Refractive Index Modification Induced by Femtosecond Laser Filament

A self-consistent, multi-dimensional three-temperature plasma model for laser-induced non-equilibrium plasma is integrated with a semi-classical model for refractive index calculations to explore refractive index modifications in femtosecond laser-induced filaments. The investigation focuses on single filaments in nitrogen at atmospheric pressure, presenting a detailed analysis of the spatiotemporal evolution of the refractive index field during the filament's decay. The fast temporal evolution in the refractive index at early times results from the exponential decay nature of electron density. Equally important factors include the dissociation and vibrational excitation of molecular nitrogen. A comprehensive analysis of the relative contributions to the refractive index among different components depends on the initial electron density, dissociation fraction, electron temperature, and the relaxation of electron energy to vibrational states. A thorough analysis is presented, spanning a range of plasma parameters, providing a quantitative understanding of the impact of thermodynamic nonequilibrium, dissociation, and hydrodynamic effects.