Deposition of energy of high-intensity laser radiation in photo-excited wide band-gap dielectrics

摘要

In this paper we analyze space distribution of energy deposited from high-intensity laser radiation in wide band-gap materials ionized by the radiation. Considering the case of ultra-short laser pulses, we assume photo-ionization to be a dominating mechanism of non-linear absorption. The photo-ionization is described by recently derived formula for the photo-ionization rate rather than by the Keldysh formula. The energy deposition is studied by means of numerical modeling for a system of two coupled nonlinear equations - equation of intensity transfer for laser radiation and rate equation for time evolution of free-electron density. Two characteristic regimes of the ultra-fast laser-solid interactions are analyzed: 1) the regime of photo-ionization suppression at intensity above few TW/cm~2; 2) the regime of photo-ionization singularity occurring at intensity close to 10 TW/cm~2. In the first regime specific propagation conditions are provided with successful transfer of radiation energy over a large distance at low loss through free-electron absorption and weak avalanche ionization. The singularity effect results in extremely intensive generation of free electrons in subsurface layer of irradiated dielectric and locking of the ionizing radiation in a very thin surface layer. The singularity threshold corresponds to the threshold of the radiation locking. Presented results are compared with those obtained with the formula for the photo-ionization rate in the approximation of parabolic energy bands.