Self-action of tightly focused femtosecond laser radiation in air in a filamentation regime: Laboratory and numerical experiments

Abstract

The influence of self-focusing of strong femtosecond laser radiation on the spatial energy distribution of a laser beam was studied. Experimental data are provided on the transverse energy density distribution of the tightly focused radiation of the Ti-sapphire laser following its filamentation in air. The results are interpreted within the framework of the nonlinear Schrödinger equation model. The best agreement between the theory and the experiment is found in the model that takes into account the change of character of the nonlinear response of the medium induced by high-intensity radiation in the nonlinear focus, which includes the change of the ionization mechanism of molecules from multiphoton ionization to tunnel ionization, the completely instantaneous Kerr nonlinearity, and saturation of the latter due to higher-order nonlinearity. The mechanism of the ionization of molecules can change from purely multiphoton ionization to tunnel ionization.