Kansas State University

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? • Limitationsfor producing such pulses • Our approachof obtaining a higher energy • Measurement of the pulse • Further possibilities

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

4. Experimental Observation HHG Spectrum: Discrete Spectral Lines Attosecond pulse train by HHG tunnel ionization + re-collision e- 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 e- 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. 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. 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 setupGeneration 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 no+n2I (r) Self-focusing 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 Reducingfield 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 pulsesFROG- Experimental setup Wavelength (nm) Time (f s) 50% BS BBO Crystal I (t-ז) Filter Time Delay Stage I(t) lens Spectrometer and CCD

27. Measurement of pulsesmeasured FROG traces Input pulse Output pulse Time delay ( fs ) Time delay ( fs ) Wavelength (nm) Wavelength (nm) 1 pixel horizontal = 0.291 nm, 1pixel vertical = 0.716 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 Frequency (d) 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