Terahertz Applications byTHz Time Domain Spectroscopy

Ashida lab.

Iwasa Takuya

Nature Materials 1, 26 - 33 (2002)

Bradley Ferguson and Xi-Cheng Zhang

Contents

About THz THz Spectroscopy System

FTS THz-TDS

Data of THz wave detection by EO-sampling

THz Applications Summary

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.

THz Spectroscopy SystemFourier 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.

THz Spectroscopy SystemTime 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.

THz-TDS pump probe system

tTHz wave

Probe laser

E

tTime delay

Principle of THz-TDS

THz Emitter

fslaser THz

wave

Photoconductive antenna

Biasvoltage

Nonlinear crystal

THz wavefs laser

ω1ω2

frequency

Inte

nsit

y Inte

nsit

y

ω ３

213

Fourier Fourier transform

frequency

Difference frequency generation

t

tJtE

)(

)(

J(t): current

THz Detector

fslaser

THzwave

Photoconductive antenna

Biasvoltage

EO crystal

Probe la

ser

THz wav

e

Balance detector

Wallastonprism

λ/4 plate

A

EO-sampling

')'()'()( dtttNtEetJ

μ: mobility of electron

N: the number of carrier

Electro-optic sampling (EO-sampling)

EO crystal

Probe la

ser

THz wav

e

Balance detector

Wallastonprism

λ/4 plate

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.

EO-sampling

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

Data detected by EO-sampling (2004) 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.

This data is from “Ultrabroadband detection of multi-terahertz field transients with GaSe electro-optic sensors: Approaching the near infrared” American Institute of Physics 2004 C.Kubler

Appl. Phys. Lett. 85, 3360 (2004)

THz Applications

Semiconductor and high-temperature superconductor characterization

Tomographic imaging Cellular level imaging and chemical and

biological sensing

THz ApplicationsTomographic imaging

a. An optical image of the test structure.b. The target was imaged using the tomography system, and the

refractive index of each cross-sectional slice was reconstructed. The central slice is shown.

c. The cross-sectional slices are combined to form 3D image.

THz Applications

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.

Cellular level imaging and chemical and biological sensing

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.