Electron dynamics in transparent materials under high-intensity laser irradiation

Abstract

The energy of a laser beam, irradiating a surface, is primarily absorbed by electrons within the solid. In actual transparent materials, absorption is low. High-intensity lasers may, however, be absorbed by initially bounded electrons through nonlinear processes. The increase of free electron density leads eventually to dielectric breakdown and the material becomes highly absorbing. We present theoretical studies on the dynamics of electrons in dielectrics under irradiation with a visible high-intensity laser pulse. We consider microscopic processes determining absorption, redistribution of the energy among electrons and transfer of energy to the crystal lattice. We review different aspects of electronic excitation, studied with time-resolved models as the Boltzmann kinetic approach and the time and spatial resolved multiple rate equation. Further we investigate criteria for damage thresholds. Two concepts compare, namely a critical free electron density and the melting threshold of the lattice. We show that in dielectrics both criteria are fulfilled simultaneously. Optical parameters depend on the density of free electrons in the conduction band of the solid, so the free electron density directly leads to an increased energy absorption causing material modification. We present results on the spatial dependence of dielectric breakdown.

Simulation of the laser-material interaction of ultrashort pulse laser processing of silicon nitride workpieces and the key factors in the ablation process

“…In the case of activation of the MPI mechanism, some part of energy leads to the generation of free electrons [12]. Free electrons are supplied by elements in the material.…”

“…According to the study of Brenk et al [12], the plasma frequency is a function of the density of free electrons (N free ):…”

“…In this case, electron-phonon coupling volumetric energy (E eph ) is transferred to the material as a result of a collision between heated free electrons and atomic structure. Equation (19) provides the mathematical calculation of E eph [12]:…”

“…There is a I L − crt that differentiates the laser-material interaction mechanism and a principal factor in finding the temperature values. In I L > I L − crt , due to the considerable laser energy transferred to the material surface at about 100 fs, the free electrons temperature values become much higher than the material temperature in the radiation time [12]. The raise of the electrons temperature and free electron density due to ionisation of material leads to repulsive forces, destroying the whole irradiated surface.…”

“…Brenk and Rethfeld [12] studied the electron warming model for silicon oxides. Here, the workpiece was an insulator without the free electrons in the conduction band.…”