1. Terahertz Applications by THz Time Domain Spectroscopy
Nature Materials 1, (2002)
Bradley Ferguson and Xi-Cheng Zhang
Ashida lab.
Iwasa Takuya

2. Contents About THz THz Spectroscopy System FTS THz-TDS
Data of THz wave detection by EO-sampling
THz Applications
Summary

3. About THz THz radiation is from 1011 to 1013Hz approximately.
THz wave goes through materials such as plastic, paper, ceramic and semiconductor and has low energy.
It has been difficult to detect frequencies in this region.

4. THz Spectroscopy System Fourier Transform Spectroscopy (FTS)
In FTS a sample is illuminated with a incoherent broadband thermal source.
(ex. an arc lamp)
A direct detector such as a helium-cooled bolometer is used to detect the interference signals.

5. THz Spectroscopy System Time Domain Spectroscopy（THz-TDS）
THz-TDS uses short pulses of broadband THz radiation.
The transmitted THz electric field is measured coherently, which provides both high sensitivity and time-resolved phase information. t
THz wave
Probe laser E
Time delay
THz-TDS pump probe system
Principle of THz-TDS

6. THz Emitter Photoconductive antenna Difference frequency generation
Nonlinear crystal
Bias
voltage
fs laser
THz wave fs
laser
Fourier transform
THz
wave
Intensity
Intensity
J(t): current ω2 ω1 ω３
frequency
frequency

7. THz Detector Photoconductive antenna EO-sampling A
Balance detector
Bias
voltage A
Wallaston
prism fs
laser
THz
wave
λ/4 plate
EO crystal
Probe laser
μ: mobility of electron
N: the number of carrier
THz wave

8. Electro-optic sampling (EO-sampling)
EO crystal becomes birefringent under the irradiation of THz wave.
λ/4 plate change probe laser’s polarization linear into circular.
Wallaston prism divides vertical and horizontal polarization.
Balance detector
Wallaston
prism
λ/4 plate
EO crystal
Probe laser
THz wave

9. Data detected by EO-sampling (2002)
Frequency spectrum of the THz field.
The absorption resonance at 5.3THz is due to phonon modes in the ZnTe crystals.
Temporal waveform of a THz pulse.
Emitter: 27μm ZnTe
Detector: 30μm ZnTe

10. Data detected by EO-sampling (2004)
This data is from “Ultrabroadband detection of multi-terahertz field transients with GaSe electro-optic sensors: Approaching the near infrared” American Institute of Physics C.Kubler
Emitted with 20-μm-thick GaSe and detected with 30-μm-thick GaSe.
The amplitude spectrum peaks at 33.8THz and extends from 7 to beyond 120THz.
Appl. Phys. Lett. 85, 3360 (2004)

11. THz Applications
Semiconductor and high-temperature superconductor characterization
Tomographic imaging
Cellular level imaging and chemical and biological sensing

12. THz Applications An optical image of the test structure.
Tomographic imaging
An optical image of the test structure.
The target was imaged using the tomography system, and the refractive index of each cross-sectional slice was reconstructed. The central slice is shown.
The cross-sectional slices are combined to form 3D image.

13. THz Applications
Cellular level imaging and chemical and biological sensing
Detected by electro-optic sampling in a 30-μm-thick ZnTe crystal.
This spatial resolution is less than 50μm.
THz image of an onion cell membrane.

14. Summary
THz-TDS (especially used by EO-sampling) has been developed and observed broader region.
It made a lot of applications including tomographic imaging and biological sensing.