Laser-plasma-based THz Radiation Sources and Their ... · THz Generation by Laser and Plasma Plasma...
Transcript of Laser-plasma-based THz Radiation Sources and Their ... · THz Generation by Laser and Plasma Plasma...
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