Spatiotemporal dynamics of a femtosecond laser pulse filamentation in condensed media

The evolution of the spatial and temporal distribution of the intensity of the light field and the induced refractive index changes in a condensed medium upon filamentation of femtosecond laser radiation have been studied. It was shown numerically and experimentally confirmed that the filamentation of femtosecond laser radiation at a wavelength of 800 nm in fused silica results in forming subpulses which diverge from each other while propagating. The time interval between subpulses is more for radiation at a wavelength of 400 nm than at 800 nm due to a more significant contribution of the group velocity dispersion in the change of the pulse shape under self-phase modulation in a nonlinear medium. It was shown that on the axis of a femtosecond filament positive increase of the refractive index due to the Kerr nonlinearity on the leading edge of the pulse is replaced by a negative increment in the laser-induced plasma in the tail of the pulse. Distribution of the nonlinear refractive index increment in the pulse cross section shows that the radius of the plasma channel is much smaller than the size of the spatial domain where the energy in the filament is localized. The results obtained confirm the authors' model, according to which the saturation of intensity in the filament is a consequence of the fact that the optical powers of the lenses, which are formed by the increment of the refractive index induced by the Kerr and plasma nonlinearities, become equal. This model essentially develops the existing ideas about the process which limits the peak laser intensity in a filament - the main characteristics of the filamentation phenomenon.

In studies of the interaction of the filaments it was found that the interference of the light fields in convergent beams plays the main role in the space-time evolution of the pulse and laser induced plasma channels under multiple filamentation. If the formation of the filaments takes place before the region of overlapping beams, then there is an interference of the fields diverging due to defocusing in laser-induced plasma. If the formation of the filaments takes place inside the region, then there is an interference of the incident beams. Multidimensional picture of the distribution of the fluence and the concentration of electrons in laser-induced plasma under multiple filamentation in two, three and four beams intersecting at a small angle in the sapphire crystal has been obtained. It was shown that in the case of in-phase beams at the beginning central most extended filament is formed due to constructive interference. It is accompanied by the appearance of shorter filaments in side interference maxima. The values of the peak intensity of the femtosecond filament, which is formed in crossing beams, are only slightly higher than the value in a single beam with the same parameters. It is shown that under multiple filamentation in air change of the phase shift and the delay between pulses in crossing beams can be used to control the spatial distribution of high fluence areas on propagating pass.

Numerical studies on the project were made by the Russian participants using HPC MSU clusters "Lomonosov" and "Chebyshev". Experimental investigations were carried out by Ukrainian participants of the project in the shared center "Femtosecond Laser Center" in Institute of Physics, National Academy of Sciences of Ukraine.