Spatio-temporally induced pulse self-compression in a white-light filament

In this paper we discuss the spatio-temporal structures of the self-compressed pulses, both experimentally and theoretically. Most of our experiments have been conducted with a Ti:sapphire amplifier system, delivering 45-fs pulses at up to 5 mJ pulse energy at a 1 kHz repetition rate. Pulses from this source are loosely focused into a gas cell, which is filled with a noble gas, typically argon or krypton at 0.3 to 0.5 atmospheres. With pulse energies of 1 mJ or more, a filament is formed inside the gas cell. This filament can extend up to 50 cm or more, which typically reflects optimum compression conditions. The resulting pulses are analyzed by spectral phase interferometry for direct electric-field reconstruction. Pulses as short as 7.4 fs have been measured at pulse energies of 2 mJ. The spectrum displays significant broadening into the blue, whereas only little broadening is observed in the red. For a deeper analysis of the spectro-temporal structure of the self-compressed pulses, we computed XFROG spectrograms from the experimental and numerical data.