Ultrabroadband detection of THz radiation and the sensitivity estimation of photoconductive antennaItoh labMichitaka Bitoh

H. Shimosato et al. Ultrafast Optics V, (Springer, Berlin, 2007) pp. 317-323

Outline

• Introduction THz region

　　 THz time-domain spectroscopy (THz-TDS) THz emission THz detection　　 Motivation

• Experimental results and discussions ultrabroadband detection from 0.1 to 100 THz

Investigation of the sensitivity of the PC antenna

• Summary

Terahertz region

3mm 30mm300mm 3mm30mm

1THz = 1012Hz ⇔ 300mm ⇔ 4.1meV

THz region0.1~100THz

1THz 10THz 100THz0.1THz10GHz

light・ intermolecular vibration・ lattice oscillation・ superconducting gap　　　　　　　　　　　　　　　　 etc

In THz region

21

tyconductivi optical

i

6K17.5K24K27K30K33K30K<Tc<33K

In THz region, a depletion of the real part of the conductivity which correspond to the opening of a superconducting gap can be observed 　 like this.

Real part of conductivity σ1(ω) for the MgB2 film normalized to its normal state value σ1N(40K).

electric wave

THz Time-domain Spectroscopy (THz-TDS)

inn ~

fs pulse laser

Beamsplitter

Delay stageTHz emitter

Sample

THz detector

Pump pulse

Probe pulse

ref

~E sam

~E

Complex refractive index

From n and κ, many optical coefficient can be obtained.

)vacuumsample(1~

~2

)samplevacuum(1~

2

n

nt

nt

sv

vs

)(

THz

)(

)(2

1)(

~

i

ti

eE

dtetEE

Amplitude( 振幅 )phase( 位相 )

c

dnitt

E

Et

1~exp

~

~~

svvs

ref

sam

Fresnel coefficients tvs, tsv

nn

i

2,

~:constantdielectricComplex

222

1

21

/4:tcoefficien Absorption

4/)1(,2/

~:tyconductivi Optical22

21

21

nn

i

THz emission from PC antenna

fs laser pulse

Biastransient current J (t)

Forward THz waveBackward THz wave

t

tJtE

)(

)(THz

Decay time of J : ~100fs ETHz(ω):~10THz

(LT-GaAs substrate)

THz emission via DFGDFG (Difference-frequency generation) ・・・差周波発生

THz wave

I(t,ω,Δω)ETHz(t,ωTHz)

NL crystalfs laser pulse

∬

Inte

nsity

0

Frequency spectrum of

fs pulse laser

frequency 0

Inte

nsity

Δω

Frequency spectrum of THz wave

from NL crystal

213

2

Differential frequency generation

ω3 =0 　⇒ optical rectification

1

Δω

Integration of

different frequency

frequency

THz detection using PC antenna

fs laser pulse

THz wave

A

t = t1 t = t2 > t1 t = t3 > t2

t : time delay

transient current J(t)

fieldelectricincident:

tyconductivioptical:

tE

t

tdtEtttJ

Motivation

Authors demonstrated 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 mid-infrared region.

THz region0.1~100THz

1THz 10THz 100THz0.1THz10GHz

electric wave light

MgB2:Δ=3.4THzYBCO:Δ=4.9THz

superconducting gap

lattice vibration

spectral range of THz-TDS

Outline

• Introduction THz region

THz emission THz detection THz time-domain spectroscopy (THz-TDS) Motivation

• Experimental results and discussions Ultrabroadband detection from 0.1 to 100 THz

Investigation of the sensitivity of the PC antenna

• Summary

Setup

A5um400um

LT-GaAs substrate

emitter detector

10fs

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 discussions 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)

tdtEttNe

tdtEtttJ

The current by the incident THz radiation at delay time t is described as

ENJ Correspond to the sensitivity

According to the convolution theorem of the Fourier transformation:

tdttRtItN

I(t): the shape of probe laser pulseR(t): time response of carriers

The carrier number N(t) is described by the following equation:

RIN This is transformed into the following equation:

tNet

E(t): the incident electric fieldN(t): number of photocarrierse: the elementary electric chargeμ: the electron mobilityσ: optical conductivity

I(t)

-20 0 20Time(fs)

N(t)

R(t)

2.01.00.0Time(ps)

The PC antenna sensitivity (model calculation)

d

thtI

76.1

sec 2

6589.0,2069.0

4.2exp8.5exp

BA

tBtAtR

Authors assumed the shape of probe pulse as the following equation

τd : the pulse width

R(t) is estimated from the transient reflection measurement.

I(t) 　 and R(t) are transformed into I(ω) and R(ω) respectively

10fs

τd=10(fs)

1.0

0.8

0.6

0.4

0.2

0.0

Inte

ncity

(a.u

.)

-30 -20 -10 0 10 20 30

Time(fs)

Experimental date and calculated one

Good agreement between experimental data and calculated one with τd=10fs.

Noise level

This results indicates the feasibility of ultrabroadband detection over 130 THz using the PC antenna if broader and stronger THz radiation obtained.

By using 5fs pulse laser, the more sensitive and broadband detection is expected with the PC antenna.

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

By using 5fs pulse laser, the more sensitive and broadband detection is expected with the PC antenna.

Time (fs)

Inte

nsity

(a.u

.)

My study

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

My study

Absorption of DAST itself

THz radiation from 100 to 170 THz was obtained

My study

There is good agreement between experimental data 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

0.1

1

10

100

Inte

ncit

y(a.

u.)

150100500

Frequency(THz)

antennaGaSeDAST