Kansas State University

High energy 6.2 fs pulses

J. R. Macdonald Laboratory

Shambhu Ghimire, Bing Shan, and Zenghu Chang

Kansas Light Source Group

Summary

• A shorter pulse ?• A higher energy pulse?• Limitations for producing such pulses• Our approach of obtaining a higher

energy• Measurement of the pulse• Further possibilities

Applications of attosecond pulses

Time scale

as psfszs10-18 s 10-15 s 10-12 s10-21 s

Progress

HHG

e-

Attosecond pulse train by HHG

Half Cycle

HHG as pulse Train

Driving fs Pulse

Gastunnel

ionization+ re-collision

e-

E(t)

Experimental ObservationHHG Spectrum: Discrete Spectral

Lines

Discrete harmonic orders in the plateau

-Spatial analogy of pulse train interference

Single slit

Double slit

Multi slit

Diffraction patterns(spatial frequency)

Discrete pattern at plateau analogy to multi-slit diffraction

Shorter fs-pulse to get a single atto-pulse

Harmonic generation

??

Super continuum at near cutoff

With ~25 fs pulses

With ~10 fs pulses

With ~5 fs pulses

fs-pulses Atto-pulses

??

Traditional method : generation of single atto-second pulses

P. B. Corkum, N. H. Burnett, and M. Y. Ivanov, Opt. Lett. 19, 1870 (1994)V. T. Platonenko and V. V. Strelkov J. Opt. Soc. Am. B 16, 435 (1999)

p

Left Circular Pulse

Right Circular Pulse

Td

e-

e-

e-

Polarization gating for a single atto-pulse

Ellipticity dependent pulse

1) Polarization gating method

Shorter gate

Using a shorter pulse

Using a longer delay

High energy, ultra-short pulse

d

p

Tt

2

2.0 3.0

available short pulse

2) Traditional method

To cover broader wavelength range of a atto-pulse

Limits

final energy at gate

Our goal

To scale up the energy of a few cycle pulses

I t( ) Io e

4 ln 2( )t

2

n I( ) n0

n2I t( ) t

2 n

2 I t( ) L

o

o

Spectral broadening by SPM

I (t)

n(t)

no

wtd

d

t

o

t( )

t( )

Non linear medium

I (w)

I (w)

Pulse compression by - GVD

Self Phase modulation

Compressor

Experimental setupGeneration of a few cycle pulses

FROG

f = 1mO D = 6mmI D = 0.4mm

f = 2.5 mAr- gas

Hollow core fiber/ chirp compressor technique

Previous work and Limitations

• Self focusing along with self phase modulation• Self de-focusing by plasma formation

Higher Energy ~ 0.5 mJ1

Shorter Pulse Duration ~ 5 fs 1

• Achievable spectral broadening by SPM • Bandwidth limitation of compressor technique

1S. Sartania, Z. Cheng, M. Lenzner, G. Tempea, Ch. Spielmann, and F. Krausz, Optic Letters, 22,20 (1997).

Limitations

I (r)

I (r)

no+n2I (r)

no - n e

Self-focusing

Self-defocusing

f=1m

f=1m

n (I) = no + n2 I

n (I) = no - Ne/ 2Ncr

Our approach

Higher Energy

Shorter Pulse Duration

• Broader spectrum with higher input circular polarization

Linear polarization input Circular polarization input

• lower self-focusing with circular polarization• Reduced ionization greatly reduces self de-

focusing

Reducing self focusing

n (r) = no + nL2 I (r)

n (r) = no + nC2 I (r)

Radial distance in micrometer

0

2 1013

4 1013

6 1013

6 1013

0

I r( )

300300 r

190 80 0 140 2500

1 10 6

2 10 6

3 10 6

4 10 6

5 10 6

4.704 106

1.378 107

nL r( )

nC r( )

250250 r

A Gaussian pulse

Nonlinear index of refraction

nC2

nL2= 1.5

Preserving self phase modulation

b4 a n

2 I L 0

F f 1b

a

2

n (r) = no + nL2 . I (r) n (r) = no + nC2

. I (r)

Chirp parameter Output band width

Linear polarization input Circular polarization input

=

Reducing self defocusing

F

2 Fo

F.A. Ilkov et al. J. Phys. B, 25 (1992)A.M. Perelomov et al. JETP, 24, (1966)

Selection rule : Less ionization channels in circular

Reducing field by 0.7 reduces ionization by >1 order

Multiphoton ionization Tunneling ionization

Reducing self defocusing

)()( 0 InnIn cr

gas

N

NIIPIn

.2

).()(

Lower field of circular input lead to decrease in ionization

By using circular polarization input

nC F( ) 0.0086IPC F( )nL F( ) 0.0086IPL F( ) IPL

Combined effects : SF and SDF

0)( nRrn

SDFSF

60 107.4)0( nrn 008.0)0( 0 nrn

0)( nRrn 0)( nRrn

Linear input

Circular input

0)0( nrn

0)( nRrn

0)0( nrn

Complicated profile

Single mode

Measured spatial profiles

a) b) c) d)

Input Energy

0.55 mJ 1.0 mJ 1.2 mJ1.2 mJ

vacuum Ar-gas Ar-gas Ar-gas

Linear

Circular

Throughput energy in vacuum

0.4 0.6 0.8 1.0 1.20.3

0.4

0.5

0.6

0.7

0.8

0.9

Outp

ut E

nerg

y (

mJ)

Input Energy (mJ)

Linear Circular

Scaling up energy by circular polarization

Ar-gas pressure at ~ 1 atm

0.6 0.8 1.0 1.20.35

0.40

0.45

0.50

0.55

0.60

0.65

Linear-Threshold

Circular-Threshold

Outp

ut E

nerg

y (

mJ)

Input Energy (mJ)

Circular Linear

Scaling up energy by circular polarization

0.6 0.8 1.0 1.2 1.4

0.4

0.5

0.6

0.7

0.8

Linear-Threshold

Circular-Threshold

Outp

ut E

nerg

y (

mJ)

Input Energy (mJ)

Linear Circular

Ar-gas pressure at ~ 0.5 atm

Broader spectrum for circular input

Ar-gas pressure at ~ 0.5 atm

600 700 800 9000.0

0.5

1.0

Inte

nsi

ty (

norm

aliz

ed)

Wavelength (nm)

At circularthreshold

At linearthreshold

CircularAt linearthreshold

Time Delay Stage

Spectrometer and CCD

BBO Crystal

I(t)

lens

50% BS

I (t-ז)

Tim

e (f

s)

Wavelength (nm)

Filter

Measurement of the pulses

FROG- Experimental setup

Measurement of pulsesmeasured FROG traces

Input pulse Output pulse

Wavelength (nm) Wavelength (nm)

Tim

e d

ela

y (

fs

) Tim

e d

elay

( fs )

1 pixel horizontal = 0.291 nm, 1pixel vertical = 0.716 fs

Measurement of input pulses

-100 -50 0 50 1000.0

0.5

1.0

33 fs

Intensity Phase

Time (fs)

Inte

nsity (

norm

aliz

ed)

0

5

10

15

20

Phase (ra

d)

Measurement of input pulses

725 750 775 800 825 850 8750.0

0.5

1.0

Reconstructed measured

Wavelength (nm)

Inte

nsity

(no

rmal

ized

)

Time

Time

(d)

Fre

que

ncy

Fre

que

ncy

Measured

Reconstructed

Measurement of output pulses

-20 -10 0 10 200.0

0.5

1.0

6.2 fs

Intensity Phase

Time (fs)

Inte

nsity (

norm

aliz

ed)

0

2

4

6

8

10

Phase (ra

d)

0.6 mJ, 6.2 fs pulses at rep. rate of 1k Hz

Measurement of output pulses

600 700 800 900 10000.0

0.5

1.0

Inte

nsity (

no

rma

lized

)

Wavelength (nm)

Retrieved Measured

(d)

Measured

Reconstructed

Fre

que

ncy

Fre

que

ncy

Time

Time

Results

• 0.6 mJ, 6.2 fs pulses by Hollow Core/Chirp compressor technique

• Scaling up the pulse energy by a factor of 1.5 by using circular input

• Demonstration of measurement of sub-10 fs pulses with SHG-FROG

Further possibilities

Obtaining even higher energy and a shorter pulse

• By lowering gas pressure and further increasing input energy

• By using Ne gas instead of Ar• By improving the compressor technique to

compress a broader spectra