Abstract

Recently, femtosecond pulse compression utilizing femtosecond filaments in noble gases have revived the interest in nonlinear pulse shaping effects in self-guided geometries [1]. In contrast to hollow-fiber compressor filaments do not require coupling into a passive waveguide structure. This eliminates coupling losses and limitations on the pulse energy due to potential destruction of the guiding structure. In the following we will discuss the resulting spatio-temporal structures of the self-compressed pulses, both experimentally and theoretically [2 Most of our experiments have been conducted with a Ti:sapphire amplifier system, delivering 45-fs pulses at u 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 c more, a filament is formed inside the gas cell. This filament can extend up to 50 cm or more, which typical] reflects optimum compression conditions. The resulting pulses are analyzed by spectral phase interferometry for direct electric-field reconstruction (SPIDER, [3], Fig. 1(a)). Pulses as short as 7.4 fs have been measure at pulse energies of 2 mJ, see Fig. 1(b). 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. This analysis is depicted in Fig. 1(c). These plots clearly indicate that the self-compressed pulses generated inside the filament are often composed of a short pulse generated in the blue wing of the spectrum and a second pedestal-like structure encompassing the spectral region of the generating pulse. Despite the complexity of filament formation, the numerical simulations pinpoint the major pulse shaping effect that dominantly compress* the pulse during filamentary confinement after the linear focal point. It appears that in this propagation regin leading and trailing time-slices diffract much faster than the high-intensity center of the pulse, which is spatial] confined by the optical Kerr effect after a plasma defocusing stage. On axis, this process results in the formation of a temporally compressed structure. We found this confirmed by analyzing the radial dependence of the pulse duration in the numerical simulations, see Fig. 1(d). Integrating over the central 100 μm of the beam clearly shows self-compression down to ~ 10 fs pulse duration, while radial averaging over the total beam profile indicates little to no compression. This has important implications for applications of filament compressor Compressed filament sources do not appear ideal for driving low-order nonlinear processes, but for high order processes (5 photons or more) only the compressed high-intensity components of the pulse are important an low intense pedestals can be ignored.

© 2007 IEEE