Laser-Induced Breakdown in Dielectrics: Strong Electron Superheating

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, ε
_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 ε
_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, Δε
_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
_h
= E
_cr
(dε
_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).