P1252109102ZSGfp

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Intense Terahertz Generation and Spectroscopy of
Warm Dense Plasmas
Kiyong Kim University of Maryland, College Park
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Collaborators Kishore Yellampalle George
Rodriguez Toni Taylor Jim Glownia
LOS ALAMOS NATIONAL LABORATORY
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Outline

• Background
• - Terahertz (TH) science.
• Intense THz generation
• - Two-color photoionization.
• THz spectroscopy
• - Warm dense plasmas.

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Phenomena at terahertz (THz) frequencies
1 THz 1012 Hz 1 ps 300 ?m 0.004 eV 33.3
cm-1
molecules
Rydberg atoms
Gaseous and solid-state plasmas
Semiconductor nanostructures
Figure courtesy of Klaas Wynne
Biomolecules proteins
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Strong THz sources
Large facility THz sources
FEL
Synchrotrons
Linacs
Photo courtesy DESY
Stanford, UCSB, FELIX
SLAC, JLab,BNL
ALS (BNL)
Free electron lasing
Coherent synchrotron radiation
synchrotron radiation
M. S. Sherwin et al., DOE-NSF-NIH Workshop on
Opportunities in THz Science
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Intense THz generation Two-color photoionization
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Two-color photoionization
?
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THz generation mechanism
?
2?
?
BBO crystal
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THz energy measurement
THz energy vs pressure
THz energy vs laser energy

• ETHz 5 ?J/pulse with Kr (C.E. gt 10-4)

K. Y. Kim et al., Nature Photonics doi2008.153
(2008).
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THz spectrum measurement
Field autocorrelations
Fourier-transform spectra
(a)
(a)
(b)
(b)
(c)
(c)
THz generation up to 75 THz ( 4 ?m)
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THz spectroscopy Warm Dense Matter
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Electrical conductivity measurements of WDM
Optical probe
WDM
?0
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.
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).
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THz conductivity measurements of WDM
Target (Aluminum)
The quasi-DC electrical conductivity can be
directly determined from THz probe reflectivity
measurements.
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Experimental results I
THz reflectivity for various pump energies
Possible pseudogap formation at the Fermi energy
???
K. Y. Kim et al., Phys. Rev. Lett. 100, 135002
(2008).
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Experimental results II
THz reflectivity vs delay
Room temp. Al ?r 4.1 ? 107 ?-1m-1 3.7 ?1017
s-1 ??-1m-1 1.1 ? 10-10 ?s-1
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Experimental results III
THz reflectivity vs intensity
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Summaries

• THz generation via two-color photoionization
• Generated intense (gt5 ?J), super-broadband TH
  radiation (gt75 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.

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Backup slides
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Experimental setup
THz energy measurement
THz spectrum measurement
B-dot probe 3? measurement
Pyroelectric detector
BBO
d
B-dot probe
3? filter
P.D.
THz pulse
Si window
BBO
Plasma
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Strong THz field 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

• Rapid THz imaging
• Biomedical and security imaging

ETHz gt 1 MV/cm

Strong THz sources

• High magnetic field effects
• 1 MV/cm ? 0.3 T
• Pulsed electron spin resonance
• THz spintronics

• Nonlinear THz Optics
• THz 2nd, 3rd nonlinear effects.
• Extreme nonlinearity with ponderomotive energy gt
  photon energy
• THz-optical nonlinear mixing

M. S. Sherwin et al., DOE-NSF-NIH Workshop on
Opportunities in THz Science
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Plasma current model I
Laser field
2? field
? field
? relative phase ? photoionization phase
? (? 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)
K. Y. Kim et al., Opt. Express 15, 4577 (2007).
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Plasma current model II
The nonlinearity arises from extremely nonlinear
tunneling ionization localized near the laser
peaks.
Laser field Ionization rate Ea
atomic field Plasma current THz
field for Ea gt E? gtgt E2?
and Ng gtgt Ne The function f(E?) is highly
nonlinear, not necessarily quadratic dominant.
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Simulation results I
ADK tunneling ionization and subsequent classical
electron motion in the laser field are
considered.
Simulation with ? 0
Simulation with ? ? /2
I? 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.
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Experimental setup I
Electro-optic THz detection
Balanced detector
Laser pulse
or
WP
QWP
ZnTe
P
Pellicle
THz pulse
BBO (Type I)
Si window
Air plasma
Max. 8 conversion efficiency with
polarization
An amplified Tisapphire laser system delivering
815 nm, 50 fs, 25 mJ pulses at a 10 Hz repetition
rate was used.
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Experimental result I
THz spectrum
THz waveform
Detection bandwidth is limited by dispersion and
absorption in our 1-mm thick ZnTe crystal.
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Experimental result II
To check the validity of our plasma current
model, we studied ? dependence of THz yield
BBO
?
? ?(n??n2?)d/c
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3? measurements
Simulation
Experiment
2? polarization angle
K. Y. Kim et al., Nature Photonics (submitted).
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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.
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Single-shot THz detection
Chirped spectral interferometric technique
Spectrometer
Chirped optical pulse
CCD
Electro-optic crystal (ex. ZnTe)
Polarizer
ETHz (t)
THz field
Polarizer
Pellicle beam combiner
Optical pulse
THz pulse
Delay (time)
K. Y. Kim et al., Appl. Phys. Lett. 88, 041123
(2006) Z. Jiang et al., Appl. Phys. Lett. 72,
1945 (1998)
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Experimental setup
THz generation pulse
Chirped optical probe
Polarizer
ZnTe
Optical pump
Teflon
Al target
ZnTe
Polarizer
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Experimental setup
Al disk
Laser-ablated spots
Gratings
ZnTe
Sample
Pellicle
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Experimental result IV THz generation from
ablation
Aluminum
Transient current
e-

Optical pump pulse
e-

e-

THz waveform
1 ps
Coherent THz generation from a current surge in
the laser-produced plasma
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THz propagation simulation
To determine the THz skin depth, we solve the
Helmholtz equation.
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).