Evolution of laser microfilaments in the wake of a femtosecond driving pulse

Abstract

A theoretical model for subnanosecond evolution of a nonequilibrium, inhomogeneous free-electron gas in a laser filament or microfilament wake channel is presented. The spatial distributions of electron density and temperature calculated in axially symmetric geometry as a function of time reveal dynamics on the picosecond time scale that is principally driven by a combination of thermal conduction in the electron gas and impact ionization of residual neutral atoms. At high laser intensity, the electron density evolves into a widened distribution with a sharp edge while the temperature distribution evolves to a central peak surrounded by a wide plateau. At low laser intensity, little change is seen in the electron density while the temperature again evolves to a wide plateau. The calculations show that the homogeneous electron-density approximation becomes progressively invalid at higher laser intensity. Pump-probe experiments addressing Fraunhofer diffraction patterns, four-wave mixing, and dynamic Rabi sidebands are proposed for experimental verification of the results.