Intertwined relaxation processes maintain athermal electron distribution in laser-excited dielectrics

We study the relaxation dynamics of laser-excited non-equilibrium electron distributions in the valence- and conduction band of a dielectric. We apply Boltzmann collision integrals to trace the influence of different scattering mechanisms on the energy- and particle density of electrons and holes. Our results show a two-timescale behavior of the equilibration process: Thermalization within each band towards a respective Fermi distribution as well as equilibration of the band-resolved temperatures occur within a few femtoseconds. In contrast, the equilibration of the respective chemical potentials, driven by scattering processes involving particle exchange like impact ionization and Auger recombination, requires timescales in the range of hundreds of femtoseconds. We evaluate the effect of our model assumptions for distinct material parameters on the extracted specific relaxation times. All these timescales, however, strongly increase, when the additional scattering channel with the cold phonon system is considered: Our simulations demonstrate that an athermal non-Fermi electron distribution can be maintained well up to the picosecond range.