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Transcript
Generation and detection of
ultrabroadband terahertz radiation
Y. C. Shen, et al., Appl. Phys. Lett. 85, 164 (2004)
Itoh laboratory
Taisuke Katashima
Contents
・Introduction of THz wave
・Applications of THz wave
・Generation and detection of THz wave
・Previous experiments
・Experiments
・Experimental results
・Summary
What is a THz wave ?
THz region:0.1~10THz
Visible X-ray γ-ray
Microwaves
Frequency(Hz)
103
killo
Example
106
mega
Radio
109
giga
1012
tera
Radar
1015
peta
・Wavenumber
1021
zetta
Optical Medical Astrophysics
・Frequency: 1THz=1012Hz
・Wavelength: 1THz
1018
exa
300µm
33cm-1
Applications of THz
・THz spectroscopy
・THz Time-domain Spectroscopy (TDS)
・THz imazing
・3D-rendered imaging
・Imaging of teeth, cancerous tissue and flames ・・・ etc.
・Security screening
THz -TDS
splitter
mirror
We can get the amplitude and phase of electromagnetic wave
at the same time!!
The principle of THz-TDS
THz wave
At x=d
  n0d

Ei ( )  E0 exp i 
  

 c
  n~( )d

Et ( )  E0 exp i 
  
c


Et ( )
T ( ) 
Ei ( )
Refractive index: n( ) 
2
Refractive index : n0
E t ()
~n
Sample
0
Ei ()
d
x
 ( )  t ( )  i ( )
c
c
 ( )  n0 :  ( )  
ln T ( )
Extinction cofficient
d
2d
Complex index of refraction : n~( )  n( )  i ( )
THz-imaging
http://www.teraview.com/home_index.htm
Imaging of teeth
THz imaging is safer than X-ray imaging
http://www.agri.tohoku.ac.jp/thz/jp/41_h15.htm
Security screening
Generation and detection of THz wave
Two major elements to generate THz wave with using fs laser
・photoconductive antenna (PC antenna)
・nonlinear crystal(NL crystal)
Generation: Photoconductive antenna
fs laser pulse
THz wave
Photoconductive antenna
i (t )
ETHz (t ) 
t
fs laser pulse is focused on the gap
Gap
Generation of carriers
Transient current
Electrode
Emission of THz wave
Detection:Photoconductive antenna
THz wave
fs laser pulse
J (t )  ETHz (t )
Ã
preamplifier
①fs laser pulse is focused on the gap
②Generation of carriers
Lock-in amp
PC
③Carriers are accelerated by THz electric field
Carriers movement
Current signal of ETHz(t)
Experiments: previous study
Y. C. Shen, et al Appl., Phys. Lett. 83, (2003)
Generation: LT-GaAs photoconductive antenna
Detection: 20-µm-thick ZnTe crystal using electro-optic sampling
(EO sampling)
Frequency components over 30THz were observed!!
Experimental results:previous study
(1)Fourier-transform amplitude spectrum
・Over 30THz were observed!!
Low frequency components
were not observed!!
The dips at 5.2THz and the peaks at 6.2THz are TO and LO phonon
resonance in ZnTe, respectively.
(TO =Transverse-optical, LO =Longitudinal-optical )
This spectral distribution up to 8THz is not ideal for spectroscopy.
Purpose of this study
・Generate and detect ultrabroadband THz wave
To reduce the phonon resonances of ZnTe crystal
(This complicated measurement of THz signals in previous study)
Using LT-GaAs PC emitter and receiver instead of PC emitter and ZnTe receiver
・Apply this system for spectroscopy
Sample: Maltose pellet
Measuring method: detecting transmitted light
Experiments
・LT-GaAs PC antenna
Gap: 400μm
Gap
0.53-mm-thick GaAs substrate
1.0μm-thick LT-GaAs layer
・Samples
Electrode
・Mixed maltose polycrystalline power with polyethylene powder pellet
(in a mass ratio 1:10)
・Thickness 1.3mm
Experimental set up
Parabolic mirror
PC antenna
PC antenna
sample
Lock-in amp
Ã
PC
Pump beam:400mW
fs pulsed Ti-sapphire
Probe beam:30mW
Beam splitter
delay stage
・15fs duration
・Repetition 76MHz
・Center wavelength 790nm
: Vacuum-tight box
purged with dry nitrogen gas
Experimental results
・Low frequency components were observed.
They can’t be observed in previous study.
・Distinct dips at 5.2THz and peaks at 6.2THz
were reduced!!
Distinct dips at 8.0THz and peaks at 8.7THz
are TO and LO phonon resonance in GaAs.
(a) Temporal THz wave form
We can observe smooth spectral distribution up to 8THz!!
Experimental results (spectroscopy)
・Conventional THz-TDS・・・frequency range 0.3~3.0THz
・This system・・・・・・・・・・・frequency range 0.3~8.0THz
pure polyethylene
14 vibrational modes are observed
at room temperature
maltose/polyethylene
Corresponding to interand intra-molecular interactions
maltose
Fourier-transform amplitude spectrum of pure polyethylene and maltose/polyethylene
Summary
・PC-generation/PC-detection scheme
leads to smooth spectral distribution up to 8THz.
・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 scheme is an ideal system
for THz-TDS in the frequency range 0.3-8.0THz.
Comparison with THz-TDS and other spectroscopy
Fourier-transform infrared spectroscopy:5vibrational
modes
Raman spectroscopy:14vibrational modes
Neutron inelastic scattering spectroscopy:all vibrational modes can be observed
in principal
THz-TDS: 19 vibrational modes of cytidine were observed
in the frequency range 1-20THz
mary
Generation:Optical rectification of NL crystal
Optical rectification
fs laser pulse:
1
second-order non-linear optical effects
2
3 THz wave
fs laser pulse:
①fs laser pulses falls on the NL-crystal
Non-linear crystal
②Nonlinear polarization generates
( 2)
pi (ω3)   j .k  x , y , z  ijk
(3 ; 1  2 ) j (1 )k (2 )
③Electromagnetic wave ω3 is emitted
*ω3= ω1-ω2
Detection: Optical rectification of EO crystal
EO crystal
λ/4 plate
THz wave
:preamplifier
beam splitter
photo diode
:lock-in amp
:delay stage
:sampling pulse (fs-laser pulse)
PC
・Electric field of THz induces anisotropy of refraction index in a crystal
sampling pulse proves change in intensity
(b)
Abstract
・Recent years terahertz science and technology have developed
・THz technology
Longitudinal-optical (LO)
phonon resonance in ZnTe
Experimental results:previous study
(1)Temporal THz waveform
Full width at half maxima is 40fs
It’s the shortest THz pulse
with using PC emitter
(2)Fourier-transform amplitude spectrum
Over 30THz are observed!!
・The dips at 5.2THz
TO phonon resonance in ZnTe
(TO =Transverse-optical)
・The peaks at 6.2THz
LO phonon resonance in ZnTe
(LO =Longitudinal-optical )
3D-renderd image(two plastic cylinders)
Optical image
Refractive index of each cross sectional slice
Surface-rendered image
・Carrier lifetime Emitter:0.1ps
Reciever:10ps
・Electrodes
Reciever:Ti/Pd/Au
Emitter:NiCr/Au
E t ()
d 
 ~
 exp  in ()  n 0 

E i ()
c 
