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Intense Terahertz Generation and Spectroscopy of Warm Dense Plasmas. Kiyong Kim University of Maryland, College Park. Collaborators: Kishore Yellampalle George Rodriguez Toni Taylor Jim Glownia. LOS ALAMOS NATIONAL LABORATORY. Outline:. Background: - Terahertz (TH) science. - PowerPoint PPT Presentation
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Intense Terahertz Generation andSpectroscopy of Warm Dense Plasmas
Kiyong Kim
University of Maryland, College Park
Collaborators:
Kishore Yellampalle
George Rodriguez
Toni Taylor
Jim Glownia
LOS ALAMOS NATIONAL LABORATORY
• Background:- Terahertz (TH) science.
• Intense THz generation:
- Two-color photoionization.
• THz spectroscopy:- Warm dense plasmas.
Outline:
Biomolecules & proteinsFigure courtesy of Klaas Wynne
Rydberg atoms
molecules
Semiconductor nanostructures
Gaseous and solid-state plasmas
Phenomena at terahertz (THz) frequencies:
1 THz = 1012 Hz =1 ps = 300 m = 0.004 eV = 33.3 cm-1
Strong THz sources:
FEL SynchrotronsLinacs
Stanford, UCSB, FELIX SLAC, JLab,BNL ALS (BNL) Free electron lasing
* M. S. Sherwin et al., DOE-NSF-NIH Workshop on Opportunities in THz Science
Photo courtesy: DESYPhoto courtesy: ALS
Large facility THz sources*
Coherent synchrotron radiation
synchrotron radiation
Intense THz generation:
Two-color photoionization
Lens SHGTHz pulse
Two-color photoionization:
* M. Kress et al, Opt. Lett. 29, 1120 (2004); T. Bartel et al, Opt. Lett. 30, 2805 (2005); X. Xie et al, Phys. Rev. Lett. 96, 075005 (2006).
Four-wave mixing * THz = + - 2
2
plasma
But the third order nonlinearity originating from bound electrons of ions ((3)
ions) and free electrons ((3)
free-electrons) via ponderomotive or thermal effects is too small to explain the measurements.
(3)plasma = (3)
ions + (3)free-electrons
2
BBO crystal
THz generation mechanism:
e- e- e- e- e-
e- e- e-
THz
Current surge THz generation
Directional quasi- DC current
THz energy measurement:
0 200 400 6000
200
400
600
800
1000
He
Air
N2
Ar
Kr
Pyr
oele
ctric
sig
nal (
a.u.
)
Pressure (torr)
0 5 10 15 200
200
400
600
800
Si + Teflon filter
Si filter
Si + P.E. filter
Pyr
oele
ctric
sig
nal
(m
V)
Laser energy (mJ)
THz energy vs pressure THz energy vs laser energy
• ETHz ~ 5 J/pulse with Kr (C.E. > 10-4)
K. Y. Kim et al., Nature Photonics doi:2008.153 (2008).
THz spectrum measurement:
0.0 0.3 0.6 0.90.5
1.0 Air 580 torr
Pyr
oele
ctric
sig
nal
(nor
m.)
Time delay (ps)
0.0
0.5
1.0 Ar 100 torr
Pyr
oele
ctric
sig
nal
(nor
m.)
0.0
0.5
1.0 Ar 10 torr
Pyr
oele
ctric
sig
nal
(nor
m.)
(a)
(b)
(c)
0 20 40 600
1
Spe
ctra
l pow
er(n
orm
.)
Frequency (THz)
Air 580 torr
0
1
Spe
ctra
l pow
er(n
orm
.)
Ar 100 torr
0
1
Spe
ctra
l pow
er(n
orm
.)
Ar 10 torr(a´)
(b´)
(c´)
Field autocorrelations Fourier-transform spectra
THz generation up to 75 THz (= 4 m)
THz spectroscopy:
Warm Dense Matter
0
r
)1()(
220
r
Drude model
Optical pump pulse
WDM
Electrical conductivity measurements of WDM:
Optical probe
H. M. Milchberg et al., Phys. Rev. Lett. 61, 2364 (1988).A. Ng et al., Phys. Rev. Lett. 72, 3351 (1994). A. N. Mostovych et al., Phys. Rev. Lett. 79, 5094 (1997).
AC (0)
0
(THz)
THz
Measure probe reflectivity
From the reflectivity, one can measure the electrical conductivity at the probe frequency. With THz probing, one can measure
quasi-DC conductivity directly.
THz conductivity measurements of WDM:
The quasi-DC electrical conductivity can be directly determined from THz probe reflectivity measurements.
Target (Aluminum)
Pump pulse
D ~ 1 mm
THz probe
To single-shot THz diagnostic
0.5 1.0 1.50.8
1.0
1.2
1.4
1.6
Al
GaAs (x10-1+0.8)
TH
z re
flect
ivity
rat
io, R
' p/R
p
Frequency (THz)
Al, 3 mJ Al, 4 mJ Al, 5 mJ Al, 9 mJ Al, 10 mJ
GaAs, 5 J/cm2
Experimental results I:
THz reflectivity for various pump energies
Breakdown of Drude model Possible pseudogap formation at the Fermi energy ???
K. Y. Kim et al., Phys. Rev. Lett. 100, 135002 (2008).
-20 0 20 40 60 80 100 120-0.2
-0.1
0.0
0.1
1013
1014
1015
1016
1017
1018
1019
Ref
lect
ivity
, R
/R
Delay (ps)
Simulation, 1013 W/cm2
Simulation, 1.5 x 1013 W/cm2
Reflectivity
Con
duct
ivity
r (
s-1)
Conductivity r
Experimental results II:
THz reflectivity vs delay
Conductor-to-insulator-like transition
Room temp. Al: r = 4.1 107 -1m-1 = 3.7 1017 s-1 [-1m-1] = 1.1 10-10 [s-1]
0.2 0.4 0.6 0.8 1.0
-0.08
-0.06
-0.04
-0.02
0.00
0.02
1014
1015
1016
1017
Ref
lect
ivity
, R
/R
Peak intensity (1013 W/cm2)
Reflectivity
Conductivity r
Con
duct
ivity
r (
s-1)
THz reflectivity vs intensity
Resistivity saturation
Experimental results III:
• THz generation via two-color photoionization:
– Generated intense (>5 J), super-broadband TH radiation (>75 THz).
– Developed a transient photocurrent model.
– Potential application for nonlinear THz optics and spectroscopy.
• THz spectroscopy for WDM:
– Directly measured the quasi-DC electrical conductivity of warm dense aluminum.
– Complements optical and x-ray diagnostics for WDM studies.
Summaries:
Backup slides:
Experimental setup:
THz energy measurement
BBO
THz pulse
THz spectrum measurement
3 filter
Si window
B-dot probe
B-dot probe & 3 measurement
BBO P.D
.
P.D.
Pyroelectric detector
d
P.D
.
Plasma
Strong THz field science*:
Nonlinear THz Optics• THz 2nd, 3rd nonlinear effects. • Extreme nonlinearity with ponderomotive energy > photon energy• THz-optical nonlinear mixing
Rapid THz imaging• Biomedical and security imaging
High magnetic field effects• 1 MV/cm 0.3 T• Pulsed electron spin resonance• THz spintronics
Strong THz sources
ETHz > 1 MV/cm
Photo courtesy: the Star Tiger
* M. S. Sherwin et al., DOE-NSF-NIH Workshop on Opportunities in THz Science
THz pump experiments• THz pumping of metals, insulators, and correlated electron materials.• Coherent band-gap distortion & phase transition.• THz-pump optical-probe experiments.• THz coherent control
Plasma current model I:
Electron drift velocity
)2sin(2
sin 21
ee
d m
eE
m
eE
])(2cos[)cos()( 21 tEtEtEL
Laser field
field
2 field
( = 800 nm) and 2 ( = 400 nm) lasers with relative
intensity of I = 1015 W/cm2 and I2 = 2 1014 W/cm2
(assuming 20% efficiency of frequency doubling)
-10
0
10
Lase
r fie
ld(1
08 V
/cm
)
(c)
(b)
(a)
0 3 6 9
-20
0
20
e- dis
plac
emen
t(n
m)
Time (fs)0 3 6 9
Time (fs)
-2 0 2
-10
0
10
e- dri
ft v
eloc
ity
(108 c
m/s
)
Phase (rad)
EL
e
-3 0 3
EL
e
Phase (rad)
= 0 = /2
: relative phase
: photoionization phase
K. Y. Kim et al., Opt. Express 15, 4577 (2007).
* Laser field:
* Ionization rate: Ea: atomic field
* Plasma current:
* THz field:
for Ea > E >> E2 and Ng >> Ne
* The function f(E) is highly nonlinear, not necessarily quadratic dominant.
The nonlinearity arises from extremely nonlinear tunneling ionization localized near the laser peaks.*
Plasma current model II:
)2cos(cos)( 2 tEtEtEL
)(3
2exp
)(4)(
tE
E
tE
Etw
L
a
L
aa
)()()( tvteNtJ e
sin)()(
2THz EEfdt
tdJE
a
aa
E
E
E
E
E
EEf
3
3
2exp)(
Simulation results I:
Simulation with = 0
ADK tunneling ionization and subsequent classical electron motion in the laser field are considered.
-40 -20 0 20 40 60 80
-2
0
2
0
2
4
6
Ele
ctro
n cu
rren
t(a
.u.)
Time (fs)
-0.2
0.0
0.2
Ne
(1019
cm
-3)
Lase
r fie
ld(E
/Eat)
(ii)
(i)
Simulation with = /2
-40 -20 0 20 40 60 80
-2
0
2
0
2
4
6
Time (fs)
-0.2
0.0
0.2
Ne
(1019
cm
-3)
(ii)
(i)
Quasi-DC currentI = 1015 W/cm2, I2 = 2 1014 W/cm2, 50 fs (FWHM)
Assumptions: No rescattering effect, No electron-ion or electron-neutral collisional processes, No space charge effect, No electron transport.
ZnTe
Balanced detector
QWP
Laser pulse
BBO(Type I)
Pellicle
THz pulse
WP
Si window
Air plasma
Experimental setup I:
or
CCD
P
ZnTe
P
An amplified Ti:sapphire laser system delivering 815 nm, 50 fs, 25 mJ pulses at a 10 Hz repetition rate was used.
Electro-optic THz detection
Max. 8% conversion efficiency
with polarization
4.4 mm
THz imaging
Experimental result I:
0 2 4 6
-6
-3
0
3
6
THz
field
(a.u
.)
Time (ps)
0 1 20
1
2
3
4
Spec
tral int
ensity
(a.u
.)Frequency (THz)
THz waveform THz spectrum
Detection bandwidth is limited by dispersion and absorption in our 1-mm thick ZnTe crystal.
Strong THz absorption by water vapor in air
Experimental result II:
0 2 4 6 8 10 12
0
20
40
60
TH
z yi
eld
(a.u
.)
Distance (d) (cm)
As d 0, THz yield 0
Current model :
Four-wave mixing :
To check the validity of our plasma current model, we studied
dependence of THz yield
]2cos[)cos()( 21 tEtEtEL
BBO
= (nn2)d/c
d
0
3 measurements:
K. Y. Kim et al., Nature Photonics (submitted).
0
30
60
90
120
150
180
SHG THG THz
0
30
60
90
120
150
180
Experiment Simulation
2 polarization angle
Anti-correlation of THz and THG
0.0 0.2 0.4 0.6 0.8 1.0 1.210-13
10-9
10-5
10-1
32
Sp
ectr
al P
ow
er
(a.u
.)
Frequency (PHz)
= 0, = /2, alone
Warm Dense Matter (WDM):
WDM lies between a solid state and an ideal plasma state. It is too hot to be described by solid-state physics and too dense to be depicted by the classical plasma theory.
WDM: warm (0.1~100 eV) dense (0.1~10 times the solid density) matter which is a strongly coupled (e kBT) and Fermi degenerate (F ~ kBT) plasma.
WDM
Brown dwarfs
NASA
Jupiter Laser-heated solids
NASA
Chirped spectral interferometric technique *
THz pulse
Pellicle beam combiner
Chirped optical pulse
Spectrometer
Polarizer
Polarizer
Electro-optic crystal (ex. ZnTe)
CCD
Delay (time)
THz field
Optical pulse
ETHz (t)
* K. Y. Kim et al., Appl. Phys. Lett. 88, 041123 (2006); Z. Jiang et al., Appl. Phys. Lett. 72, 1945 (1998);
Single-shot THz detection:
Experimental setup:
800 820 840
-0.2
0
0.2
Diff
ere
nce
spe
ctru
m (
a.u
.)
Wavelength (nm)
0 1 20
1
2
Spe
ctru
m (
a.u.
)
Freq (THz)
(c)
(a) (b)
Altarget
ZnTe
ZnTe
THz generation pulseChirped optical probe
Optical pump
Polarizer
Teflon
Imagingspectrometer
CCD
Polarizer
Experimental setup:Al disk
Sample
Gratings
ZnTe
Pellicle
Laser-ablated spots
Aluminum
Optical pump pulse
Transient current
e-
e-
e-
+++
Coherent THz generation from a current surge in the laser-produced plasma
800 810 820 830 840
-0.6
-0.4
-0.2
0
Wavelength (nm)
Diff
eren
ce s
pect
rum
(a.
u.)
THz waveform
1 ps
Experimental result IV: THz generation from ablation
-100 -50 0 50 100 150 2000.0
0.5
1.0
0
1
2
3
ITHz
at 1 ps I
THz at 10 ps
TH
z in
ten
sity
Distance (nm)
0.0
0.5
1.0
1.5
(b)
(a)
e- te
mp
era
ture
Te
(eV
)
Te at 1 ps
Te at 10 ps
at 1 ps at 10 ps
Ma
ss d
en
sity
(g
/cm
3 )
THz propagation simulation:
To determine the THz skin depth, we solve the Helmholtz equation.
0)sin())(( 22122 BkdxdBdxddxBd
At 1ps:Te ~ 0.9 eV, ~ 2.6 g/cm3,
r ~ 1016 s-1
At 10 ps:Te ~ 0.6 eV, ~ 1.6 g/cm3,
r ~1015 s-1
K. Y. Kim et al., Phys. Rev. Lett. 100, 135002 (2008).