Laser-Induced Breakdown in Dielectrics: Strong Electron Superheating

Abstract

Laser-induced breakdown hinders the operation of modern lasers, photonic elements and devices, and can be also employed as an important operational regime for high-intensity laser technology of new materials [1,2]. In this report we generalize the two-temperature model of laser-matter interaction [3] to simulate the regimes of laser-induced breakdown in dielectrics. This generalization allow us to associate the onset of breakdown with a sequence of the step-wise increase and decrease of the mean electron energy, e e , and related evolution of the free electron density, n e . The model set of the rate equations includes photo-ionization and recombination kinetics, radiation absorption, energy release and exchange effects defining the time evolution of e e to the band gap energy E G , and critical energy of ionization, E cr ≈1.5 E G [4]. Additionally, our model includes an effect of a strong electron superheating, Δe e , above E cr . Namely, in treating the onset of impact ionization by generated free electrons the related impact ionization rate [1,2], w imp = τ−1, is associated with the time of the electron heating to the critical energy, τ Η = E cr (de e /dt)−1 ≈ 1 /(αl) (where α = σ λ /E cr is the avalanch coefficient, σ λ is the related absorption cross-section of free electrons and I is the radiation intensity).