo>,</mo><mn>1</mn><mo>,</mo></mrow></mtd><mtd><mrow><mrow><mrow><mi>if</mi><mo></mo><mstyle><mspace width="0.8em" height="0.8ex"/></mstyle><mo></mo><msup><mi>U</mi><mfrac><mn>2</mn><mn>3</mn></mfrac></msup></mrow><mo></mo><msup><mi>V</mi><mfrac><mn>2</mn><mn>3</mn></mfrac></msup></mrow><mo>></mo><msup><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mn>0</mn></msub></mrow><mo>)</mo></mrow><mfrac><mn>2</mn><mn>3</mn></mfrac></msup></mrow></mtd></mtr><mtr><mtd><mrow><mroot><mrow><mi>β</mi><mo>+</mo><msqrt><mrow><msup><mi>β</mi><mn>2</mn></msup><mo></mo><msup><mi>α</mi><mn>3</mn></msup></mrow></msqrt></mrow><mn>3</mn></mroot><mo>+</mo><mroot><mrow><mi>β</mi><mo></mo><msqrt><mrow><msup><mi>β</mi><mn>2</mn></msup><mo></mo><msup><mi>α</mi><mn>2</mn></msup></mrow></msqrt></mrow><mn>3</mn></mroot></mrow></mtd><mtd><mrow><mrow><mrow><mi>if</mi><mo></mo><mstyle><mspace width="0.8em" height="0.8ex"/></mstyle><mo></mo><msup><mi>U</mi><mfrac><mn>2</mn><mn>3</mn></mfrac></msup></mrow><mo></mo><msup><mi>V</mi><mfrac><mn>2</mn><mn>3</mn></mfrac></msup></mrow><mo>≤</mo><msup><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mn>0</mn></msub></mrow><mo>)</mo></mrow><mfrac><mn>2</mn><mn>3</mn></mfrac></msup></mrow></mtd></mtr></mtable></mrow></mtd><mtd><mrow><mo>(</mo><mn>52</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where α≡[U<sup>2</sup>−V<sup>2</sup>−(1+U<sub>o</sub>)<sup>2</sup>]/3,β≡UV(1+U<sub>o</sub>), θ=tan<sup>−1</sup>√{square root over (α<sup>3</sup>/β<sup>2</sup>−1)} and U<sup>2/3</sup>−V<sup>2/3</sup>>(1+U<sub>o</sub>)<sup>2/3</sup><img id="CUSTOMCHARACTER00033" he="2.46mm" wi="3.13mm" file="US20090284083A120091119P00003.TIF" imgcontent="character" imgformat="tif"/>α<sup>3</sup>−β<sup>2</sup>>0<img id="CUSTOMCHARACTER00034" he="2.46mm" wi="3.13mm" file="US20090284083A120091119P00003.TIF" imgcontent="character" imgformat="tif"/>α>0. Note that, in the first two peaks of the transmission curve are not equal for V>0, but the one at higher frequencies (v=0<img id="CUSTOMCHARACTER00035" he="2.46mm" wi="2.79mm" file="US20090284083A120091119P00002.TIF" imgcontent="character" imgformat="tif"/> positive detuning) corresponding to the odd system normal mode is higher, as should be expected, since the odd mode is the one that radiates less. Finally, by substituting D<sub>o </sub>into η<sub>P </sub>from Eq. (52), then from η′<sub>P</sub>(U<sub>o</sub>)=0 we get that, for fixed U and V, the efficiency can be maximized for
<FORM>U<sub>0</sub>*=√{square root over ((1+U<sup>2</sup>)(1−V<sup>2</sup>))}{square root over ((1+U<sup>2</sup>)(1−V<sup>2</sup>))} and D<sub>0</sub>*=UV. (53)</FORM>
The dependence of the efficiency on D<sub>o </sub>for different U<sub>o </sub>, (including the new ‘criticalcoupling’ condition) are shown in FIGS. 16(a,b). The overall optimal power efficiency using Eqs. (53) is
<maths id="MATHUS00049" num="00049"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>η</mi><msup><mi>P</mi><mo>*</mo></msup></msub><mo>≡</mo><mrow><msub><mi>η</mi><mi>P</mi></msub><mo></mo><mrow><mo>(</mo><mrow><msub><mi>D</mi><mrow><mn>0</mn><mo>*</mo></mrow></msub><mo>,</mo><msub><mi>U</mi><mrow><mn>0</mn><mo>*</mo></mrow></msub></mrow><mo>)</mo></mrow></mrow></mrow><mo>=</mo><mfrac><mrow><msup><mi>U</mi><mn>2</mn></msup><mo>+</mo><msup><mi>V</mi><mn>2</mn></msup></mrow><mrow><msup><mrow><mo>(</mo><mrow><msub><mi>U</mi><mrow><mn>0</mn><mo>*</mo></mrow></msub><mo>+</mo><mn>1</mn></mrow><mo>)</mo></mrow><mn>2</mn></msup><mo>+</mo><mrow><msup><mi>U</mi><mn>2</mn></msup><mo></mo><msup><mi>V</mi><mn>2</mn></msup></mrow></mrow></mfrac></mrow></mtd><mtd><mrow><mo>(</mo><mn>54</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
which depends only on U, V and is depicted in FIGS. 16(c,d), increasing with V for a fixed U, and actually η<sub>P</sub>→1 as V→1 for all values of U.
In some examples, a goal can be to minimize the power reflection at the side of the generator S<sub>11</sub><sup>2 </sup>and the load S<sub>22</sub><sup>2</sup>. The symmetry upon interchanging 1⇄2 is again preserved and, using then Eq. (17), one would require the ‘impedance matching’ condition
<FORM>(1−iD<sub>0</sub>)<sup>2</sup>−U<sub>0</sub><sup>2</sup>+(U+iV)<sup>2</sup>=0 (55)</FORM>
from which again we easily find that the values of D<sub>o </sub>and U<sub>o </sub>that cancel all reflections are exactly those in Eqs. (53).
In some examples, it can be of interest to minimize the power radiated from the system, since e.g. it can be a cause of interference to other communication systems, while still maintaining good efficiency. In some examples, the two objects can be the same, and then, using Eq. (41), we find
<maths id="MATHUS00050" num="00050"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>η</mi><mi>rad</mi></msub><mo>≡</mo><mfrac><msub><mi>P</mi><mi>rad</mi></msub><msup><mrow><mo></mo><msub><mi>S</mi><mrow><mo>+</mo><mn>1</mn></mrow></msub><mo></mo></mrow><mn>2</mn></msup></mfrac></mrow><mo>=</mo><mfrac><mtable><mtr><mtd><mrow><mrow><mn>4</mn><mo></mo><mrow><msub><mi>U</mi><mn>0</mn></msub><mo></mo><mrow><mo>(</mo><mrow><msup><mrow><mo></mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mn>0</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>D</mi><mn>0</mn></msub></mrow></mrow><mo></mo></mrow><mn>2</mn></msup><mo>+</mo><msup><mrow><mo></mo><mrow><mi>U</mi><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>V</mi></mrow></mrow><mo></mo></mrow><mn>2</mn></msup></mrow><mo>)</mo></mrow></mrow><mo></mo><mfrac><mi>Q</mi><msub><mi>Q</mi><mi>rad</mi></msub></mfrac></mrow><mo></mo></mrow></mtd></mtr><mtr><mtd><mrow><mn>2</mn><mo></mo><mrow><mi>V</mi><mo></mo><mrow><mo>(</mo><mrow><mi>V</mi><mo>+</mo><msub><mi>VU</mi><mn>0</mn></msub><mo>+</mo><msub><mi>UD</mi><mn>0</mn></msub></mrow><mo>)</mo></mrow></mrow></mrow></mtd></mtr></mtable><msup><mrow><mo></mo><mrow><msup><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><msub><mi>U</mi><mn>0</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>D</mi><mn>0</mn></msub></mrow></mrow><mo>)</mo></mrow><mn>2</mn></msup><mo>+</mo><msup><mrow><mo>(</mo><mrow><mi>U</mi><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>V</mi></mrow></mrow><mo>)</mo></mrow><mn>2</mn></msup></mrow><mo></mo></mrow><mn>2</mn></msup></mfrac></mrow></mtd><mtd><mrow><mo>(</mo><mn>56</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
Then, to achieve our goal, we maximize η<sub>P</sub>/η<sub>rad </sub>and find that this can be achieved for
<FORM>U<sub>0</sub>**√{square root over (1+U<sup>2</sup>−V<sub>rad</sub><sup>2</sup>U<sup>2</sup>+V<sup>2</sup>−2VV<sub>rad</sub>)} and D<sub>0**</sub>=UV<sub>rad</sub>, (57)</FORM>
where V<sub>rad</sub>≡Λ/√{square root over (Γ<sub>1,rad</sub>Γ<sub>2,rad</sub>)}, as defined in Eq. (47), we call the interference factor and quantifies the degree of farfield interference that the system experiences compared to the radiative loss, thus
<maths id="MATHUS00051" num="00051"><math overflow="scroll"><mrow><mrow><msub><mi>V</mi><mi>rad</mi></msub><mo>=</mo><mrow><mrow><mi>V</mi><mo></mo><msqrt><mrow><mfrac><msub><mi>Q</mi><mrow><mn>1</mn><mo>,</mo><mi>rad</mi></mrow></msub><msub><mi>Q</mi><mn>1</mn></msub></mfrac><mo></mo><mfrac><msub><mi>Q</mi><mrow><mn>2</mn><mo>,</mo><mi>rad</mi></mrow></msub><msub><mi>Q</mi><mn>2</mn></msub></mfrac></mrow></msqrt></mrow><mo>≥</mo><mi>V</mi></mrow></mrow><mo>,</mo></mrow></math></maths>
and V=V<sub>rad </sub>when all loss is radiative, in which case Eq. (57) reduces to Eq. (53).
In this description, we suggest that, for any temporal energytransfer scheme and given some achieved couplingtoloss ratio, the efficiency can be enhanced and the radiation can be suppressed by shifting the operational frequency away from exact resonance with each object's eigenfrequency and closer to the frequency of the odd normalmode, which suffers less radiation due to destructive farfield interference. It is the parameters
<maths id="MATHUS00052" num="00052"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mi>U</mi><mo>=</mo><mrow><mfrac><mi>κ</mi><msqrt><mrow><msub><mi>Γ</mi><mn>1</mn></msub><mo></mo><msub><mi>Γ</mi><mn>2</mn></msub></mrow></msqrt></mfrac><mo>=</mo><mrow><mi>k</mi><mo></mo><msqrt><mrow><msub><mi>Q</mi><mn>1</mn></msub><mo></mo><msub><mi>Q</mi><mn>2</mn></msub></mrow></msqrt></mrow></mrow></mrow><mo></mo><mstyle><mspace width="0.8em" height="0.8ex"/></mstyle><mo></mo><mstyle><mtext></mtext></mstyle><mo></mo><mi>and</mi><mo></mo><mstyle><mtext></mtext></mstyle><mo></mo><mrow><mi>V</mi><mo>=</mo><mrow><mfrac><mi>Λ</mi><msqrt><mrow><msub><mi>Γ</mi><mn>1</mn></msub><mo></mo><msub><mi>Γ</mi><mn>2</mn></msub></mrow></msqrt></mfrac><mo>=</mo><mrow><msub><mi>V</mi><mi>rad</mi></msub><mo></mo><msqrt><mrow><mfrac><msub><mi>Q</mi><mn>1</mn></msub><msub><mi>Q</mi><mrow><mn>1</mn><mo>,</mo><mi>rad</mi></mrow></msub></mfrac><mo></mo><mfrac><msub><mi>Q</mi><mn>2</mn></msub><msub><mi>Q</mi><mrow><mn>2</mn><mo>,</mo><mi>rad</mi></mrow></msub></mfrac></mrow></msqrt></mrow></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>58</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
that are the figuresofmerit for any system under consideration for wireless energytransfer, along with the distance over which large U, V can be achieved. Clearly, also V can be a decreasing function of distance, since two sources of radiation distant by more than a few wavelengths do not interfere substantially. It is important also to keep in mind that the magnitude of V depends on the degree to which radiation dominates the objects' losses, since it is only these radiative losses that can contribute to interference, as expressed from V<sub>rad</sub>≧V.
To achieve a large stronginterference factor V, in some examples, the energytransfer application preferably uses again subwavelength resonances, because, for a given sourcedevice distance, the interference factor V<sub>rad </sub>will increase as frequency decreases, since naturally the odd mode of two coupled objects, distant much closer than a wavelength, will not radiate at all.
To achieve a large stronginterference factor V, in some examples, the energytransfer application preferably uses resonant modes of high factors Q/Q<sub>rad</sub>. This condition can be satisfied by designing resonant modes where the dominant loss mechanism is radiation. As frequency decreases, radiation losses always decrease and typically systems are limited by absorption losses, as discussed earlier, so Q/Q<sub>rad </sub>decreases; thus, the advantage of interference can be insignificant at some point compared to the deterioration of absorptionQ.
Therefore, V will be maximized at some frequency ω<sub>V</sub>, dependent on the sourcedevice distance, and this optimal frequency will typically be different than ω<sub>U</sub>, the optimal frequency for U. As seen above, the problem of maximizing the energytransfer efficiency can require a modified treatment in the presence of interference. The choice of eigenfrequency for the source and device objects as ω<sub>U</sub>, where U is maximum, can not be a good one anymore, but also V needs to be considered. The optimization of efficiency occurs then at a frequency ω<sub>η</sub> between ω<sub>U </sub>and ω<sub>V </sub>and is a combined problem, which will be demonstrated below for few examples of electromagnetic systems.
Moreover, note that, at some fixed distance between the source and device objects, the figures U, V can not be maximized for the same set of system parameters; in that case, these parameters could be chosen so that the efficiency of Eq. (54) is maximized.
In the following section, we calculate a magnitude of efficiency improvement and radiation reduction for realistic systems at midrange distances between two objects, by employing this frequency detuning and by doing a joint optimization for U, V.
In the case of two objects 1, 2 supporting radiative electromagnetic resonant modes of the same eigenfrequency ω<sub>1</sub>=ω<sub>2</sub>≡ω<sub>o </sub>and placed at distance D between their arbitrarilychosen centers, so that they couple in the near field and interfere in the far field, the interference factor V<sub>rad </sub>is predicted from antenna theory (AT) to be that in Eq. (47).
We have also seen above how to compute the resonance quality factors Q and Q<sub>rad</sub>, for some example structures, and thus we can compute the factor Q/Q<sub>rad</sub>.
We will demonstrate the efficiency enhancement and the radiation suppression due to interference for the two examples of capacitivelyloaded conducting loops and dielectric disks. The degree of improvement will be shown to depend on the nature of the system.
Consider two loops 1, 2 of radius r with N turns of conducting wire with circular crosssection of radius a at distance D, as shown in FIG. 10. It was shown in Section 2.2 how to calculate the quality, coupling and strongcoupling factors for such a system.
Their coupling factor is shown in FIG. 17(a) as a function of the relative distance D/r, for three different dimensions of singleturn (N=1) loops. Their strongcoupling factor at the eigenfrequency ω<sub>Q</sub><sub><sub2>1</sub2></sub><sub>Q</sub><sub><sub2>2 </sub2></sub>is shown in FIG. 17(b). The approximate scaling k, U ∝(r/D)<sup>3</sup>, indicated by Eqs. (26) and (27), is apparent.
We compute the interference parameter between two coupled loops at distance D, using the AT analysis Eq. (47), leading to
Consider two loops 1, 2 of radius r with N turns of conducting wire with circular crosssection of radius a at distance D, as shown in FIG. 10. It was shown in Section 2.2 how to calculate the quality, coupling and strongcoupling factors for such a system. Their coupling factor is shown in FIG. 17(a) as a function of the relative distance D/r, for three different dimensions of singleturn (N=1) loops. Their strongcoupling factor at the eigenfrequency ω<sub>Q</sub><sub><sub2>1</sub2></sub><sub>Q</sub><sub><sub2>2 </sub2></sub>is shown in FIG. 17(b). The approximate scaling k, U ∝(r/D)<sup>3</sup>, indicated by Eqs. (26) and (27), is apparent. We compute the interference parameter between two coupled loops at distance D, using the AT analysis Eq. (47), leading to
<maths id="MATHUS00053" num="00053"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>V</mi><mi>rad</mi></msub><mo>=</mo><mrow><mfrac><mn>3</mn><msup><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow><mn>3</mn></msup></mfrac><mo></mo><mrow><mo>[</mo><mrow><mrow><mi>sin</mi><mo></mo><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow></mrow><mo></mo><mrow><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow><mo></mo><mrow><mi>cos</mi><mo></mo><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow></mrow></mrow></mrow><mo>]</mo></mrow></mrow></mrow><mo>,</mo></mrow></mtd><mtd><mrow><mo>(</mo><mn>59</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
for the orientation of optimal coupling, where one loop is above the other. Their interference factor is shown in FIG. 18 as a function of the normalized distance D/λ, where it can be seen that this factor has nulls only upon reaching the radiative regime. Since the resonant loops are highly subwavelength (in many examples λ/r≧50), at midrange distances (D/r≦10), we expect D/λ≦0.2 and thus the interference factor to be very large (V<sub>rad</sub>>0.8).
At a fixed resonant frequency, in some examples, the factor Q/Q<sub>rad </sub>can be increased by increasing the radii r of the loops. In some examples, the factor Q/Q<sub>rad </sub>can be increased by increasing the number N of turns of the loops. In some examples, the factor Q/Q<sub>rad </sub>can be increased by increasing the radius a of the conducting wire of the loops or by using Litz wire or a ribbon to reduce the absorption losses and thus make radiation more dominant loss mechanism.
We also plot in FIG. 19, for the example r=30 cm and a=2 cm, the strongcoupling factor U, the interference factor V<sub>rad </sub>and the stronginterference factor V as a function of the resonant eigenfrequency of the loops, for a fixed distance D=5r. Indeed, for this example, V<sub>rad </sub>decreases monotonically with frequency in this subwavelength regime and is always great than 0.8, but V exhibits a maximum, since the term Q/Q<sub>rad </sub>is increasing towards 1 with frequency, as losses become more and more radiation dominated. It can be seen that the resonant eigenfrequencies f<sub>U </sub>and f<sub>V</sub>, at which U and V become maximum respectively, are different. This implies that the efficiency will now riot necessarily peak at the eigenfrequency f<sub>U</sub>, at which U is maximized, as would be the assumption based on prior knowledge, but at a different one f<sub>η</sub> between f<sub>U </sub>and f<sub>V</sub>. This is shown below.
In FIG. 20 the efficiency η<sub>P </sub>is plotted as a function of the resonant eigenfrequency of the loops for two different examples of loop dimensions r=30 cm, a=2 cm and r=1 m, a=2 cm, at two different loop distances D=5r and D=10r, and for the cases:
 (i) (solid lines) including interference effects and detuning the driving frequency from the resonant frequency by D<sub>o</sub>=UV from Eq. (53) to maximize the powertransmission efficiency and similarly using U<sub>o </sub>from Eq. (53), which thus implies optimal efficiency as in Eq. (54).
 (ii) (dashdotted lines) including interference effects and detuning the driving frequency from the resonant frequency by D<sub>o</sub>=UV<sub>rad </sub>from Eq. (57) to maximize the ratio of power transmitted over power radiated and similarly using U<sub>o </sub>from Eq. (57).
 (iii) (dashed lines) including interference effects but not detuning the driving frequency from the resonant frequency and using U<sub>o </sub>from Eq. (14), as one would do to maximize efficiency in the absence of interference.
 (iv) (dotted lines) truly in the absence of interference effects and thus maximizing efficiency by not detuning the driving frequency from the resonant frequency and using U<sub>o </sub>from Eq. (14), which thus implies efficiency as in Eq. (15).
In FIG. 21 we show the amount of drivingfrequency detuning that is used in the presence of interference either to maximize efficiency (case (i) (solid lines) of FIG. 20—D<sub>o</sub>=UV) or to maximize the ratio of power transmitted over power radiated (case (ii) (dashdotted lines) of FIG. 20—D<sub>o</sub>=UV<sub>rad</sub>). Clearly, this drivingfrequency detuning can be a nontrivial amount.
It can be seen from FIG. 20 that, for all frequencies, the efficiency of case (i) (solid lines) is larger than the efficiency of case (iii) (dashed lines) which is in turn larger than the efficiency of case (iv) (dotted lines). Therefore, in this description, we suggest that employing farfield interference improves on the powertransmission efficiency (improvement from (iv) (dotted) to (iii) (dashed)) and, furthermore, employing destructive farfield interference, by detuning the driving frequency towards the lowradiationloss odd normal mode, improves on the powertransmission efficiency even more (improvement from (iii) (dashed) to (i) (solid)).
If f<sub>72 </sub> is the eigenfrequency, at which the efficiency of case (i) (solid) is optimized, then, in some examples, the resonant eigenfrequency can be designed to be larger than f<sub>η</sub>, namely in a regime where the system is more radiation dominated. In this description, we suggest that at such eigenfrequencies, there can be a significant improvement in efficiency by utilizing the destructive farfield interference effects and driving the system at a frequency close to the odd normal mode. This can be seen again from FIG. 20 by comparing the solid lines to the corresponding dashed lines and the dotted lines.
In general, one would tend to design a system resonant at the frequency f<sub>U </sub>where the strongcoupling factor U is maximal. However, as suggested above, in the presence of interference, FIG. 20 shows that the maximum of η<sub>P </sub>is at an eigenfrequency f<sub>η</sub> different than f<sub>U</sub>. In some examples, f<sub>η</sub>>f<sub>U</sub>. This is because at higher eigenfrequencies, losses are determined more by radiation than absorption, therefore destructive radiation interference can play a more significant role in reducing overall losses and thus f<sub>V</sub>>f<sub>U </sub>and the efficiency in increased at f<sub>η</sub>>f<sub>U</sub>. In this description, we suggest that, in some examples, the resonant eigenfrequency can be designed to be close to the frequency f<sub>η</sub> that optimizes the efficiency rather than the different f<sub>U</sub>. In particular, in FIG. 22(a) are plotted these two frequencies f<sub>η</sub> (solid line) and f<sub>U </sub>(dashed line) as a function of relative distance D/r of two r=30 cm loops. In FIG. 22(b) we show a graded plot of the optimal efficiency from Eq. (54) in the U−V plane. Then, we superimpose the U−V curve of case (i) (solid), parametrized with distance D, for two r=30 cm loops resonant at the optimal frequency f<sub>η</sub> for each D. From the path of this curve onto the graded plot the efficiency as a function of distance can be extracted for case (i) (solid). We then also superimpose in FIG. 22(b) the U−V curve of case (iii) (dashed), parametrized with distance D, for two r=30 cm loops resonant at f<sub>U</sub>, and the U range of case (iv) (dotted), parametrized with distance D, for two r=30 cm loops resonant at f<sub>U </sub>(note that in this last case there is no interference and thus V=0). In FIG. 22(c) we then show the efficiency enhancement factor achieved by the solid curve of FIG. 22(b), as a function of distance D/r, compared to best that can be achieved without drivingfrequency detuning (dashed) and without interference whatsoever (dotted). The improvement by employing interference can reach a factor of 2 at large separation between the loops.
In FIG. 23 we plot the radiation efficiency η<sub>rad</sub>, using Eq. (39), as a function of the eigenfrequency of the loops for the two different loop dimensions, the two different distances and the four different cases examined in FIG. 20. It can be seen from FIG. 23 that, for all frequencies, η<sub>rad </sub>of case (ii) (dashdotted lines) is smaller than η<sub>rad </sub>of case (i) (solid lines) which is in turn smaller than η<sub>rad </sub>of case (iii) (dashed lines) and this smaller than η<sub>rad </sub>of case (iv) (dotted lines). Therefore, in this description, we suggest that employing farfield interference suppresses radiation (improvement from (iv) (dotted) to (iii) (dashed)) and, furthermore, employing destructive farfield interference, by detuning the driving frequency towards the lowradiationloss odd normal mode, suppress radiation efficiency even more (improvement from (iii) (dashed) to (i) (solid) and (ii) (dashdotted)), more so in case (ii), specifically optimized for this purpose.
In some examples, the resonant eigenfrequency can be designed to be larger than f<sub>72 </sub>, namely in a regime where the system is more radiation dominated. In this description, we suggest that at such eigenfrequencies, there can be a significant suppression in radiation by utilizing the destructive farfield interference effects and driving the system at a frequency close to the odd normal mode. The case (ii)=(dashdotted) accomplishes the greatest suppression in radiation and, as can be seen in FIG. 20, there is a range of eigenfrequencies (close to f<sub>V</sub>), for which the efficiency that this configuration can achieve is only little compromised compared to the maximum possible of configuration (i).
In one example, two singleturn loops of r=30 cm and a=2 cm are at a distance D/r=5 in the orientation shown in FIG. 10 and they are designed to resonate at 30 MHz. In the absence of interference, the powertransmission efficiency is 59% and the radiation efficiency is 38%. In the presence of interference and without detuning the driving frequency from 30 MHz, the powertransmission efficiency is 62% and the radiation efficiency is 32%. In the presence of interference and detuning the driving frequency from 30 MHz to 31.3 MHz to maximize efficiency, the powertransmission efficiency is increased to 75% and the radiation efficiency is suppressed to 18%.
In another example, two singleturn loops of r=30 cm and a=2 cm are at a distance D/r=5 in the orientation shown in FIG. 10 and they are designed to resonate at 10 MHz. In the absence of interference or in the presence of interference and without detuning the driving frequency from 10 MHz, the powertransmission efficiency is approximately 81% and the radiation efficiency is approximately 4%. In the presence of interference and detuning the driving frequency from 10 MHz to 10.22 MHz to maximize transmission over radiation, the powertransmission efficiency is 42%, reduced by less than a factor of 2, while the radiation efficiency is 0.4%, suppressed by an order of magnitude.
In another example, two singleturn loops of r=1 m and a=2 cm are at a distance D/r=5 in the orientation shown in FIG. 10 and they are designed to resonate at 10 MHz. In the absence of interference, the powertransmission efficiency is 48% and the radiation efficiency is 47%. In the presence of interference and without detuning the driving frequency from 10 MHz, the powertransmission efficiency is 54% and the radiation efficiency is 37%. In the presence of interference and detuning the driving frequency from 10 MHz to 14.8 MHz to maximize efficiency, the powertransmission efficiency is increased to 66% and the radiation efficiency is suppressed to 24%.
In another example, two singleturn loops of r=1 m and a=2 cm are at a distance D/r=5 in the orientation shown in FIG. 10 and they are designed to resonate at 4 MHz. In the absence of interference or in the presence of interference and without detuning the driving frequency from 4 MHz, the powertransmission efficiency is approximately 71% and the radiation efficiency is approximately 8%. In the presence of interference and detuning the driving frequency from 4 MHz to 5.06 MHz to maximize transmission over radiation, the powertransmission efficiency is 40%, reduced by less than a factor of 2, while the radiation efficiency is approximately 1%, suppressed by almost an order of magnitude.
Consider two dielectric disks 1 and 2 of radius r and dielectric permittivity ε placed at distance D between their centers, as shown in FIG. 15(b). Their coupling as a function of distance was calculated in Section 2.4, using analytical and finiteelementfrequencydomain (FEFD) methods, and is shown in FIG. 24.
To compute the interference factor between two coupled disks at distance D, we again use two independent methods to confirm the validity of our results: numerically, the FEFD calculations again give Λ (and thus V) by the splitting of the lossrates of the two normal modes; analytically, calculation of the AT prediction of Eq. (47) gives
<maths id="MATHUS00054" num="00054"><math overflow="scroll"><mtable><mtr><mtd><mtable><mtr><mtd><mrow><mi>m</mi><mo>=</mo><mrow><mn>1</mn><mo></mo><mstyle><mtext>:</mtext></mstyle></mrow></mrow></mtd><mtd><mrow><msub><mi>V</mi><mi>rad</mi></msub><mo>=</mo><mrow><mfrac><mn>2</mn><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow></mfrac><mo></mo><mrow><msub><mi>J</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow></mrow></mrow></mrow></mtd></mtr><mtr><mtd><mrow><mi>m</mi><mo>=</mo><mrow><mn>2</mn><mo></mo><mstyle><mtext>:</mtext></mstyle></mrow></mrow></mtd><mtd><mrow><msub><mi>V</mi><mi>rad</mi></msub><mo>=</mo><mrow><mfrac><mn>8</mn><msup><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow><mn>3</mn></msup></mfrac><mo></mo><mrow><mo>{</mo><mrow><mrow><mn>3</mn><mo></mo><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow><mo></mo><mrow><msub><mi>J</mi><mn>0</mn></msub><mo></mo><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow></mrow></mrow><mo>+</mo><mrow><mrow><mo>[</mo><mrow><msup><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow><mn>2</mn></msup><mo></mo><mn>6</mn></mrow><mo>]</mo></mrow><mo></mo><mrow><msub><mi>J</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow></mrow></mrow></mrow><mo>}</mo></mrow></mrow></mrow></mtd></mtr><mtr><mtd><mrow><mi>m</mi><mo>=</mo><mrow><mn>3</mn><mo></mo><mstyle><mtext>:</mtext></mstyle></mrow></mrow></mtd><mtd><mrow><msub><mi>V</mi><mi>rad</mi></msub><mo>=</mo><mrow><mfrac><mn>6</mn><msup><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow><mn>5</mn></msup></mfrac><mo></mo><mrow><mo>{</mo><mtable><mtr><mtd><mrow><mrow><mrow><mo>[</mo><mrow><mrow><mn>24</mn><mo></mo><msup><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow><mn>3</mn></msup></mrow><mo></mo><mrow><mn>320</mn><mo></mo><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow></mrow></mrow><mo>]</mo></mrow><mo></mo><mrow><msub><mi>J</mi><mn>0</mn></msub><mo></mo><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow></mrow></mrow><mo>+</mo></mrow></mtd></mtr><mtr><mtd><mrow><mrow><mo>[</mo><mrow><mrow><mn>3</mn><mo></mo><msup><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow><mn>4</mn></msup></mrow><mo></mo><mrow><mn>128</mn><mo></mo><msup><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow><mn>2</mn></msup></mrow><mo>+</mo><mn>640</mn></mrow><mo>]</mo></mrow><mo></mo><mrow><msub><mi>J</mi><mn>1</mn></msub><mo></mo><mrow><mo>(</mo><mi>kD</mi><mo>)</mo></mrow></mrow></mrow></mtd></mtr></mtable><mo>}</mo></mrow></mrow></mrow></mtd></mtr></mtable></mtd><mtd><mrow><mo>(</mo><mn>60</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
The results for the interference of two same disks, for exactly the same parameters for which the coupling was calculated in FIG. 24, are presented in FIG. 25, as a function of frequency (due to varying ε) at fixed distances. It can be seen that also the stronginterference factor V can have nulls, which can occur even before the system enters the radiativecoupling regime, namely at smaller frequencies than those of U at the same distances, and it decreases with frequency, since then the objects become more and more absorption dominated, so the benefit from radiative interference is suppressed. Both the above effects result into the fact that, for most distances, U (from FIG. 24(b)) and V (from FIG. 25(b)) can be maximized at different values of the frequency (f<sub>U </sub>and f<sub>V </sub>respectively), and thus different can also be the optimal frequency f<sub>η</sub> for the final energytransfer efficiency of Eq. (54), which is shown in FIG. 26 again for the same set of parameters. From this plot, it can be seen that interference can significantly improve the transfer efficiency, compared to what Eq. (15) would predict from the calculated values of the coupling figureofmerit U.
Furthermore, not only does a given energytransfer system perform better than what a prediction which ignores interference would predict, but also our optimization design will typically lead to different optimal set of parameters in the presence of interference. For example, for the particular distance D/r=5, it turns out from FIG. 26 that the m=1 resonant modes can achieve better efficiency than the m=2 modes within the available range of ε, by making use of strong interference which counteracts their weaker U, as viewed in FIG. 24, from which one would have concluded the opposite performance. Moreover, even within the same mbranch, one would naively design the system to operate at the frequency f<sub>U</sub>, at which U is maximum. However, the optimization design changes in the presence of interference, since the system should be designed to operate at the different frequency f<sub>η</sub>, where the overall efficiency η peaks. In FIG. 27(a), we first calculate those different frequencies where the strongcoupling factor U and the efficiency η (which includes interference) peak, as distance D is changing for thechoice of the m=2 disk of FIG. 24, and observe that their difference is actually significant. Then, in FIG. 27(b) we show the peak efficiency for the various frequency choices. For large distances, where efficiency is small and could use a boost, the improvement factor reaches a significant 2 for the particular system under examination. The same result is shown in FIG. 27(c) as a plot of the path of the efficiency on the U−V map, as distance is changing. Similar results are derived for the modes of different morder. Physically, moving to higher frequencies increases role of radiative losses compared to absorption and thus interference can have a greater influence. At the optimal frequency f<sub>η</sub> radiated power including interference is close to what it is at f<sub>U</sub>, but absorbed power is much less, therefore the efficiency has been improved.
In some examples, instead of improving efficiency, one might care more about minimizing radiation. In that case, we calculate at the frequency f<sub>U </sub>how much power is radiated when optimized under the conditions Eq. (57) compared to the power radiated when simply operating on resonance (D<sub>o</sub>=0) in the cases with and without interference (the latter case can be describing a case where the two disks do not interfere, because they are dissimilar, or due to decoherence issues etc.). We find in FIG. 28 that radiation can be suppressed by a factor of 1.6 by detuning the operating frequency towards the odd subradiant mode.
In general, the overall performance of an example of the resonancebased wireless energytransfer scheme depends strongly on the robustness of the resonant objects' resonances. Therefore, it is desirable to analyze the resonant objects' sensitivity to the near presence of random nonresonant extraneous objects. One appropriate analytical model is that of “perturbation theory” (PT), which suggests that in the presence of an extraneous perturbing object p the field amplitude a<sub>1</sub>(t) inside the resonant object 1 satisfies, to first order:
<maths id="MATHUS00055" num="00055"><math overflow="scroll"><mtable><mtr><mtd><mrow><mfrac><mrow><mo></mo><msub><mi>a</mi><mn>1</mn></msub></mrow><mrow><mo></mo><mi>t</mi></mrow></mfrac><mo>=</mo><mrow><mrow><mrow><mo></mo><mrow><mi></mi><mo></mo><mrow><mo>(</mo><mrow><msub><mi>ω</mi><mn>1</mn></msub><mo></mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>Γ</mi><mn>1</mn></msub></mrow></mrow><mo>)</mo></mrow></mrow></mrow><mo></mo><msub><mi>a</mi><mn>1</mn></msub></mrow><mo>+</mo><mrow><mrow><mi></mi><mo></mo><mrow><mo>(</mo><mrow><mrow><mi>δ</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>κ</mi><mrow><mn>11</mn><mo></mo><mrow><mo>(</mo><mi>p</mi><mo>)</mo></mrow></mrow></msub></mrow><mo>+</mo><mrow><mi></mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><mi>δ</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle><mo></mo><msub><mi>Γ</mi><mrow><mn>1</mn><mo></mo><mrow><mo>(</mo><mi>p</mi><mo>)</mo></mrow></mrow></msub></mrow></mrow><mo>)</mo></mrow></mrow><mo></mo><msub><mi>a</mi><mn>1</mn></msub></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>61</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths>
where again ω<sub>1 </sub>is the frequency and Γ<sub>1 </sub>the intrinsic (absorption, radiation etc.) loss rate, while δκ<sub>11</sub>(p) is the frequency shift induced onto 1 due to the presence of p and δΓ<sub>1(p) </sub>is the extrinsic due to p (absorption inside p, scattering from p etc.) loss rate. δ⊖<sub>1(p) </sub>is defined as δΓ<sub>1(p)</sub>≡Γ<sub>1(p)</sub>−Γ<sub>1</sub>, where Γ<sub>1(p) </sub>is the total perturbed loss rate in the presence of p. The firstorder PT model is valid only for small perturbations. Nevertheless, the parameters δκ<sub>11(p)</sub>, δΓ<sub>1(p) </sub>are well defined, even outside that regime, if a<sub>1 </sub>is taken to be the amplitude of the exact perturbed mode. Note also that interference effects between the radiation field of the initial resonantobject mode and the field scattered off the extraneous object can for strong scattering (e.g. off metallic objects) result in total Γ<sub>1,rad(p) </sub>that are smaller than the initial Γ<sub>1,rad </sub>(namely δΓ<sub>1,rad(p) </sub>is negative).
It has been shown that a specific relation is desired between the resonant frequencies of the source and deviceobjects and the driving frequency. In some examples, all resonant objects must have the same eigenfrequency and this must be equal to the driving frequency. In some examples, when trying to optimize efficiency or suppress radiation by employing farfield interference, all resonant objects must have the same eigenfrequency and the driving frequency must be detuned from them by a particular amount. In some implementations, this frequencyshift can be “fixed” by applying to one or more resonant objects and the driving generator a feedback mechanism that corrects their frequencies. In some examples, the driving frequency from the generator can be fixed and only the resonant frequencies of the objects can be tuned with respect to this driving frequency.
The resonant frequency of an object can be tuned by, for example, adjusting the geometric properties of the object (e.g. the height of a selfresonant coil, the capacitor plate spacing of a capacitivelyloaded loop or coil, the dimensions of the inductor of an inductivelyloaded rod, the shape of a dielectric disc, etc.) or changing the position of a nonresonant object in the vicinity of the resonant object.
In some examples, referring to FIG. 29a, each resonant object is provided with an oscillator at fixed frequency and a monitor which determines the eigenfrequency of the object. At least one of the oscillator and the monitor is coupled to a frequency adjuster which can adjust the frequency of the resonant object. The frequency adjuster determines the difference between the fixed driving frequency and the object frequency and acts, as described above, to bring the object frequency into the required relation with respect to the fixed frequency. This technique assures that all resonant objects operate at the same fixed frequency, even in the presence of extraneous objects.
In some examples, referring to FIG. 29(b), during energy transfer from a source object to a device object, the device object provides energy or power to a load, and an efficiency monitor measures the efficiency of the energytransfer or powertransmission. A frequency adjuster coupled to the load and the efficiency monitor acts, as described above, to adjust the frequency of the object to maximize the efficiency.
In other examples, the frequency adjusting scheme can rely on information exchange between the resonant objects. For example, the frequency of a source object can be monitored and transmitted to a device object, which is in turn synched to this frequency using frequency adjusters, as described above. In other embodiments the frequency of a single clock can be transmitted to multiple devices, and each device then synched to that frequency using frequency adjusters, as described above.
Unlike the frequency shift, the extrinsic perturbing loss due to the presence of extraneous perturbing objects can be detrimental to the functionality of the energytransfer scheme, because it is difficult to remedy. Therefore, the total perturbed quality factors Q<sub>(p) </sub>(and the corresponding perturbed strongcoupling factor U<sub>(p) </sub>and the perturbed stronginterference factor V<sub>(p)</sub>) should be quantified.
In some examples, a system for wireless energytransfer uses primarily magnetic resonances, wherein the energy stored in the near field in the air region surrounding the resonator is predominantly magnetic, while the electric energy is stored primarily inside the resonator. Such resonances can exist in the quasistatic regime of operation (r<<λ) that we are considering: for example, for coils with h<<2r, most of the electric field is localized within the selfcapacitance of the coil or the externally loading capacitor and, for dielectric disks, with ε>>1 the electric field is preferentially localized inside the disk. In some examples, the influence of extraneous objects on magnetic resonances is nearly absent. The reason is that extraneous nonconducting objects p that could interact with the magnetic field in the air region surrounding the resonator and act as a perturbation to the resonance are those having significant magnetic properties (magnetic permeability Re{μ}>1 or magnetic loss Im{μ}>0). Since almost all everyday nonconducting materials are nonmagnetic but just dielectric, they respond to magnetic fields in the same way as free space, and thus will not disturb the resonance of the resonator. Extraneous conducting materials can however lead to some extrinsic losses due to the eddy currents induced inside them or on their surface (depending on their conductivity). However, even for such conducting materials, their presence will not be detrimental to the resonances, as long as they are not in very close proximity to the resonant objects.
The interaction between extraneous objects and resonant objects is reciprocal, namely, if an extraneous object does not influence a resonant object, then also the resonant object does not influence the extraneous object. This fact can be viewed in light of safety considerations for human beings. Humans are also nonmagnetic and can sustain strong magnetic fields without undergoing any risk. A typical example, where magnetic fields B˜1 T are safely used on humans, is the Magnetic Resonance Imaging (MRI) technique for medical testing. In contrast, the magnetic nearfield required in typical embodiments in order to provide a few Watts of power to devices is only B˜10<sup>−4 </sup>T, which is actually comparable to the magnitude of the Earth's magnetic field. Since, as explained above, a strong electric nearfield is also not present and the radiation produced from this nonradiative scheme is minimal, the energytransfer apparatus, methods and systems described herein is believed safe for living organisms.
In some examples, one can estimate the degree to which the resonant system of a capacitivelyloaded conductingwire coil has mostly magnetic energy stored in the space surrounding it. If one ignores the fringing electric field from the capacitor, the electric and magnetic energy densities in the space surrounding the coil come just from the electric and magnetic field produced by the current in the wire; note that in the far field, these two energy densities must be equal, as is always the case for radiative fields. By using the results for the fields produced by a subwavelength (r<<λ) current loop (magnetic dipole) with h=0, we can calculate the ratio of electric to magnetic energy densities, as a function of distance D<sub>p </sub>from the center of the loop (in the limit r<<D<sub>p</sub>) and the angle θ with respect to the loop axis:
<maths id="MATHUS00056" num="00056"><math overflow="scroll"><mtable><mtr><mtd><mtable><mtr><mtd><mrow><mfrac><mrow><msub><mi>w</mi><mi>e</mi></msub><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow><mrow><msub><mi>w</mi><mi>m</mi></msub><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow></mfrac><mo>=</mo><mi/><mo></mo><mfrac><mrow><msub><mi>ɛ</mi><mi>o</mi></msub><mo></mo><msup><mrow><mo></mo><mrow><mi>E</mi><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow><mo></mo></mrow><mn>2</mn></msup></mrow><mrow><msub><mi>μ</mi><mi>o</mi></msub><mo></mo><msup><mrow><mo></mo><mrow><mi>H</mi><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow><mo></mo></mrow><mn>2</mn></msup></mrow></mfrac></mrow></mtd></mtr><mtr><mtd><mrow><mrow><mo>=</mo><mi/><mo></mo><mfrac><mrow><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><mfrac><mn>1</mn><msup><mi>x</mi><mn>2</mn></msup></mfrac></mrow><mo>)</mo></mrow><mo></mo><msup><mi>sin</mi><mn>2</mn></msup><mo></mo><mi>θ</mi></mrow><mrow><mrow><mrow><mo>(</mo><mrow><mfrac><mn>1</mn><msup><mi>x</mi><mn>2</mn></msup></mfrac><mo>+</mo><mfrac><mn>1</mn><msup><mi>x</mi><mn>4</mn></msup></mfrac></mrow><mo>)</mo></mrow><mo></mo><mn>4</mn><mo></mo><msup><mi>cos</mi><mn>2</mn></msup><mo></mo><mi>θ</mi></mrow><mo>+</mo><mrow><mrow><mo>(</mo><mrow><mn>1</mn><mo></mo><mfrac><mn>1</mn><msup><mi>x</mi><mn>2</mn></msup></mfrac><mo>+</mo><mfrac><mn>1</mn><msup><mi>x</mi><mn>4</mn></msup></mfrac></mrow><mo>)</mo></mrow><mo></mo><msup><mi>sin</mi><mn>2</mn></msup><mo></mo><mi>θ</mi></mrow></mrow></mfrac></mrow><mo>;</mo><mrow><mi>x</mi><mo>=</mo><mrow><mn>2</mn><mo></mo><mi>π</mi><mo></mo><mfrac><msub><mi>D</mi><mi>p</mi></msub><mi>λ</mi></mfrac></mrow></mrow></mrow></mtd></mtr></mtable></mtd><mtd><mrow><mo>(</mo><mn>62</mn><mo>)</mo></mrow></mtd></mtr><mtr><mtd><mrow><mrow><mrow><mrow><mo>⇒</mo><mfrac><mrow><munder><mo>∯</mo><msub><mi>S</mi><mi>p</mi></msub></munder><mo></mo><mrow><mrow><msub><mi>w</mi><mi>e</mi></msub><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow><mo></mo><mrow><mo></mo><mi>S</mi></mrow></mrow></mrow><mrow><munder><mo>∯</mo><msub><mi>S</mi><mi>p</mi></msub></munder><mo></mo><mrow><mrow><msub><mi>w</mi><mi>m</mi></msub><mo></mo><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow></mrow><mo></mo><mrow><mo></mo><mi>S</mi></mrow></mrow></mrow></mfrac></mrow><mo>=</mo><mfrac><mrow><mn>1</mn><mo>+</mo><mfrac><mn>1</mn><msup><mi>x</mi><mn>2</mn></msup></mfrac></mrow><mrow><mn>1</mn><mo>+</mo><mfrac><mn>1</mn><msup><mi>x</mi><mn>2</mn></msup></mfrac><mo>+</mo><mfrac><mn>3</mn><msup><mi>x</mi><mn>4</mn></msup></mfrac></mrow></mfrac></mrow><mo>;</mo><mrow><mi>x</mi><mo>=</mo><mrow><mn>2</mn><mo></mo><mi>π</mi><mo></mo><mfrac><msub><mi>D</mi><mi>p</mi></msub><mi>λ</mi></mfrac></mrow></mrow></mrow><mo>,</mo></mrow></mtd><mtd><mstyle><mspace width="0.3em" height="0.3ex"/></mstyle></mtd></mtr></mtable></math></maths>
where the second line is the ratio of averages over all angles by integrating the electric and magnetic energy densities over the surface of a sphere of radius D<sub>p</sub>. From Eq. (62) it is obvious that indeed for all angles in the near field (x<<1) the magnetic energy density is dominant, while in the far field (x>>1) they are equal as they should be. Also, the preferred positioning of the loop is such that objects which can interfere with its resonance lie close to its axis (θ=0), where there is no electric field. For example, using the systems described in Table 4, we can estimate from Eq. (62) that for the loop of r=30 cm at a distance D<sub>p</sub>=10r=3 m the ratio of average electric to average magnetic energy density would be ˜12% and at D<sub>p</sub>=3r=90 cm it would be ·1%, and for the loop of r=10 cm at a distance D<sub>p</sub>=10r=1 m the ratio would be ˜33% and at D<sub>p</sub>=3r=30 cm it would be ˜2.5%. At closer distances this ratio is even smaller and thus the energy is predominantly magnetic in the near field, while in the radiative far field, where they are necessarily of the same order (ratio→1), both are very small, because the fields have significantly decayed, as capacitivelyloaded coil systems are designed to radiate very little. Therefore, this is the criterion that qualifies this class of resonant system as a magnetic resonant system.
To provide an estimate of the effect of extraneous objects on the resonance of a capacitivelyloaded loop including the capacitor fringing electric field, we use the perturbation theory formula, stated earlier, δΓ<sub>1,abs(p)</sub>=ω<sub>1</sub>/4·∫d<sup>3</sup>r IM{ε<sub>p</sub>(r)}E<sub>1</sub>(r)<sup>2</sup>/W with the computational FEFD results for the field of an example like the one shown in the plot of FIG. 5 and with a rectangular object of dimensions 30 cm×30 cm×1.5 m and permittivity ε=49+16i (consistent with human muscles) residing between the loops and almost standing on top of one capacitor (˜3 cm away from it) and find δQ<sub>abs(human)</sub>˜10<sup>5 </sup>and for ˜10 cm away δQ<sub>abs(human)</sub>˜5·10<sup>5</sup>. Thus, for ordinary distances (˜1 m) and placements (not immediately on top of the capacitor) or for most ordinary extraneous objects p of much smaller losstangent, we conclude that it is indeed fair to say that δQ<sub>abs(p)</sub>→∞. The only perturbation that is expected to affect these resonances is a close proximity of large metallic structures.
Selfresonant coils can be more sensitive than capacitivelyloaded coils, since for the former the electric field extends over a much larger region in space (the entire coil) rather than for the latter Oust inside the capacitor). On the other hand, selfresonant coils can be simple to make and can withstand much larger voltages than most lumped capacitors. Inductivelyloaded conducting rods can also be more sensitive than capacitivelyloaded coils, since they rely on the electric field to achieve the coupling.
For dielectric disks, small, lowindex, lowmaterialloss or faraway stray objects will induce small scattering and absorption. In such cases of small perturbations these extrinsic loss mechanisms can be quantified using respectively the analytical firstorder perturbation theory formulas
<FORM>[67 Q<sub>1,rad(p)</sub>]<sup>−1</sup>≡2δΓ<sub>1,rad(p)</sub>/ω<sub>1 </sub>∝∫d<sup>3</sup>r[RE{ε<sub>p</sub>(r)}E<sub>1</sub>(r)]<sup>2</sup>/W </FORM>
<FORM>[δQ<sub>1,abs(p)</sub>]<sup>−1</sup>≡2δΓ<sub>1,abs(p)</sub>/ω<sub>1</sub>=∫d<sup>3</sup>rIm{ε<sub>p</sub>(r)}E<sub>1</sub>(r)<sup>2</sup>/2W </FORM>
where W=∫d<sup>3</sup>rε(r)E<sub>1</sub>(r)<sup>2</sup>/2 is the total resonant electromagnetic energy of the unperturbed mode. As one can see, both of these losses depend on the square of the resonant electric field tails E1 at the site of the extraneous object. In contrast, the coupling factor from object 1 to another resonant object 2 is, as stated earlier,
<FORM>k<sub>12</sub>=2κ<sub>12</sub>/√{square root over (ω<sub>1</sub>ω<sub>2</sub>)}≈∫d<sup>3</sup>rε<sub>2</sub>(r)E<sub>2</sub>*(r)E<sub>1</sub>(r)/∫d<sup>3</sup>rε(r)E<sub>1</sub>(r)<sup>2 </sup></FORM>
and depends linearly on the field tails E<sub>1 </sub>of 1 inside 2. This difference in scaling gives us confidence that, for, for example, exponentially small field tails, coupling to other resonant objects should be much faster than all extrinsic loss rates (κ<sub>12</sub>>>δΓ<sub>1,2(p)</sub>), at least for small perturbations, and thus the energytransfer scheme is expected to be sturdy for this class of resonant dielectric disks.
However, we also want to examine certain possible situations where extraneous objects cause perturbations too strong to analyze using the above firstorder perturbation theory approach. For example, we place a dielectric disk close to another offresonance object of large Re{ε}, Im{ε} and of same size but different shape (such as a human being h), as shown in FIG. 30a, and a roughened surface of large extent but of small Re{ε}, Im{ε} (such as a wall w), as shown in FIG. 30b. For distances D<sub>h,w</sub>/r=10−3 between the diskcenter and the “human”center or “wall”, the numerical FDFD simulation results presented in FIGS. 30a and 30b suggest that, the disk resonance seems to be fairly robust, since it is not detrimentally disturbed by the presence of extraneous objects, with the exception of the very close proximity of highloss objects. To examine the influence of large perturbations on an entire energytransfer system we consider two resonant disks in the close presence of both a “human” and a “wall”. Comparing Table 8 to the table in FIG. 30c, the numerical FDFD simulations show that the system performance deteriorates from U˜150 to U<sub>(hw)</sub>˜0.510, i.e. only by acceptably small amounts.
In general, different examples of resonant systems have different degree of sensitivity to external perturbations, and the resonant system of choice depends on the particular application at hand, and how important matters of sensitivity or safety are for that application. For example, for a medical implantable device (such as a wirelessly powered artificial heart) the electric field extent must be minimized to the highest degree possible to protect the tissue surrounding the device. In such cases where sensitivity to external objects or safety is important, one should design the resonant systems so that the ratio of electric to magnetic energy density w<sub>e</sub>/w<sub>m </sub>is reduced or minimized at most of the desired (according to the application) points in the surrounding space.
The nonradiative wireless energy transfer techniques described above can enable efficient wireless energyexchange between resonant objects, while suffering only modest transfer and dissipation of energy into other extraneous offresonant objects. The technique is general, and can be applied to a variety of resonant systems in nature. In this Section, we identify a variety of applications that can benefit from or be designed to utilize wireless power transmission.
Remote devices can be powered directly, using the wirelessly supplied power or energy to operate or run the devices, or the devices can be powered by or through or in addition to a battery or energy storage unit, where the battery is occasionally being charged or recharged wirelessly. The devices can be powered by hybrid battery/energy storage devices such as batteries with integrated storage capacitors and the like. Furthermore, novel battery and energy storage devices can be designed to take advantage of the operational improvements enabled by wireless power transmission systems.
Devices can be turned off and the wirelessly supplied power or energy used to charge or recharge a battery or energy storage unit. The battery or energy storage unit charging or recharging rate can be high or low. The battery or energy storage units can be trickle charged or float charged. It would be understood by one of ordinary skill in the art that there are a variety of ways to power and/or charge devices, and the variety of ways could be applied to the list of applications that follows.
Some wireless energy transfer examples that can have a variety of possible applications include for example, placing a source (e.g. one connected to the wired electricity network) on the ceiling of a room, while devices such as robots, vehicles, computers, PDAs or similar are placed or move freely within the room. Other applications can include powering or recharging electricengine buses and/or hybrid cars and medical implantable devices. Additional example applications include the ability to power or recharge autonomous electronics (e.g. laptops, cellphones, portable music players, household robots, GPS navigation systems, displays, etc), sensors, industrial and manufacturing equipment, medical devices and monitors, home appliances (e.g. lights, fans, heaters, displays, televisions, countertop appliances, etc.), military devices, heated or illuminated clothing, communications and navigation equipment, including equipment built into vehicles, clothing and protectivewear such as helmets, body armor and vests, and the like, and the ability to transmit power to physically isolated devices such as to implanted medical devices, to hidden, buried, implanted or embedded sensors or tags, to and/or from rooftop solar panels to indoor distribution panels, and the like.
In some examples, farfield interference can be utilized by a system designer to suppress total radiation loss and/or to increase the system efficiency. In some examples, systems operating optimally closer to the radiative regime can benefit more from the presence of farfield interference, which leads to reduced losses for the subradiant normal mode of the coupled objects, and this benefit can be substantial.
A number of examples of the invention have been described. Nevertheless, it will be understood that various modifications can be made without departing from the spirit and scope of the invention.