Self-referencing frequency comb based on high-order sideband generation

• US 10,490,974 B2
• Filed: 03/30/2018
• Issued: 11/26/2019
• Est. Priority Date: 03/31/2017
• Status: Active Grant

• Current Assignee: Regents of the University of California (University of California)
• Original Assignee: Regents of the University of California (University of California)
• Inventors: Sherwin, Mark, Banks, Hunter, Valovcin, Darren
• Primary Examiner(s): Peace, Rhonda S
• Application Number:
  US15/941,777
• Publication Number:
  US 20180301868A1
• Time in Patent Office: 606 Days
• Field of Search: None
• CPC Class Codes: G02F 1/35 Non-linear optics
• CPC Class Codes: G02F 2203/56 Frequency comb synthesizer
• CPC Class Codes: H01S 5/041 Optical pumping
• CPC Class Codes: H01S 5/3408 characterised by specially shaped wells, e.g. triangular
• CPC Class Codes: H01S 5/34313 with a well layer having only As as V-compound, e.g. AlGaAs, InGaAs
• CPC Class Codes: H01S 5/3432 the whole junction comprising only (AI)GaAs
• CPC Class Codes: H01S 5/3434 with a well layer comprising at least both As and P as V-compounds
• CPC Class Codes: H01S 5/34346 characterised by the materials of the barrier layers
• CPC Class Codes: H01S 5/34353 based on (AI)GaAs
• CPC Class Codes: H01S 5/3438 based on In(Al)P
• CPC Class Codes: H03K 4/06 having triangular shape

Abstract

A frequency comb generator including a semiconductor, wherein the semiconductor outputs a frequency comb in response to frequency mixing of an optical field and at terahertz field in the semiconductor using a high order sideband (HSG) mechanism. The frequency comb spans a bandwidth sufficient for self-referencing and may be used in optical clock applications, for example.

Claims

1. A frequency comb generator, comprising:

a semiconductor outputting a frequency comb in response to frequency mixing of an optical field and a terahertz field in the semiconductor, wherein;

the semiconductor has a bandstructure, including a valence band and a conduction band, such that the semiconductor outputs the frequency comb in response to the terahertz field driving electrons and holes in the bandstructure, after the optical field excites the electrons into the conduction band and excites the holes into the valence band.

2. The frequency comb generator of claim 1, wherein the frequency comb has a bandwidth spanning at least one octave.

3. The frequency comb generator of claim 1, wherein the frequency comb has a bandwidth spanning at least 67% of an octave.

4. The frequency comb generator of claim 1, wherein the frequency comb has a bandwidth sufficient for 3f-2f self referencing.

5. The frequency comb generator of claim 1, further comprising a transmission line on the semiconductor, wherein the transmission line may include a resonator that enhances the terahertz field so that the transmission line couples the terahertz field into semiconductor with an electric field strength of at least 10 kV/cm.

6. The frequency comb generator of claim 1, wherein a THz frequency and electric field strength of the terahertz field are chosen by maximizing the product of the THz frequency and electric field strength available at that frequency.

7. The frequency comb generator of claim 1, wherein:

the semiconductor comprises a bulk semiconductor, and

the bulk semiconductor outputs the frequency comb in response to the terahertz field driving electrons and holes in the bulk semiconductor, after the optical field excites the electrons into the conduction band of the bulk semiconductor and excites the holes into the valence band of the bulk semiconductor.

8. The frequency comb generator of claim 7, wherein the bulk semiconductor comprises bulk GaAs, bulk Al_xGa_1-xAs (0<

x≤

1), or bulk In_xGa_1-xAs_yP_1-y (0≤

x≤

1, 0≤

y≤

1) and where the thickness of the bulk semiconductor is between 0.2 and 20 microns thick.

9. The frequency comb generator of claim 7, wherein the bulk semiconductor comprises In_0.53Ga_0.47As that is about 1 μ

m thick.

10. The frequency comb generator of claim 1, wherein the bandstructure comprises a quantum well structure.

11. The frequency comb generator of claim 10, wherein the quantum well structure comprises GaAs quantum wells and Al_xGa_1-xAs barriers.

12. The frequency comb generator of claim 10, wherein the quantum well structure comprises In_xGa_1-xAs_yP_1-y lattice-matched to InP.

13. The frequency comb generator of claim 10, wherein the quantum well structure comprises one or more In_xGa_1-xAs wells with 0.4<

x<

0.6, the In_xGa_1-xAs wells having In_yAl_1-yAs barriers with 0.4<

y<

0.6 or InP barriers.

14. The frequency comb generator of claim 10, wherein the composition and width of the quantum wells are chosen in order to achieve, simultaneously:

a. a desired two dimensional (2D) band gap (the energy difference between a top of the highest valence subband band and a bottom of the lowest conduction subband) is between 500 meV and 1000 meV;

b. an offset between the bottom of the lowest electron subband (in the quantum well structure) and the continuum states (in the barrier between quantum wells) that is at least 0.5 times the 2D band gap so that the electrons with kinetic energy sufficient to emit the sidebands with a photon energy at least 1.5 times the band gap do not escape from the quantum wells.

15. The frequency comb generator of claim 1 coupled to an optical clock.

16. The frequency comb generator of claim 15, wherein:

one tooth of the frequency comb is locked to a microwave frequency standard, and

another tooth of the frequency comb is locked to an optical frequency standard, such that the optical frequency standard stabilizes the microwave frequency standard via the frequency comb.

17. The frequency comb generator of claim 1 coupled to:

a source emitting the terahertz field having a frequency in a range of 200-900 GHz, and

a source emitting the optical field having a wavelength of 700 nm-3 micrometers.

18. The frequency comb generator of claim 1 coupled to a source comprising a frequency multiplier chain outputting the terahertz field, wherein the frequency multiplier chain converts a microwave field into the terahertz field.

19. The frequency comb generator of claim 1, wherein:

the semiconductor has crystallographic directions,

the optical field and terahertz field each have a linear polarization, and

a direction of the linear polarization of the terahertz field with respect to the crystallographic directions, and a direction of the linear polarization of the optical field with respect to the terahertz field, are chosen together to maximize a bandwidth of the frequency comb.

20. The frequency comb generator of claim 1, wherein the optical field and the terahertz field each have a linear polarization and the optical field is polarized at an angle θ

with respect to the terahertz field, with 45 degrees<

θ

<

135 degrees.

21. A method of generating a frequency comb, comprising:

obtaining a semiconductor comprising a bandstructure including a valence band, a conduction band, and a bandgap;

irradiating the semiconductor with an optical field having a frequency corresponding to an energy equal to or larger than the bandgap, so that the optical field excites electrons into the conduction band and holes into the valence band;

using a terahertz field to drive the electrons in the conduction band and drive the holes in the valence band; and

selecting an intensity of the terahertz field, a timing of the terahertz field with respect to the optical field, and the bandstructure such that the semiconductor outputs a frequency comb having a bandwidth sufficient for self-referencing.

22. A frequency comb generator, comprising:

a semiconductor outputting a frequency comb in response to frequency mixing of an optical field and a terahertz field in the semiconductor using a high order sideband generation (HSG) mechanism, wherein;

the semiconductor has a bandstructure, including a valence band and a conduction band, such that the optical field excites electrons into the conduction band and excites holes into the valence band;

the HSG mechanism comprises;

the terahertz field;

ionizing the electrons and the holes, and

accelerating the electrons and the holes first away from each other and then back towards each other, and

when the electrons and the holes recombine, emitting excess kinetic energy of the electrons and holes in sidebands with higher frequency than the optical field; and

the frequency comb comprises the sidebands.