Phase-matched scalable THz generation in two-color filamentation

Kiyong Kim

Institute for Research in Electrons and Applied Physics, University of Maryland, College Park, MD 20742

• Background- THz generation via two-color laser mixing- Plasma current model

• THz generation with high-power lasers

• THz phase matching in long filaments

• Summary

Outline:

Lens BBOTHz pulse

Two-color photoionization:

2

plasma

2

BBO crystal

THz generation mechanism:

e- e- e- e- e-

e- e- e-

THz

Current surge THz generation

Directional quasi-DC current

Tunneling ionization:

U(x)

x

-Ui

U(x)

-Ui

x

U(x)

-Ui

x

Multiphoton ionization

Tunneling ionization

Over-the-barrier ionization

The nonlinearity arises from photoionization!

However, single-color photoionization can not effectively generate THz radiation.

I < 1012 W/cm2 I > 1014 W/cm2 I > 1016 W/cm2

Plasma current model I:

K. Y. Kim et al, Opt. Express 15, 4577 (2007)K. Y. Kim et al, IEEE J. Quantum Electron. 48, 797 (2012)

2( ) cos( ) cos(2 )LE t E t E t

: relative phase

Laser field:

-4

-2

0

2

4x 10

18

e- den

sity

, Ne (c

m-3

)

-50 0 50 100

-100

0

100

Cur

rent

, J (a

.u.)

Time (fs)

= 0

Plasma current model II:Tunneling ionization and subsequent classical electron motion in the laser field are considered.

I = 1014 W/cm2, I2 = 2 1013 W/cm2, 50 fs (FWHM)

Strong field approximation; ignored rescattering, collisional processes, plasma oscillations.

-4

-2

0

2

4x 10

18

e- den

sity

, Ne (c

m-3

)

-50 0 50 100-50

0

50

Cur

rent

, J (a

.u.)

Time (fs)

= /2

Quasi-DC current

0 0.2 0.4 0.6 0.8 1 1.2

10-10

10-5

100

Inte

nsity

(a.u

.)

Frequency (PHz)

= /2 = 0 alone

2

THz 3

Radiation spectrum

* Laser field:

* Ionization rate:

* Plasma current:

* THz field:

* 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 III:

)2cos(cos)( 2 tEtEtEL

)(32exp

)(4)(

tEE

tEEtw

L

a

L

aa

)()()( tvteNtJ e

sin)()(2THz EEf

dttdJE

a

aa

EE

EE

EEEf

3

32exp)(

• Ionization model:Tunneling vs multiphoton ionization

• Plasma current calculation:Semiclassical vs quantum-mechanical calculation

• Additional effects:Plasma oscillations, collisional (electron-ion, electron-neutral) effects, rescattering with parents’ ions

• Propagation effects:Self- and cross-phase modulations, spectral shifting and broadening, Kerr-induced polarization rotation, phase-and group velocity walk-offs

More effects to consider:

Broadband EM generation & control:

1 THz 10 THz 100 THz 1 PHz 10 PHz300 m 30 m 3 m 300 nm 30 nm

Spectral broadening(SPM & blue shift)

rescattering

Plasma current

Plasma oscillation &Collisional effects

Ion current

0 1 2 3

-1

0

1

Time (ns)

B-d

ot s

igna

l (V

) B-dot probe

0 5 10 15 20 25 30 35

10-4

10-3

10-2

10-1

100

Frequency(THz)

Pow

er(a

rb. u

nits

)

EFISHFTIR

5 mJ, 25 fs, 1 kHz Ti:S laser

Laser upgrade @ Kim Lab:

15 mJ, 1 kHz Ti:S laser(Peak power ~0.6 TW)

New cryo-amplifier

New cryogenic amplifier:

Beam profile

Ti:S crystal

Cryo-chamber

15 mJ laser interaction with gaseous targets

10 mm

Filamentation in air

Interaction with Ar cluster gas jets

THz generation with 15 W laser @1 kHz:

2 3 4 5 6 7 8 9 10 110

0.5

1

1.5

2

2.5

3

3.5

4

Laser energy (mJ)

THz

yiel

d (a

.u.)

SiHDPE x2Teflon x15

Gas jetLaser pulse

BBO

THz

e–

e–

Lens Pyroelectric detector

THz generation with 20 TW laser

In vacuumMonomers or clusters

20 TW laser

Parabolodal mirror

0

1

2

3

4

5

0 100 200 300 400 500 600

pressure (psi)py

ro s

igna

l (V)

N2

Ar

• Several microjoule THz radiation per pulse.

• THz energy saturation.

• Clusters provides less THz output.

Experimental Result:THz energy vs pressureTHz energy vs laser energy

0

0.5

1

1.5

0 50 100 150

Pyr

o s

ign

al (

V)

Laser energy (mJ)

Phase matching in THz generation:

Microscopic source

Macroscopic phase

matching

2

e-e-e-e-e-

e- e- e-

Directional current surge

2

THz

2

2

Gaseous medium

Dephasing between and 2:This occurs due to different refractive indices at and 2in air and plasma

BBO

022,,12,, /])()[( cLnnLnn filamentfilamentairair

filament

THz

l = P3 – (P1 + P2) = (m + 1/2)

cos 1 – /(2ld)

Phase matching in a long filament:

Condition for constructive interference:

THz phase matching:

THz far field:

' ' ',Ω , sin ( ') exp Ω ,g THzP r A r z in k z i t

Source:

''3 '

,( ,Ω)

'

THzi k r r

V

P r eE r d r

r r

22 2 222 ' 2 2 11 2 1 2 02

2 β ( ), ,Ω , 2 cos 2Ja lE r A r

r

THz intensity:

1,21,2

1,2

sin( )

1,2 cos

2 2THz

gd

k l nl

2 sina

cos 12p

dl

angle(degree)

angl

e(de

gree

)

-10 -5 0 5 10

-10

-5

0

5

10

angle(degree)

angl

e(de

gree

)

-10 -5 0 5 10

-10

-5

0

5

10

angle(degree)

angl

e(de

gree

)

-10 -5 0 5 10

-10

-5

0

5

10

angle(degree)

angl

e(de

gree

)

-10 -5 0 5 10

-10

-5

0

5

10

Simulated THz profiles:

Ne ~ 3 1016 cm-3

0.1 ~ 1 THz 1 ~ 30 THz

Short (~1 cm) plasma

Long (~3 cm) plasma

THz radiation profile measurement:

Pyroelectric detector

BBO

THz

Lens

FilamentRaster

scanning

Laser pulse

Teflon & Si filtersor

Ge filter

horizontal ()

verti

cal ()

-5 0 5

-4

0

4

0.1

0.2

0.3

0.4

0.5

horizontal ()

verti

cal ( )

-5 0 5

-4

0

4

0

0.2

0.4

0.6

0.8

horizontal ()

verti

cal ( )

-5 0 5

-5

0

50.5

1

1.5

2

2.5

horizontal ()

verti

cal ( )

-5 0 5

-5

0

5 2

4

6

8

10

12

14

16

18

Low THz High THz

Short (<1 cm) plasma

Long (>3 cm) plasma

Measured THz profiles:

Measured optical and THz profiles:

~700 nm

(blue-shifted 800 nm)400 nm + 800 nm

THz focusing issue:

Focused THz profiles

~6 mm long filament

THz yield vs filament length:

BBO

lfaperture

THz yield vs filament length:

THz output increases almost linearly with the plasma length

• THz generation via two-color laser mixing:– Ideal for broadband intense THz spectroscopy

• High-energy THz generation:– Used 15 W, 1 kHz and 20 TW, 10 Hz lasers

– Produced several J THz radiation but observed saturation

• Scalable THz generation:– Demonstrated THz phase matching in long filaments

– THz output energy increases with the filament length

Summaries:

Acknowledgements:

Yong Sing You

Taek Il Oh

Luke Johnson

Yungjun Yoo

Jeffrey Magill

Tomas Antonsen

UINVERSITY OF MARYLAND