overview of high power thz sources from laser …overview of high power thz sources from...

Overview of high power THz sources

from laser-plasma interaction

Lecture at the 5th ASS&S

SIOM-CAS, Shanghai

August 16-20, 2010

Z.M. Sheng

Department of Physics, Shanghai Jiao Tong

University / Institute of Physics, CAS

Outline

What to do with intense THz sources

Intense THz sources with electron bunches

Power THz sources from laser plasmas-experiments

Power THz sources from laser plasmas-theory

@Emission from laser wakefield by linear mode conversion

@Emission from a thin plasma layer by transient currents

@Model of emission from laser filaments

@Emission by residual transverse currents from field ionization

@Effect of external DC/AC fields applied to plasma

Summary

What is THz radiation?

G. Williams, Rep. Prog. Phys.69, 301 (2006)

Molecular rotations, Collisions between gas phase molecules, Electron

response in semiconductors and their nanostructures, Superconducting

energy gaps, Collective modes of proteins vibration,…..

Applications of Strong THz fields

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

Courtesy of X.J. Wang

Nonlinear optics: Time-dependent Cross Phase

Modulation (XPM)

Y. Shen et al.,PRL,

99: 043901 (2007);

Y. Shen et al. PRA,

78, 043813 (2008).

Outline

Why to do with intense THz sources

Intense THz sources from electron bunches








Summary

Strong THz sources from large

facilities

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

Coherent synchrotron

radiation

synchrotron radiation

THz Free electron lasers

From “Opportunities in THz Science”, Report of a DOE-NSF-NIH Workshop held

February 12 – 14, 2004, Arlington, VA, Ed. by M. S. Sherwin et al.

THz generation from ultrashort electron

bunches

G. L. Carr et al., Nature 420, 153 (2002).

M. Abo-Bakr et al., PRL 90, 094801 (2003)

coherent

Incoherent

THz radiation source at the Compact ERL (energy

recovery linac)

K. Harada et al. / Infrared Physics & Technology 51 (2008) 386–389

Terahertz coherent

synchrotron radiation

THz from laser-plasma produced

ultrashort electron bunches

Jeroen van Tilborg, PhD Thesis, 2006

Phys. Rev. Lett. 96(1), 014801 (2006)

Outline









Summary

Different plasma-based THz generation techniques

M. D. Thomson, M. Kress et al., Laser & Photon. Rev. 1, No. 4, 349–368 (2007)

THz emission associated with laser driven

plasma oscillations

H. Hamster et al., Phys. Rev. Lett. 71, 2725 (1993); Phys. Rev. E 49, 671 (1994).

THz emission from laser-driven

filaments in air

G. Mechain et al., Appl. Phys.

B 77, 707 (2003); S.

Tzortzakis et al., Opt. Lett.

27, 1944 (2002);

Conical Forward THz Emission from Femtosecond-

Laser-Beam Filamentation in Air

C. D’Amico et al., PRL

98, 235002 (2007).

Transition-Cherenkov emission

from the plasma space charge

moving behind the ionization

front at light velocity.

Efficient way of THz generation

D. J. Cook and R. M. Hochstrasser, "Intense

terahertz pulses by four-wave rectification in

air," Opt. Lett. 25, 1210-1212 (2000)

w+2w THz

Xu Xie, J. Dai, and X.-C. Zhang, PRL 96, 075005 (2006)

THz emission from a dc-biased laser

filaments in air

Yanping Chen et al., APL 95, 101101 (2009)

Super-broad THz spectrum in laser-gas

interaction

0 200 400 6000

200

400

600

800

1000

He

Air

N2

Ar

Kr

Pyro

ele

ctr

ic s

ignal (a

.u.)

Pressure (torr)

THz energy vs pressure

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

K. Y. Kim et al., Nature Photonics doi:2008.153 (2008).

0 20 40 600

1

Spectr

al pow

er

(norm

.)

Frequency (THz)

Air 580 torr

0

1

Spectr

al pow

er

(norm

.)

Ar 100 torr

0

1

Spectr

al pow

er

(norm

.)

Ar 10 torr

Fourier-transform spectra

THz Radiation from Cerenkov wake in a magnetized

plasma

N. Yugami et al., Phys. Rev. Lett. 89,

065003 (2002). N. Spence et al., Phys.

Plasmas 8, 4995 (2001).

vg

w/k, w ~ wp

Laser

pulse

plasm

a

B

THz emission associated with ultrashort electron

bunches: transition radiation

W. P. Leemans et al., Phys. Rev. Lett. 91,

074802 (2003); E. Esarey et al., Phys. Rev.

E 69, 016501 (2004); J. van Tilborg, et al.,

Phys. Rev. Lett. 96(1), 014801 (2006)Plasma E-Beam

Transition radiation

Outline










Summary

Recent experimental results on laser

wakefield acceleration: 1GeV in 3cm!

W. Leemans et al., Nature Physics (2007)

An electron plasma wave is

potentially a coherent high-power

THz source

Plasma waves that can be driven by ultrashort laser pulses

oscillate typically at the THz range (e.g., ne=1018cm-3,

wp/2p=9THz).

The field strength before wave-breaking is as high as

GV/cm for ne=1018cm-3. For example, LBL 1GeV in ~cm.

• How can an electrostatic wave (E//k) be converted

to an electromagnetic wave?

(Usually env is cancelled by -dE/dt )

Dispersion of electromagnetic waves and

electron plasma waves

2222

peck ww +

w

klDe

wpe

1

Slope c

Langmuir

waves Slope 31/2vte

2222 3 peevk ww +

They meet each other only at k=0.

EM wave

ES wave

THz radiations from a vacuum-plasma interface by

introducing an inhomogeneous plasma region

22 4/

/2

emn

Lp

pw

pww

3161011.1,12 - cmnTHz e

pew

p

L

w ~2p/L

ZM Sheng et al., Phys. Rev. E 69, 025401(R) (2004).

ZM Sheng et al., Phys. Rev. Lett. 94, 095003 (2005).

ZM Sheng et al., Phys. Plasmas.12, 123103 (2005).

HC Wu, ZM Sheng et al., Phys. Rev. E, 75 (2007), 016407.

Wave vector of a plasma wave in

inhomogeneous plasmas

3/0

2

3),(

,)/(),/(

0

0

0

0

2/1

0000

tvxfork

x

tvxtxk

LxLxnn

g

p

g

pp

-

w

ww

g

p

g

pp

vxtSincek

xL

xtvxLtxk

LxLxnn

/0

)(2

)()(2),(

,)/1(),/1(

0

0

00

0

2/1

0000

-

-+-

--

w

ww

Wakefield driven by ultrashort laser pulses:

longitudinal electric field (a=0.5, T=20, n/L=0.01nc/60l)

A key aspect: evolution of the wave vector

0 0 0

0

( , ) ( )( / ),

/ 0

p gx t x t x v

k x k

w

-

Linear model conversion

q

nccos2q nc

EM

ES

ES

EM

Resonance absorption

Energy conversion efficiency scaling

C mainly depends upon the incident angle and the pulse profile.

dLL

n0

3

00

2

0

2

0

00

5

0 ~~

w

w

ll

l

Laa

n

n

L

dC c

L

energy

Coherent Wake Emission

e-

Conical emission for normal incidence of a

laser pulse with a finite beam diameter

(a=0.5, T=20, n/L=0.01nc/60l)

Effect of transverse beam size

W=10l, t150 W=20l, t150

Two-dimensional simulation of

oblique incidence

W=10l, q15o, t160 W=20l, q15o, t160

Outline










Summary

Single-Cycle emission from thin plasma

layers

Mechanisms: net transient

currents at the vacuum-plasma

boundaries.

H. C. Wu, Z. M. Sheng et al.,

Phys. Rev. E 77, 046405

(2008).

Scaling with the laser intensity and

incident angle

The THz field amplitude

1/ 2 2

0 sinT Le n a q

Weakly relativistic theory model

2 1/ 2

0 / ea n

1/ 2 2

0 sine en v n a q

en

q

If an electron bunches

interacting with a solid target,

there appears similar effects.

Single cycle emission from steep

plasma density profiles

When wp>2p/L

dL=cL, kL=2p/dL

XG Dong et al., PRE 2009

Outline

Why can one do with intense THz sources









Summary

A model for emission from

laser filamens

P. Sprangle et al., Phys. Rev. E 69, 066415 (2004).

2 1/ 2

0 / ea n

2

0

2/1 anBer

Why It is emitted in single cycle

H.C. Wu, et al., submitted (2009)

Second harmonic

Outline










Summary

THz emission from residual currents

Residual transverse currents

Ways to increase the residual currents:

With two pulses

With a chirped pulse

With a few cycle pulse

A three-step model for THz emission

from field ionization

1. Electrons are freed from atoms by the

tunneling ionization.

2. Free electrons get transverse momenta while

the laser pulse passes by.

3. All moving electrons form an oscillating

electric dipole, which emits THz waves.

H.C. Wu, J. Meyer-ter-Vehn, Z.M. Sheng,

New J. Phys. 10, 043001 (2008).

THz emission in relevance with the CEP

These THz emissions are sensitive to the relative phase

of the two color waves or the absolute phase or carrier

envelope phase (CEP) of the few-cycle pulse.

[1] D. J. Cook and R. M. Hochstrasser, Opt. Lett. 25, 1210 (2000).

[2] M. Kress et al., Opt. Lett. 29, 1120 (2004).

[5] M. Kress et al., Nat. Phys. 2, 327 (2006).

Dj

PIC simulation for linearly-polarized light

90 degree

Circularly polarized light produces

linearly polarized THz emission.

Simulation with two-color lasers

a1=0.06

THz field scaling for the two-color laser

scheme

THz field scaling with chirped laser pulses

A Chirped pulse

~ a0

v

W. M. Wang, Z. M. Sheng et al., Opt. Express 16, 16999 (2008).

Outline










Summary

Emission by external DC/AC fields applied

to plasma

＋V

W.M. Wang et al., J. Appl. Phys. 107,023113 (2010)

Numerical simulation

Emitted field scales linearly with the

applied field

Relation with plasma scalelength

L

Relation with plasma sizes

T=167fs T=400fs

Summary

Laser-plasma can be an efficient intense THz source

Most experiments performed can be explained by existing theories.

High power radiation in the THz range by driving high amplitude EPW: mode conversion

High conversion efficiency, MW in the power.

Single cycle MW THz pulses with a gas jet in a diameter. The mechanism is the transient net currents generation.

Residual currents from field ionization responsible for the THz emission found in laser-gas interaction. A biased field tends to increase the currents and thus increase the emission amplitude.

With a chirped pulse or few cycle pulse, the residual currents can be increased significantly to produce a high amplitude THz wave.

Acknowledgements

H. C. Wu, M. Chen, W. M. Wang, X.G.

Dong, H. W. Du, C. Li, Y. T. Li, J. Zhang

Institute of Physics, CAS and Dept. Phys.,

Shanghai Jiao Tong Univ.

Thank you for your attention!

overview of high power thz sources from laser …overview of high power thz sources from...

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