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).