Generation of terahertz pulses through optical

High efficiency generation and
detection of terahertz pulses
using laser pulses at telecommunication 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(n,l)
Detection with DAST crystal
Detection with ZnTe crystal
• Summary
Terahertz region
1GHz 10GHz 0.1THz 1THz
THz region
0.1~10THz
electric wave
300mm
30mm
10THz 0.1PHz
3mm
300mm 30mm
1THz = 1012Hz ⇔ 300mm ⇔ 4.1meV
•
•
1PHz
light
visible ray
3mm 300nm
Telecommunication wavelength
1500~1560nm
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.
THz Time-domain Spectroscopy (THz-TDS)
Beamsplitter
fs pulse laser
Probe pulse
Pump pulse
Delay stage
THz emitter
THz detector
Sample
•
•
•
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.
~   n   i  
n
refractive index
extinction coefficie
complex dielectric constant
absorption coefficient
optical conductivity
Various optical constants are obtained easily !
Generation of THz pulse
Difference-frequency generation (DFG)
Pump pulse
DAST (4-N ,N-dimethylamino-4’-N’methyl stilbazolium tosylate)
DFG
Nonlinear optical crystal
I 3   d 2 I 1 I 2 
Intensity
3 =｜1 - 2｜
3
1
2
Gigantic nonlinear optical coefficient d
(The highest of all known materials)
dDAST:290
dZnTe :90
dGaAs:54
DFG
Frequency
An efficient emitter of THz pulse
with DFG
Coherence length lc
velocity-matching condition is given by
1   2  3
k  k  2   k 1   k 3 
Coherence length lc is given by
lc 


k
c
3 n g l   nlTHz 
Intensity
Coherence length lc is the parameter of velocity-matching
between the optical and the THz pulse.
3
1
2
Frequency
l: Optical wavelength
ng(l): refractive index of group velocity
n(l): refractive index
l: length of nonlinear crystal
1,2: angular frequency of optical pulse
3: angular frequency of THz pulse
When lc is greater than l, THz pulse
is generated efficiently.
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 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
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
The oscillation persists for t > 1 ps in
time-domain and the numerous dips in
frequency-domain
Influence of water vapor absorption
Source crystal (DAST): 0.6mm thick
Detection crystal (DAST): 0.69mm thick
・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 mm as theoretical calculation in DAST.
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
• 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
Detection of THz wave
THz-induced lensing (TIL)
Electro-optic (EO) sampling
Probe pulses
Photo
Diodes
Waveplate
THz pulses
EOC
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。
PBS
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
Nonlinear dielectric polarization
P  P L  P NL
  0  (1)  E   0  ( 2) : EE   0  (3)  EEE  
Er, t   E   exp it  k  r   E   expit  k  r 
E    E  
P NL r, t    0  2  2    ,2k  k  k   E  E   exp i   t  k  k   r 
  0  2  0    ,0  k  k   E  E  *  c.c
~    1    i 2  
 1    n 2   2 ,  2    2n
 2  
4

,    
l
4

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