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

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 zs as fs ps 10-21 s 10-18 s 10-15 s 10-12 s Time scale Progress

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

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)

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

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)

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

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

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

Pulse compression by - GVD Self Phase modulation Compressor

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

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).

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

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

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

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)

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)

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

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

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

Throughput energy in vacuum

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

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

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

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

Measurement of pulses measured 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

Measurement of input pulses

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

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

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