スライド 1

Ultrabroadband Detection of
THz radiation and the
sensitivity estimation of
Photoconductive antenna
Itoh lab
Michitaka Bitoh
Outline
• Introduction
THz region
THz emission
THz detection
THz time-domain spectroscopy (THz-TDS)
Motivation
• Experimental results and discussion
ultrabroadband detection from 0.1 to 100 THz
Investigation of the sensitivity of the PC antenna
• Summary
Terahertz region
10GHz 0.1THz
1THz
10THz
100THz
THz region
0.1~100THz
electric wave
30mm 3mm
300mm 30mm 3mm
1THz = 1012Hz ⇔ 300mm ⇔ 4.1meV
In THz region
light
・intermolecular vibration
・lattice oscillation
・superconducting energy gap
etc
Transmitted waveform through MgB2 (b)6K(●)20K(○)27K(◆)30K(◇)33K(■)
T=6K(solid line), T=40K(dashed line) (c)6K(●)20K(○)25K(◆)30K(◇)36K(■)
THz emission from PC antenna
fs laser pulse
Backward THz wave
Forward THz wave
transient current J (t)
Bias
J (t )
ETHz (t ) 
t
lifetime of carrier : ~1ps
THz emission via DFG
DFG (Difference-frequency generation) ・・・差周波発生
fs laser pulse
NL crystal
Differential frequency generation
THz wave
1
10 fs ～
I(t,ω,Δω)
3  1  2
2
ETHz(t,ωTHz)
ω3 =0 ⇒ optical rectification
Integration of
different frequency
∬
0
frequency
Intensity
Intensity
Δω
ωTHz
0
Frequency spectrum of
frequency
Frequency spectrum of THz wave
fs pulse laser
from NL crystal
THz detection using PC antenna
fs laser pulse

J t     t   t E t dt 

 t  : electrical conductivity
THz wave
E t  : incident electric field
A
transient current J(t)
t : time delay
t = t1
t = t2 > t1
t = t3 > t2
THz Time-domain Spectroscopy (THz-TDS)
Beamsplitter
fs pulse
laser
Delay stage
1
~
E ( ) 
2
Probe pulse
Pump pulse
THz emitter

E
THz
~
~
Eref   Esam  
THz detector
(t )e it dt

 E ( ) e
位相
振幅
~
E  
~
t    ~sam
E ref  
 n~    1d 
 t vs  t sv expi

c


i (  )
Fresnel coefficients tvs, tsv
2
t vs  ~
(vacuum sam ple)
n 1
2n~
t sv  ~
( sam ple vacuum)
n 1
Sample
Complex reflective index
~   n   i  
n
From n and κ, many optical coefficient
can be obtained.
Complexdielectricconstant: ~    1    i 2  
 1    n 2   2 ,  2    2n
Absorptioncoefficient :   4 / 
Opticalconductivity :      2   / 4
Motivation
Authors suggested the detection sensitivity beyond 100 THz
from a model calculation by using 10 fs laser, and they also
aimed to detect ultrabroad THz radiation with ultrashort pulse
laser.
They demonstrate ultrabroadband detection from 0.1 to 100
THz using the PC antenna detector, in two generation method.
Investigation of the sensitivity of the PC antenna for midinfrared region.
Outline
• Introduction
THz region
THz emission
THz detection
THz time-domain spectroscopy (THz-TDS)
Motivation
• Experimental results and discussion
Ultrabroadband detection from 0.1 to 100 THz
Investigation of the sensitivity of the PC antenna
• Summary
Setup
10fs
LT-GaAs substrate
400um
emitter
5um
detector
A
The PC antenna emission and detection
Water vapor absorption
The reflection of THz radiation by phonon
(between LT-GaAs and the air)
THz radiation from 0.1 to 25 THz was observed monotonously
(except around 8 THz)
DFG generation and the PC antenna detection
Phonon absorption of GaSe crystal
Absorption by CO2
THz radiation was observed up to 100THz
(except for the frequency range from0.1 to 10 THz)
Outline
• Introduction
THz region
THz emission
THz detection
THz time-domain spectroscopy (THz-TDS)
Motivation
• Experimental results and discussion
ultrabroadband detection from 0.1 to 100 THz
Investigation of the sensitivity of the PC antenna
• Summary
The PC antenna sensitivity (experimental date)
Response function
of this system
This system can detect the frequency up to 95 THz.
The response around 70 THz is distorted by the absorption by CO2.
The PC antenna sensitivity (experimental date)
Calibrated power spectrum
of the THz radiation
Extracted power
spectrum from…
Because of the absorption of CO2,
this region is unreliable.
Calibrated power spectrum of the THz radiation was obtained.
From (a) and (b), the sensitivity of PC antenna can be
calculated.
The PC antenna sensitivity (model calculation)
The current by the incident THz radiation at delay time t is described as

E(t): the incident electric field
J t    t   t E t dt 

N(t): number of photocarriers

e: the elementary electric charge
 e N t   t E t dt 

μ: the electron mobility
σ: electrical conductivity
 t   eN t 


According to the convolution theorem of the Fourier transformation:
J    N  E 
Correspond to the sensitivity
The carrier number N(t) is described by the following equation:

N t    I t Rt   t dt 
This is transformed into the following equation:
N    I  R 
I(t)
I(t): the shape of probe laser pulse
R(t): time response of carriers
Intencity(a.u.)

-20
0.0
0.5
0
20
Time(fs)
R(t)
N(t)
1.0
1.5
Time(ps)
2.0
The PC antenna sensitivity (model calculation)
2  1.76t 

I t   sec h 
 d 
Intencity(a.u.)
Authors assumed the shape of probe pulse1.0 as the following equation
τd : the pulse width
0.8
0.6
10fs
0.4
τd=10(fs)
0.2
0.0
-30 -20 -10
0
10
20
30
R(t) is estimated from the transient reflection measurement.
Time(fs)
Rt   A exp 5.8t   B exp 2.4t 
A  0.2069, B  0.6589
I(t) and R(t) are transformed into I(ω) and R(ω) respectively
Experimental date and calculated one
Good agreement between
experimental date and
calculated one.
Noise level
To use 5fs pulse laser, the
more sensitive detection is
expected with the PC
antenna.
This results indicates the feasibility of ultrabroadband
detection over 130 THz using the PC antenna if broader and
stronger THz radiation obtained.
Summary
• By using the PC antenna for detection,
spectrum response from 0.1 to 100 THz except
around 8 THz was obtained using a
combination of PC antenna emitter and a
GaSe crystal.
• The sensitivity of the PC antenna from a
model calculation shows that the PC antenna
is capable of detecting ultrabroadband THz
radiation beyond 100 THz with an advanced
setup.
My study
To use 5fs pulse laser, the more sensitive
detection is expected with the PC antenna.
My study
My study
Absorption of DAST itself
THz radiation from 100 to 170 THz was obtained
My study
There is good agreement between experimental date
and model calculation when the pulse duration is 5fs.
Future plan is ・・・
・To detect THz wave at broader frequency range
with this system
・To measure something with this system
THz Time-domain Spectroscopy (THz-TDS)
fs pulse
laser
Delay stage
Beamsplitter
Probe pulse
Pump pulse
THz emitter
• 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.
THz detector
Sample
~   n   i  
n
refractive index
extinction coefficient
complex dielectric constant
absorption coefficient
optical conductivity
Various optical constants are obtained easily !

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