Ultrabroadband terahertz generation using DAST single crystal

Ashida lab.

Takuya Iwasa

APPLIED PHYSICS LETTERS 97, 021105-1I.Katayama, R.Akai, M.Bito, H.Shimosato, K.Miyamoto, H,Ito and M.Ashida

Contents

• Introduction

• Motivation

• Experimental setup

• Results & discussion

• Summary

• My work

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

Electromagnetic wave

• THz radiation is from 1011 to 1013Hz approximately.

• If we generate broader radiation, we can measure from THz region to optical region in continuity.

Micro wave Submilimeter wave

Far infrared

Near infrared

Wavelength

FrequencyTHz region

optical

broader

30mm 3mm 300μm 30μm 3μm 300nm

10GHz 0.1THz 1THz 10THz 100THz 1PHz

THz emitter

• A 400-μm-thick DAST crystal was used as nonlinear crystal.

• A DAST crystal is an organic compound crystal and has large nonlinear optical coefficient.( 非線形光学係数）

Nonlinear crystal

THz wavefs laser

ω1ω2

frequency

Inte

nsit

y Inte

nsit

y

ω ３

213

Fourier Fourier transform

frequency

Difference frequency generation

DAST crystal

THz detectorPhotoconductive antenna

')'()'()( dtttNtEetJ

μ: mobility of electron

N: the number of carrier

Current meter

fslaser

THzwave

A

Motivation

• To generate the terahertz wave using a DAST crystal by considering the refractive index dispersion.

• To detect the THz wave generated by a DAST crystal using monochromator and THz-TDS.

Experimental setup

(Spectral Phase Interferometry for Direct Electric-field Reconstruction)

BaF2 (a positive dispersion medium)

Short wavelengt

h

Long wavelength

Positive dispersion medium

Time lag

Group velocity dispersion ( 郡速度分散 )

Positive chirping

pulse image

Results & discussion The data of laser phase spectra observed using SPIDER.

BaF2 thickness― 0mm― 2mm― 4mm

0mm-thick BaF2 2mm-thick BaF2 0mm-thick BaF2

Negative chirping Positive chirping

Short wavelength

Long wavelength

Results & discussion

BaF2 thickness― 0mm― 2mm― 4mm

The best of these data is the one of negative chirping.

Experimental setup

BS

Chirp mirror

Band-pass filter

Delay stage

Chopper

I/V amp lock-in amp PC

DAST400mm

Ti:Sapphire laser

PC antenna

BaF2 platePump pulse

Probe pulse

Results & discussion

In this graph, very high frequency part is present.

Results & discussion

BaF2 thickness― 0mm― 2mm― 4mm

The Fourier transform of the electric waveform. (dotted line)

Results & discussion

Short wavelength

Long wavelength The energy

is absorbed a lot.

DAST (Large positive

dispersion)

The region of generating

High-frequency THz wave

Short wavelength

Long wavelength

0mm-thick BaF2 2mm-thick BaF2

Summary

• In the DAST crystal, the group velocity dispersion is important for the generation of the ultrahigh-frequency terahertz wave.

• A waveform is detected by using photoconductive antenna.

My work

I try to measure a wave form by using this optical system when there is a sample in front of PC antenna.

Diffraction grating

SlitSlit

Concave mirror

Monochromator

HgCdTe and InAs Detector

THz pulse

SPIDER 法 (Spectral Phase Interferometry for Direct Electric-field Reconstruction)

)()(cos)()(2)()()(222

EEEES

SPIDER interferogram

Nonlinear Crystal

I (t)

Replica1Replica2

(t)

I (t)

Pulse to be measured and its duplication

Intense strongly-chirped pulse from Ti: sapphire amplifier

Sum frequency

I ()

I ()

Interferogram

21