Ultrabroadband detection of terahertz radiation

Ultrabroadband detection of terahertz
radiation from 0.1 to 100THz with
photoconductive antenna
Itho lab
M1 Taisuke katashima
Contents
・Introduction of THz wave
・experiment
experimental setup and results
(detection of ultrabroadand THz radiation)
experimental setup and results
(estimating sensitivity of PC antenna)
・summary
What is a THz wave ?
THz region:0.1~10THz
Microwaves
Visible X-ray γ-ray
Frequency(Hz)
103
killo
Example
106
mega
Radio
109
giga
Radar
101
2
tera
101
5
peta
Optical Medical
・Frequency: 1THz=1012Hz
・Wavelength: 1THz
・Wavenumber
1018
exa
300µm
33cm-1
1021
zetta
Application of THz
・THz Time-domain Spectroscopy (TDS)
This system can detect the waveform of electric field.
We can obtain the amplitude and phase of electric field
by Fourier transform.
Complex refractive index is able to be calculated
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 ( )
2
Refractive index: n0
E t ()
~n
Sample
0
Ei ()
d
x
 ( )  t ( )  i ( )
c
c
ln T ( )
 ( )  n0 Extinction cofficient:  ( )  
Refractive index: n( ) 
2d
d
Complexrefrctiveindex : n~( )  n( )  i ( )
THz –TDS setup
splitter
mirror
The background of this study
The material whose character changes in the ultrabroad frequency range
needs the system which can spectroscope this in the same range.
(example: La2-xSrxCuO4)
It is essential to generate and detect
ultrabroadband THz radiation
to realize THz-TDS in ultrabroad frequency range.
Purpose of these experiments
Previous experiments
Kono et al. reported THz radiation over 60 THz with a
ZnTe emitter and a PC antenna.
This experiments
The purpose is to detect THz radiation up to 100 THz
with a GaSe emitter and a PC antenna detector
and to investigate the sensitivity of PC antenna.
Experimental methods
Experiment 1
generation: PC antenna
detection: PC antenna
Experiment 2
generation: GaSe crystal
detection: PC antenna
Investigating the sensitivity of PC antenna
Measuring black-body radiation at 500℃
by using monochromator and MCT (HgCdTe) detector
Comparing the experimental spectrum with the Planck distribution
Response function of monochromator and MCT detector.
Experimental setup 2
Ti:Sapphire Laser
delay stage
probe
BS
pump
chopper
Si BS
GaSe emitter
PC antenna
I/V amp
lock-in amp
computer
Experimental setup 1
Ti:Sapphire Laser
probe
BS
pulse width: 10fs, center wavelength: 800nm
repetition: 78MHz
pump
chopper
A
THz
radiation
(a)
Si BS
(a)
(b)
Gap size: 5µm
(b)
I/V amp
lock-in amp
computer
PC antenna
Low-temperature grown GaAs substrate (LT-GaAs), Au electrodes
Experimental results 1
Amplitude (a.u.)
100
10
Reflection at the boundary between
an LT-GaAs substrate and the air
-1
10-2
10-3
0
5
10
15
20
25
Frequency (THz)
Fourier-transformed power spectrum of the THz electric field
Many dips are caused by the vapor absorption.
Monotonously decreasing spectral distribution
from 0.1 to 25 THz was observed.
30
Experimental results 2
8
phonon absorption of CdSe
Log intensity (a.u.)
Intensity (a.u.)
6
4
2
0
-2
-4
-6
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Time (ps)
absorption of CO2
0 20 40 60 80 100 120 140 160
Frequency (THz)
Temporal waveform of the THz electric field Fourier-transformed power spectrum
Frequency component up to 100THz was observed.
Response function of monochromator and MCT detector
Experimental setup
Si BS
Monochromator
MCT detector
Black-body furnace at 500℃
Response function of this system
is calculated.
Response of the system (a.u.) Intensity (a.u.)
Black-body radiation
CO2 absorption
This system has a sensitivity up to 95 THz.
Response function
95THz
60
70
80
90 100 110
Frequency (THz)
THz radiation spectrum detected with
monochromator and MCT detector.
30
Experimental setup
Si BS
Monochromator
Intensity (a.u.)
25
THz radiation
20
15
10
95THz
5
0
GaSe crystal
MCT detector
40 50 60 70 80 90 100 110
Frequency (THz)
THz radiation up to 95 THz was detected
with monochromator and MCT detector.
Comparison of calibrated spectrum with experimental spectrum
We calculate the calibrated spectrum with the response function.
Log intensity (a.u.)
Calibrated spectrum
PC antenna detection
0
20
40 60 80 100 120 140 160
Frequency (THz)
The calibrated spectrum and experimental one are alike.
Smooth response of the PC antenna detector
Estimating the sensitivity from a model calculation
・The photocurrent by incident THz radiation at a time delay t

J (t )  e  E (t ' ) N (t 't )dt'

J ( )  E ( )  N ( )
Fourier transform
・The currier number

N (t )   G( ) D(  t )d

N ( ) G ( ) ×
D( )
Fourier transform
・The photocurrent is described by noted above equations.
J ( )  E ( )  D( )  G( )
D(ω)・G(ω) corresponds to sensitivity of PC antenna
E(t) : THz electric field, N(t) : the number of photocarriers
G(t) : shape of probe laser pulse, D(t) : time response of carriers
Sensitivity of the PC antenna detector
d : width of probe beam
101
Experimental data
10-1
τd= 5fs
Sensitivity (arb. units)
Experimental data is consistent
with calculated one.
100
10-2
The pulse width is similar to
that of this experiment laser.
τd= 15fs
τd= 10fs
10-3
Noise level
10-4
0
20 40 60 80 100 120 140 160
Frequency (THz)
Sensitivity of the PC antenna detector
If more broad and strong THz radiation is obtained,
PC antenna has the sensitivity over 130THz.
d : width of probe beam
101
100
Experimental data
10-1
τd= 5fs
Sensitivity (arb. units)
Log intensity (a.u.)
Summary
10-2
τd= 15fs
τd= 10fs
10-3
Noise level
10-4
0 20 40 60 80 100 120 140 160
Frequency (THz)
0
20 40 60 80 100 120 140 160
Frequency (THz)
Sensitivity of the PC antenna detector
・THz radiation up to 100THz was observed
with GaSe crystal emitter and PC antenna detector.
・PC antenna is capable of detecting THz radiation beyond 100 THz.
.
We obtained a monotonously decreasing spectral response
of the PC antenna from 0.1 to 100 THz of PC antenna.
First experiment: using PC antenna emitter and PC antenna detector
Second experiment: using GaSe crystal and PC antenna detector