AFAD2018DCC in Daejeon

Laser-plasma-based THz Radiation Sources and Their Applications

Hyyong SukDepartment of Physics and Photon Science

Gwangju Institute of Science and Technology (GIST)Gwangju, Korea

Jan. 30, 2018

KSTAR-Superconducting Tokamak in Korea

Laser-plasma-based THz Generation Methods

Lensplasma

laser pulse +V

0V

EDC

(a) single color/pulse (b) external DC field is applied

ω+2ω

BBO

(c) two-color laser methodplasma

THz wave

High-power (> 1 MW) broadband THz pulses can be produced by laser-plasma interactions

20 TW/35 fs Ti:sapphire Laser System at GIST

THz Generation by Laser and Plasma

Plasma filament

Si Filter

Lens

PlasmaPhoto-diode

Probe pulse

ZnTe

QWP

Polarizer

THz

0 5 10 15 20-0.0010

-0.0008

-0.0006

-0.0004

-0.0002

0.0000

0.0002

THz

ampl

itude

(A. U

.)

Time (ps)

Enhanced THz Generation by DC Field

6 8 10 12 14 16 18 20

-1

0

1

2

0 1 2 3 40.0

0.5

1.0

Am

plitu

de (a

. u.)

Frequency (THz)

THz

field

(a. u

.)

Time (ps)

4 kV/cm 8 kV/cm 12 kV/cm 16 kV/cm 20 kV/cm

2 2cos 1 expTHz ext p cE E t t

ext

c

E

: external DC electric field : e-neutral collision frequency

(1) Beam splitter (R= 8 %, T = 92 %)(2) Delay line(3) THz generator(4) OAP(5) Silicon plate (laser beam cutting)(6) Beam dump(7) Inductively-coupled plasma

source(8) THz detector (EO sampling)(9) ZnTe crystal(10) Quarter-wave plate(11) Wollaston prism (polarizer)(12) Balanced photo-detector

Schematic for the Experiment

Lensf = 250 mm Electrode

ICP source

AnalogGauge

DigitalGauge

ZnTeQWP

Balanced PD

Gas in

Pump

OAP

OAP

Polarizer

Pump beam

Probe beam

THzSiliconplate

Beam dump

plasma source: Inductively-coupled plasma (ICP) source

Ar Plasma

and : Fourier transformed spectraof the THz field with and without the plasma

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.52

4

6

8

10

Phas

e sh

ift (r

ad)

Frequency (THz)

Measurement Fit

910 mTorr Ar with 500 W RF power ① 910 mTorr, 400 W② 910 mTorr, 500 W③ 1,040 mTorr, 500 W

①

③

②

2 22

0

, , 10 .2

p ep

e

n eL L cmc m

;Phase shift :

6 8 10 12 14 16 18 20-2

-1

0

1

2

THz

field

(a. u

.)

Time (ps)

THz without plasma THz with plasma

exp expp

r

Ei L

E

%

% pE % rE %

THz Wave with and without Plasma

• Experimental condition : pressure=910mTorr (Ar), RF power=500 W• Plasma length : 10 cm

Experiment vs. Calculation

ne = 5.31013 cm-3

6 8 10 12 14 16 18 20-2

-1

0

1

2TH

z fie

ld (a

. u.)

Time (ps)

THz without plasma Experiment Calculation

0.4 0.6 0.8 1.0 1.20

2

4

6

8

10

550 W

500 W

400 W

Elec

tron

dens

ity (1

013 /c

m3 )

Pressure (Torr)

350 W

E/g = 0.0122

22rad

ePn g p

E

P : RF power, p : gas pressureE : electric fieldΛ : inverse of the density gradient (uniformity)ηεrad : energy loss by the radiationg : constant depending on the geometrical configuration

Plasma Density vs. Pressure

300 400 500 6000

2

4

6

8

10

1.04 Torr

0.78 Torr

0.52 Torr

E/E/g = 0.0122

Elec

tron

dens

ity (1

013 /c

m3 )

Power (W)

Plasma Density vs. RF Power

Lock-in Amplifier

Inductively-coupled plasma source

BBO

ZnTe

QWP

Wollaston prism

Balanced photodetector

Electro-optic sampling methodPath delay line

Si wafer

Probe beam

Pump beam

Laser induced filament

f = 300 mm

f = 250 mm

OAP 1f = 101.6 mm

OAP 2f = 152.4 mm

Ti:Sapphire fs laser1 kHz, 3 mJ, 35 fs

Optical chopperWorking at 250 Hz

Two-color-based THz-TDS

Two-color Method : broader bandwidth

Broader bandwidth THz pulse can cover higher densities

FTIR (Fourier Transform Infrared)detection system

Single-shot Method

probe beam

pump beam

Conclusions

Strong laser-plasma interactions can generate broadband THz pulses

Laser-plasma-based THz methods can be used for high-density (1013 <ne< 1017 cm-3) plasma diagnostics

Sing-shot method based on the spectral interferometry is also possible

Thank you