TRANSIENT OPTICAL NONLINEARITIES ENGENDERED BY FEMTOSECOND LASER FILAMENTATION IN GASES

Abstract

The laser filamentation process in gases and its consequences have been at the center of interest over the three recent decades. The filament wake channel is formed by the laser pulse as a highly nonequilibrium and optically underdense plasma column. The contents of the plasma evolve towards equilibrium, giving rise to various transient optical effects. When filamentation occurs in a dense gas, it leads to the production of the excessively high density of excited atoms as compared to the density of ions. We used a kinetic model of the competing electron-collisional processes in the case of high-pressure argon gas and explored the sensitivity of the resulting excited-to-ionized atoms number density ratio to the envelope shape of the driving laser pulse. Considering three different families of the pulse shapes, we have shown that the ratio of excited atoms to ions in the dense gas can be manipulated and further increased. To further investigate the structure of the plasma column, we studied the filamentation process at the crossing of two laser beams. We have shown that in this case the process is significantly affected by the transient intensity grating caused by the beam interference in the crossing area, which leads to the formation of a microscopically structured filament wake channel. In particular, the grating of excited atom density is formed in the channel. We obtained characteristics of such excitation gratings that are controlled by the spatial and temporal characteristics of the crossing pulses. A nonlinear optical effect that is crucial in the context of excess excited atoms is the Rabi sideband generation. The Rabi sideband patterns from a one-dimensional plasma channel have already been studied. We considered theoretically the probing of the above-mentioned excitation gratings by a picosecond laser beam of 800 nm carrier wavelength and the formation of the characteristic spatial-spectral patterns of the Rabi sidebands. We demonstrated the sensitivity of these Rabi sideband patterns towards the grating characteristics, probe beam shape and wavelength and to the position of the observation screen and the observation slit on the screen. As our capstone work, we explored filamentation of long-wavelength laser pulses in atmospheric-pressure gases, as this situation effectively meets the dense gas criteria. We worked at transforming the theoretical and computational techniques that we developed for high-pressure gases at typical laser wavelengths (~800 nm) to be applicable to atmospheric-pressure gases at longer laser wavelengths (~3900 nm). Intense, ultrashort laser pulses of these latter carrier wavelength values just recently have become available for experiments and carry a great promise for applications in atmospheric optics, atmospheric chemistry, and related disciplines.