Generation and detection of ultrabroadband terahertz radiation

Itoh laboratory Taisuke Katashima

Y. C. Shen, et al., Appl. Phys. Lett. 85, 164 (2004)

Contents・・ Introduction of THz waveIntroduction of THz wave

・ Generation and detection of THz wave

・・ ExperimentsExperiments

・ SummarySummary

・ Applications of THz wave

・・ Previous experimentsPrevious experiments

・・ Experimental resultsExperimental results

What is a THz wave ?

・ Frequency: 1THz=1012Hz

・ Wavelength: 1THz 300µm

103 106 109 10181012 1015

killo mega giga tera peta exa1021

zetta

THzTHz region: region:0.1~10THz0.1~10THz

VisibleVisible X-rayX-ray γγ-ray-rayMicrowavesMicrowaves

Frequency(Hz)

ExampleExample RadioRadio RadarRadar OpticalOptical MedicalMedical AstrophysicsAstrophysics

33cm-1・Wavenumber

Applications of THz

・・ THz imazingTHz imazing

・ THz Time-domain Spectroscopy (TDS)

・ Security screening

・ 3D-rendered imaging

・・ THz THz spectroscopyspectroscopy

・ Imaging of teeth, cancerous tissue and flames ・・・ etc.

THz -TDS

We can get the amplitudeamplitude and phasephase of electromagnetic wave at the same time!!

splitter

mirror

The principle of THz-TDS

2

)(

)()(

i

t

E

ET )()()( it

)(ln2

)()()( 0

Td

cn

d

cn 　 　　 　 　 　 　 　 　 　 　 　

d

)(E i

)(E t Sample n~

THz wave

c

dniEE

c

dniEE

t

i

)(~exp)(

exp)(

0

00

0: n

)()()(~:refraction ofindex Complex inn

0x

Refractive indexAt x=d

Extinction cofficient:Refractive index:

THz-imagingTHz-imaging

Security screeningImaging of teethhttp://www.agri.tohoku.ac.jp/thz/jp/41_h15.htmhttp://www.teraview.com/home_index.htm

THz imaging is safer than X-ray imaging

Generation and detection of THz wave

Two major elements to generate THz wave with using fs laserTwo major elements to generate THz wave with using fs laser

・ nonlinear crystal(NL crystal)

・ photoconductive antenna (PC antenna)

Photoconductive antennaPhotoconductive antenna

THz waveTHz wave

Generation: Photoconductive antenna

fs laser pulsefs laser pulse

Electrode

Gap

fs laser pulse is focused on the gap

Generation of carriers

Transient current

Emission of THz wave

t

titETHz

)()(

Detection:Photoconductive antennaTHz waveTHz wave

fs laser pulsefs laser pulse

preamplifier

Lock-in amp

PC

①fs laser pulse is focused on the gap

②Generation of carriers

③Carriers are accelerated by THz electric field

Current signal of ETHz(t)Carriers movementCarriers movement

Ã

)()( tEtJ THz

Experiments: previous study

GenerationGeneration: LT-GaAs photoconductive antenna

DetectionDetection: 20-µm-thick ZnTe crystal using electro-optic sampling

Frequency components over 30THzover 30THz were observed!!

Y. C. Shen, et al Appl., Phys. Lett. 83, (2003)

(EO sampling)

Experimental results:previous study

(1))Fourier-transform amplitude spectrum Fourier-transform amplitude spectrum

The dips at 5.25.2THz THz and the peaks at 6.26.2THz THz are TO TO and LOLO phonon resonance in ZnTe, respectively.

・・ Over 30THzOver 30THz were observed!!

(TO =Transverse-optical, LO =Longitudinal-

optical )

Low frequency componentsLow frequency components 　　were not observed!!were not observed!!

This spectral distribution up to 8THz is not ideal for spectroscopy.

Purpose of this study

・ Generate and detect ultrabroadband THz waveGenerate and detect ultrabroadband THz wave

Using LT-GaAs PC emitter and receiver instead of PC emitter and ZnTe receiver

To reduce the phonon resonances of ZnTe crystalthe phonon resonances of ZnTe crystal

（ This complicated measurement of THz signals in previous study ）

・・ Apply Apply this systemthis system for spectroscopyfor spectroscopy

Sample: Maltose pellet

Measuring method: detecting transmitted light

Experiments

・・ LT-GaAs PC antennaLT-GaAs PC antenna

GapGap: 400μm

Electrode

1.0μm-thick LT-GaAs layer

0.53-mm-thick GaAs substrate

・・ SamplesSamples・ Mixed maltose polycrystalline powermaltose polycrystalline power with polyethylene powderpolyethylene powder pellet

(in a mass ratio 1:10)・ Thickness 1.3mm

Experimental set up

delay stagefs pulsed Ti-sapphire

・ 15fs duration

・ Center wavelength 790nm

・ Repetition 76MHz

Pump beam:400mW

Probe beam:30mWProbe beam:30mW

Parabolic mirror

PC antenna

PC

Lock-in amp

: Vacuum-tight box purged with dry nitrogen gas

Beam splitter

sample ÃPC antenna

(a) Temporal THz wave form(a) Temporal THz wave form

Experimental results

Distinct dips Distinct dips atat 8.0THz and peaks at 8.7THz THz and peaks at 8.7THz are are TO and LO phonon resonance in GaAs.

・・ Low frequency componentsLow frequency components were observed. were observed.

They can’t be observed in previous study.

We can observe sWe can observe smooth spectral distribution up to 8THz!!mooth spectral distribution up to 8THz!!

・・ Distinct dips Distinct dips at at 5.2THz5.2THz and peaks at and peaks at 6.2THz6.2THz were were reducedreduced!!!!

Experimental results (spectroscopy)

・・ Conventional THz-TDSConventional THz-TDS ・・・・・・ frequency range 0.3~3.0THz

・・ This systemThis system ・・・・・・・・・・・・・・・・・・・・・・ frequency range 0.3~8.0THz0.3~8.0THz

114 vibrational modes 4 vibrational modes are observed at room temperatureat room temperature

Corresponding to inter- and intra-molecular interactions

Fourier-transform amplitude spectrum of pure polyethylene and maltose/polyethylene

pure polyethylene

maltose/polyethylene

maltose

Summary

・ PC-generation/PC-detection schemePC-generation/PC-detection scheme leads to smooth spectral distribution up to 8THz8THz..

・ PC-detection provides about 8 times better signal-to-noise-ratio than EO-detection up to frequencies in excess of 8THz.

PC-generation/PC-detection schemePC-generation/PC-detection scheme is an ideal systeman ideal system for THz-TDS in the frequency range 0.3-8.0THz.

Comparison with THz-TDS and other spectroscopy

THz-TDS: 19 vibrational modes of cytidine were observed in the frequency range 1-20THz

Fourier-transform infrared spectroscopy:5vibrational modes

Raman spectroscopy:14vibrational modes

Neutron inelastic scattering spectroscopy:all vibrational modes can be observed in principal

mary

Generation:Optical rectification of NL crystal

Optical rectification second-order non-linear optical effects

Non-linear crystal

1

23 THz waveTHz wave

fs laser pulse:

fs laser pulse:

)()();()( 21213,,.

)2(3

kjzyxkj ijki

p ω

①fs laser pulses falls on the NL-crystal

②Nonlinear polarization generates

③Electromagnetic wave ω3 is emitted ＊ ω3= ω1-ω2

Detection: Optical rectification of EO crystal

:preamplifier

:delay stage

:lock-in amp

λ/4 plate

beam splitter

EO crystal

THz waveTHz wave

photo diode

PC :sampling pulse (fs-laser pulse)fs-laser pulse)

・ Electric field of THz induces anisotropy of refraction index in a crystal

sampling pulse proves change in intensity

Abstract・ Recent years terahertz science and technology have developed

・ THz technology

Longitudinal-optical (LO) phonon resonance in ZnTe

(b)

Experimental results:previous study(1)Temporal THz waveformTemporal THz waveform

(2))Fourier-transform amplitude spectrumFourier-transform amplitude spectrum

・ The dips at 5.25.2THzTHz

・ The peaks at 6.26.2THzTHz

Full width at half maxima is 40fs40fs

Over 30THzOver 30THz are observed!!

LO phonon resonance in ZnTe

TO phonon resonance in ZnTe (TO =Transverse-optical)

(LO =Longitudinal-optical )

It’s the shortestshortest THz pulse with using PC emitter

3D-renderd image(two plastic cylinders)

Optical image Refractive index of each cross sectional slice

Surface-rendered image

・ Electrodes Reciever:Ti/Pd/AuEmitter:NiCr/Au

・ Carrier lifetime Reciever:10psEmitter:0.1ps

c

dn)(n~iexp

)(E

)(E0

i

t