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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

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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
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(n,l)

Detection with DAST crystal

Detection with ZnTe crystal

  • Summary
terahertz region

THz region

0.1~10THz

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.

1GHz

10GHz

0.1THz

1THz

10THz

0.1PHz

1PHz

electric wave

light

visible ray

300mm

30mm

3mm

300mm

30mm

3mm

300nm

Telecommunication wavelength

1500~1560nm

1THz = 1012Hz ⇔ 300mm ⇔ 4.1meV

thz time domain spectroscopy thz tds
THz Time-domain Spectroscopy (THz-TDS)

Beamsplitter

Probe pulse

  • 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 laser

Pump pulse

Delay stage

THz emitter

THz detector

Sample

refractive index

extinction coefficient

complex dielectric constant

absorption coefficient

optical conductivity

Various optical constants are obtained easily !

generation of thz pulse
Generation of THz pulse

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

Difference-frequency generation (DFG)

Pump pulse

DFG

Nonlinear optical crystal

Gigantic nonlinear optical coefficient d

(The highest of all known materials)

dDAST:290

dZnTe :90

dGaAs:54

w3 =|w1 - w2|

w3

w1

w2

Intensity

DFG

An efficient emitter of THz pulse with DFG

Frequency

coherence length l c

w3

Intensity

w1

w2

Frequency

Coherence length lc

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

velocity-matching condition is given by

l: Optical wavelength

ng(l): refractive index of group velocity

n(l): refractive index

l: length of nonlinear crystal

w1,w2: angular frequency of optical pulse

w3: angular frequency of THz pulse

Coherence length lc is given by

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

motivation
Motivation

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

Why DAST for generation?

DAST has high conversion efficiency than ZnTe for generation.

Why ZnTe for detection?

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

Why 1500nm?

Telecommunication wavelength range is from 1.5 to 1.56 mm.

contour plot of the coherence length l c n l
Contour plot of the coherence length lc(n,l)

Coherence length lc in DAST

Coherence length lc in ZnTe

750nm

1500nm

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.

Combination of DAST and ZnTe

Red:DAST (no coloring lc > 1.5 mm)

Green:ZnTe (no coloring lc > 0.75 mm)

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

experimental setup
Experimental setup

For THz-induced lensing (DAST)

For EO sampling (ZnTe)

DAST

Source crystal (DAST): 0.6mm thick

Detection crystal (DAST): 0.69mm thick

Source crystal (DAST): 0.6mm thick

Detection crystal (ZnTe): 0.5mm thick

A fundamental wavelength is 1500 nm.

At room temperature.

detection with dast crystal
Detection with DAST crystal

Source crystal (DAST): 0.6mm thick

Detection crystal (DAST): 0.69mm thick

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

・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.

Influence of water vapor absorption

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

detection with znte crystal
Detection with ZnTe crystal

Source crystal (DAST): 0.6mm thick

Detection crystal (ZnTe): 0.5mm thick

The spectrum extends only to 4 THz.

・lc < l (above 3.5THz)

・Absorption of ZnTe strongly increases above 4THz.

A continuous spectrum from 1.3 to 4 THz is observed.

summary
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
Detection of THz wave

THz-induced lensing (TIL)

Electro-optic (EO) sampling

Probe pulses

Photo

Diodes

Waveplate

THz pulses

EOC

PBS

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

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

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。

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

Analysis of polarization variation

Time-domain spectrum of THz pulses

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