Tuning-fork based near field probe for spectral measurement, near-field microscope using the same, and spectral analysis method using near-field microscope

• US 9,606,052 B2
• Filed: 07/24/2014
• Issued: 03/28/2017
• Est. Priority Date: 07/29/2013
• Status: Active Grant

• Current Assignee: Postech Academy-Industry Foundation
• Original Assignee: Postech Academy-Industry Foundation
• Inventors: Han, Hae Wook, Do, Young Woong, Moon, Ki Won
• Primary Examiner(s): Green, Yara B
• Application Number:
  US14/339,955
• Publication Number:
  US 20150028210A1
• Time in Patent Office: 978 Days
• US Class Current: 1/1
• CPC Class Codes: G01N 21/35 using infrared light G01N21/39 takes precedence
• CPC Class Codes: G01N 21/3586 by Terahertz time domain spectroscopy [THz-TDS]
• CPC Class Codes: G01Q 30/02 Non-SPM analysing devices, e.g. SEM [Scanning Electron Microscope], spectrometer or optical microscope
• CPC Class Codes: G01Q 60/22 Probes, their manufacture, or their related instrumentation, e.g. holders

Abstract

The present invention is provided to remove scattering from other parts, except for an end part of a nano-probe, in a near-field microscope, and to enable a spectral analysis by delaying the generation of multiple reflections caused through the shaft of the nano-probe. A first characteristic of the present invention is to temporally delay generation of multiple reflections by manufacturing a probe portion to have a predetermined length or more in a tuning-fork based near-field probe. A second characteristic of the present invention is to provide a near-field microscope which includes a tuning-fork based near-field probe having a structure as above, and can measure a time-domain transient reaction of a scattered wave. A third characteristic of the present invention is to provide a method for performing a spectral analysis on a time-domain signal measured by the near-field microscope.

Claims

1. A tuning-fork based near-field probe for spectral measurement, comprising:

a first electrode and a second electrode arranged apart from each other; and

a wire-shaped nano-probe downward attached to a one-side end of the second electrode and configured to vibrate in a perpendicular direction with respect to a sample,

wherein the nano-probe comprises;

a shaft for receiving a terahertz pulse incident through a means for focusing light;

an end part for localizing the received terahertz pulse to interact with the sample, and scattering a terahertz pulse, into air, which has obtained local information of the sample in a localizing procedure; and

a tapered region for connecting the shaft and the end part, wherein a length of the nano-probe, which is defined as a sum of a length of the shaft and a length of the tapered region is formed to be longer than a radius of a focus of a beam by the terahertz pulse which is focused by the means for focusing light.

2. The tuning-fork based near-field probe according to claim 1, wherein a material of the nano-probe is tungsten or a conductive metal.

3. The tuning-fork based near-field probe according to claim 1, wherein, in order to stabilize vibration of the nano-probe, the first electrode is provided on a one-side end thereof with a knife edge having a length and a weight which are equal to those of the nano-probe.

4. The tuning-fork based near-field probe according to claim 3, wherein the knife edge is made of a material equal to that of the nano-probe, and has a shape of a blade at an end part thereof.

5. The tuning-fork based near-field probe according to claim 1, wherein the shaft has a diameter of 1 μ

m or more, and the end part has an available radius of 10 nm or more.

6. The tuning-fork based near-field probe according to claim 1, wherein the length of the nano-probe is equal to or greater than 3 mm.

7. The tuning-fork based near-field probe according to claim 1, wherein the length of the nano-probe is equal to or greater than c/2f₁ where the terahertz pulse is a broadband pulse light source having a frequency band in a range of f₁-f₂ (where “

c”

is the velocity of light, f₁ is a first frequency of the terahertz pulse, f₂ is a second frequency of the terahertz pulse, and f₁<

f₂).

8. The tuning-fork based near-field probe according to claim 1, wherein the length of the nano-probe is equal to or greater than “

cT/2”

(where “

c”

is the velocity of light and “

T”

is a duration of a transient reaction of the scattered terahertz pulse generated by interaction between the end part and the sample).

9. A near-field microscope using a tuning-fork based near-field probe for spectral measurement, the near-field probe microscope comprising:

an optical system configured to include a plurality of beam splitters, a mirror, and a lens for transferring a laser pulse generated by a fast-pulse laser to a terahertz pulse generation device, and to include one or more beam splitters and a plurality of mirrors for transferring a part of the laser pulse generated by the fast-pulse laser to an electric field detector;

a sample stand configured to enable a location and a height thereof to be controlled;

a plurality of first optical components for transferring and focusing a terahertz pulse generated by the terahertz pulse generation device on a sample;

a tuning-fork based near-field probe comprising a first electrode and a second electrode arranged apart from each other, and a wire-shaped nano-probe downward attached to a one-side end of the second electrode and configured to vibrate in a perpendicular direction with respect to the sample, wherein the nano-probe includes a shaft for receiving a terahertz pulse focused through the first optical components, an end part for allowing the focused terahertz pulse to interact with the sample and scattering a terahertz pulse, into air, which has been subjected to an interaction procedure, and a tapered region for connecting the shaft and the end part;

an AC voltage generation device connected to the first and second electrodes of the near-field probe in order to vibrate the near-field probe;

an electric field detector for measuring an alternating current flowing through the tuning-fork based near-field probe;

a control device for controlling a height of the sample stand to uniformly maintain a distance between the end part of the near-field probe and the sample;

a plurality of second optical components for focusing a terahertz pulse directly reflected from the sample and a part of a pulse scattered from the end part of the near-field probe onto the surface of the electric field detector;

a lock-in amplifier for demodulating a signal detected through the electric field detector with a vibration frequency of the nano-probe or harmonic of the nano-probe, thereby extracting a component modulated by vibration of the nano-probe;

a mechanical optical delay unit for measuring a transient reaction of the terahertz pulse in a time domain; and

a control personal computer (control PC) for analyzing the measured transient reaction in a frequency domain, and measuring a characteristic spectrum of the sample,

wherein a length of the nano-probe, which is a sum of a length of the shaft and a length of the tapered region, is longer than a radius of a focus of a terahertz beam focused by a means for focusing light.

10. The near-field microscope according to claim 9, wherein the near-field probe is driven with 25-32 kHz.

11. The near-field microscope according to claim 9, wherein a material of the nano-probe is tungsten or a conductive metal.

12. The near-field microscope according to claim 9, wherein, in order to stabilize vibration of the nano-probe, the first electrode is provided on a one-side end thereof with a knife edge having a length and a weight which are equal to those of the nano-probe.

13. The near-field microscope according to claim 12, wherein the knife edge is made of a material equal to that of the nano-probe, and has a shape of a blade at an end part thereof.

14. The near-field microscope according to claim 9, wherein the shaft has a diameter of 1 μ

m or more, and the end part has an available radius of 10 nm or more.

15. The near-field microscope according to claim 9, wherein the length of the nano-probe is equal to or greater than 3 mm.

16. The near-field microscope according to claim 9, wherein the length of the nano-probe is equal to or greater than c/2f₁ where the terahertz pulse is a broadband pulse light source having a frequency band in a range of f₁-f₂ (where “

c”

is the velocity of light, f₁ is a first frequency of the terahertz pulse, f₂ is a second frequency of the terahertz pulse, and f₁<

f₂).

17. The near-field microscope according to claim 9, wherein the length of the nano-probe is equal to or greater than “

cT/2”

(where “

c”

is the velocity of light and “

T”

is a duration of a transient reaction of the scattered terahertz pulse generated by interaction between the end part and the sample).

18. The near-field microscope according to claim 9, wherein the terahertz pulse generation device comprises an electro-optical crystal or a photoconductive antenna including metal electrodes facing each other on a semiconductor crystal or a semiconductor crystal, such as InAs, GaAs, or the like.

19. The near-field microscope according to claim 9, wherein the terahertz detection device comprises a photoconductive antenna or an electro-optical crystal.

20. The near-field microscope according to claim 9, wherein the direction of a magnetic field of a terahertz pulse generated by the terahertz pulse generation device is perpendicular to the probe.

21. A spectral analysis method using a near-field microscope, the method comprising the steps of:

(a) transferring an ultrafast laser pulse generated by a fast-pulse laser to a terahertz generation unit using a plurality of beam splitters, a mechanical chopper, a mechanical optical delay unit, and a mirror;

(b) transferring and focusing a terahertz pulse generated by the terahertz generation device onto a sample attached on a piezo-stage;

(c) mounting a tuning-fork based near-field probe above the sample and driving the near-field probe, the near-field probe comprising;

a wire-shaped nano-probe downward attached to a one-side end of a second electrode of first and second electrodes arranged apart from each other, and configured to vibrate in a perpendicular direction with respect to the sample; and

a wire-shaped knife edge attached to a one-side end of the first electrode to stabilize vibration of the nano-probe;

(d) measuring, through an electric field detector, an electric field of a pulse wave scattered from an end part of the near-field probe and a terahertz pulse directly reflected from the sample;

(e) demodulating a signal, which is detected through the electric field detector, with a vibration frequency of the nano-probe or a harmonic of the nano-probe through a lock-in amplifier, thereby extracting only a scattered wave of the nano-probe; and

(f) recoding a harmonic component detected through the lock-in amplifier while scanning a mechanical delay line, acquiring a time-domain signal, Fourier-transforming the acquired time-domain signal, and performing an analysis in a frequency domain, wherein characteristics of the sample are measured by removing second and third pulse waves by multiple reflections, performing a Fourier transform, and performing an analysis in a frequency domain.