1. High energy 6.2 fs pulses Shambhu Ghimire, Bing Shan, and Zenghu Chang
Kansas Light Source Group
J. R. Macdonald Laboratory
Kansas State University

2. 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

3. Applications of attosecond pulseszs as fs ps
10-21 s
10-18 s
10-15 s
10-12 s
Time scale
Progress

4. Attosecond pulse train by HHG
tunnel ionization
+ re-collision e-
Experimental Observation
HHG Spectrum: Discrete Spectral Lines
E(t)
HHG
Gas
Driving fs Pulse e-
HHG as pulse Train
Half Cycle

5. Discrete harmonic orders in the plateau -Spatial analogy of pulse train interference
Discrete pattern at plateau analogy to multi-slit diffraction
Single slit
Double slit
Multi slit
Diffraction patterns
(spatial frequency)

6. Shorter fs-pulse to get a single atto-pulse
fs-pulses
Harmonic generation
Atto-pulses
With ~25 fs pulses
With ~10 fs pulses
With ~5 fs pulses ??
Super continuum at near cutoff ??
Traditional method : generation of single atto-second pulses

7. Polarization gating for a single atto-pulse
Right Circular Pulse e- e- Td
Ellipticity dependent pulse
Left Circular Pulse
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)

8. 1) Polarization gating method
High energy, ultra-short pulse
1) Polarization gating method
Limits
Using a shorter pulse
available short pulse
Shorter gate
Using a longer delay
final energy at gate
2) Traditional method
To cover broader wavelength range of a atto-pulse

9. To scale up the energy of a few cycle pulses
Our goal
To scale up the energy of a few cycle pulses

10. Spectral broadening by SPM
I (w)
I (t)
n(t)
Non linear medium no
I (w)

11. Pulse compression by - GVD
Self Phase
modulation
Compressor

12. Experimental setup Generation of a few cycle pulses
O D = 6mm
I D = 0.4mm
f = 1m
f = 2.5 m
Ar- gas
FROG
Hollow core fiber/ chirp compressor technique

13. Previous work and Limitations
Higher Energy ~ 0.5 mJ1
Self focusing along with self phase modulation
Self de-focusing by plasma formation
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).

14. Limitations Self-focusing Self-defocusing no+n2I (r) I (r) f=1m
n (I) = no + n2 I
n (I) = no - Ne/ 2Ncr
f=1m
I (r)
no - n e
Self-defocusing

15. Our approach Higher Energy
Linear polarization input
Circular polarization input
lower self-focusing with circular polarization
Reduced ionization greatly reduces self de-focusing
Shorter Pulse Duration
Broader spectrum with higher input circular polarization

16. Reducing self focusing
Nonlinear index of refraction
A Gaussian pulse
n (r) = no + nL2 I (r)
n (r) = no + nC2 I (r)
nL2
= 1.5
nC2
Radial distance in micrometer

17. Preserving self phase modulation
Chirp parameter
Output band width
Circular polarization input
Linear polarization input =
n (r) = no + nC2. I (r)
n (r) = no + nL2 . I (r)

18. Reducing self defocusing
Reducing field by 0.7 reduces ionization by >1 order
Tunneling ionization
Multiphoton ionization
Selection rule : Less ionization channels in circular
F.A. Ilkov et al. J. Phys. B, 25 (1992)
A.M. Perelomov et al. JETP, 24, (1966)

19. Reducing self defocusing
Lower field of circular input lead to decrease in ionization
By using circular polarization input

20. Combined effects : SF and SDF
Complicated
profile
Linear input SF
SDF
Single mode
Circular input

21. Measured spatial profiles
vacuum
Ar-gas
Ar-gas
Ar-gas
Linear a) b) c) d)
Input Energy
1.0 mJ
1.2 mJ
1.2 mJ
0.55 mJ
Circular

22. Throughput energy in vacuum

23. Scaling up energy by circular polarization
Ar-gas pressure at ~ 1 atm

24. Scaling up energy by circular polarization
Ar-gas pressure at ~ 0.5 atm

25. Broader spectrum for circular input
At circular
threshold
Circular
At linear
threshold
At linear
threshold
Ar-gas pressure at ~ 0.5 atm

26. Measurement of the pulses FROG- Experimental setup
Time (f s)
Wavelength (nm)
50% BS
BBO Crystal
I (t-ז)
Filter
Time Delay Stage
I(t)
lens
Spectrometer and CCD

27. Measurement of pulses measured FROG traces
Input pulse
Output pulse
Time delay ( fs )
Time delay ( fs )
Wavelength (nm)
Wavelength (nm)
1 pixel horizontal = nm, 1pixel vertical = fs

28. Measurement of input pulses

29. Measurement of input pulses
Measured
Frequency
Time
Reconstructed
Frequency
Time
(d)

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

31. Measurement of output pulses
Measured
(d)
Frequency
Time
Reconstructed
Frequency
Time

32. 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

33. 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