Monte Carlo calculations of laser-induced free electron heating in SiO2
Abstract
We report on a theoretical study of free electron heating in SiO2 in the presence of high intensity laser excitation at 1 μm wave length. The formalism is based on a Monte Carlo integration of the Boltzmann Transport Equation which has successfully explained DC transport data. The simulations are based on experimentally determined energy dependent electron-phonon scattering rates and electron-hole pair excitation rates. The temperature and laser power dependence of the free carrier induced lattice heating and the impact ionization rate are calculated. We find that the average power loss of conduction electrons to the lattice via phonon excitations increases rapidly with incident laser power. This effect allows for strong free carrier induced lattice heating at fields well below the onset of carrier multiplication by impact ionization, as observed experimentally. When the electron, by chance, scatters from phonons in such a way as to be in phase with the alternating electric field for several oscillations, then the electron can reach large energies very quickly. These events occur frequently enough to give the electron distribution a long high energy tail. Above a critical laser intensity, electrons in this high energy tail gain enough energy to cause impact ionization even though the average electron energy is much smaller than the band gap. Contrary to free electron heating, impact ionization is found to decrease strongly with temperature. It is shown, that this temperature dependence has severe implications for the standard single shot impact ionization breakdown model. We propose a new mechanism for single shot prebreakdown laser heating which combines impact ionization and free electron heating.