High efficiency generation and detection of terahertz pulses using laser pulses at tele-communication wavelengths

A.Schneider et al.

OPTICS EXPRESS 5376/Vol.14,No.12(2006)

Itoh Lab.

Michitaka Bito

Outline

• Introduction Terahertz region THz Time-domain Spectroscopy (THz-TDS) Generation of THz pulse Coherence length lc• Experimental results and discussion Contour plot of the coherence length lc() Detection with DAST crystal Detection with ZnTe crystal

• Summary

Terahertz region

• There are many properties in THz region , for example intermolecular vibration, lattice oscillation, superconducting energy gap and so on.

• THz wave is absorbed by water.

3mm 30m

THz region0.1~10THz

300m 3m 300nm30mm300mm

1THz 10THz 0.1PHz 1PHz0.1THz10GHz1GHz

visible ray

1THz = 1012Hz ⇔ 300m ⇔ 4.1meV

electric wave light

Telecommunication wavelength1500~1560nm

THz Time-domain Spectroscopy (THz-TDS)

• THz-TDS can be carried out at room temperature and ambient air.

• Temporal waveform is detected.

• The complex refractive index of the sample can be estimated without Kramers-Kronig analysis.

fs pulse laserBeamsplitter

Delay stageTHz emitter

Sample

THz detector

Various optical constants are obtained easily !

Pump pulse

Probe pulse

inn ~

refractive index extinction coefficient

complex dielectric constantabsorption coefficientoptical conductivity

Generation of THz pulse

Difference-frequency generation (DFG)

Pump pulse DFG

Nonlinear optical crystal

Frequency

Inte

nsit

y

1 23

3 =｜ 1 - 2｜

DFG

DAST (4-N ,N-dimethylamino-4’-N’- methyl stilbazolium tosylate)

Gigantic nonlinear optical coefficient d(The highest of all known materials)

An efficient emitter of THz pulse with DFG

212

3 IIdI

dDAST:290dZnTe :90dGaAs:54

Coherence length lc

THzg

c

nn

ck

l

3

312

321

kkkk

Coherence length lc is given by

: Optical wavelengthng(): refractive index of group velocityn(): refractive indexl: length of nonlinear crystal1,2: angular frequency of optical pulse 3: angular frequency of THz pulse

Coherence length lc is the parameter of velocity-matching between the optical and the THz pulse.

velocity-matching condition is given by 1 2

3

Frequency

Inte

nsit

y

When lc is greater than l, THz pulse is generated efficiently.

Motivation

DAST has high conversion efficiency than ZnTe for generation.

ZnTe is more versatile than DAST for detection because organic crystals like DAST suffer from fast degradation and limited thickness.

The velocity-matched generation with DAST and detection with ZnTe at the wavelength 1500nm theoretically and experimentally

Why DAST for generation?

Why ZnTe for detection?

Why 1500nm?

Telecommunication wavelength range is from 1.5 to 1.56 m.

Contour plot of the coherence length lc()

Coherence length lc in DAST Coherence length lc in ZnTe

1500nm

750nm

Telecommunication wavelength are suited for generation and detection with DAST.

Frequency-doubled pulses is suited for velocity-matching at the 2 to 3 THz range in ZnTe.

Red:DAST (no coloring lc > 1.5 mm)Green:ZnTe (no coloring lc > 0.75 mm)

Combination of DAST and ZnTe

The broadband THz frequency range is accessible with a fundamental wavelength near 1500 nm.

Experimental setup

For EO sampling (ZnTe)For THz-induced lensing (DAST)

DAST

A fundamental wavelength is 1500 nm.At room temperature.

Source crystal (DAST): 0.6mm thickDetection crystal (DAST): 0.69mm thick

Source crystal (DAST): 0.6mm thickDetection crystal (ZnTe): 0.5mm thick

Detection with DAST crystal

The oscillation persists for t > 1 ps in time-domain and the numerous dips in frequency-domain

Influence of water vapor absorption

・ There are resonance frequencies of DAST at 1.1THz and 5.2THz.・ A continuous spectrum is obtained in the central part of the spectrum from 1.3 to 4.8 THz.

This shows the spectrum from 1.3 to 4.8 THz is velocity-matched to optical pulses with 1.5 m as theoretical calculation in DAST.

Source crystal (DAST): 0.6mm thickDetection crystal (DAST): 0.69mm thick

Detection with ZnTe crystal

・ lc < l (above 3.5THz)・ Absorption of ZnTe strongly increases above 4THz.

The spectrum extends only to 4 THz.

A continuous spectrum from 1.3 to 4 THz is observed.

Source crystal (DAST): 0.6mm thickDetection crystal (ZnTe): 0.5mm thick

Summary

• Authors demonstrated The velocity-matched generation with DAST and detection with ZnTe at the wavelength 1500nm theoretically and experimentally.

• The broadband THz frequency range is accessible with wavelength of 1500 nm. (theoretically)

• A continuous spectrum from 1.3 to 4 THz is obtained by EO sampling. (experimentally)

An agreement was obtained between theoretical calculation and experimental results.

Detection of THz wave

THz-induced lensing (TIL) Electro-optic (EO) sampling

Solid/dashed line: Probe beam diameter without/with THz field.

THz波 (ガウシアン )で屈折率が変化。それが平凸レンズみたいな役割。透過プローブ光の直径が変化。解析して時間波形。

Probe pulses

THz pulses

PhotoDiodes

PBSEOC

Waveplate

Anisotropy of refractive index by THz pulses. (Pockels effect)

Polarization of probe pulses vary from time variation of THz electric field.

Analysis of polarization variation

Time-domain spectrum of THz pulses

EEEEEE

PPP)3(

0)2(

0)1(

0 :

NLL

4,

4

2,

~

2

222

1

21

nn

i

Nonlinear dielectric polarization

EE

rkErkErE titit expexp,

cc

titNL

.0,0

exp2,2,*2

0

20

EEkk

rkkEEkkkrP