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Ultrafast laser irradiation of solids leads to a thermodynamic nonequilibriumwithin and between the electron and phonon subsystems of the material. Due toelectron-electron and phonon-phonon collisions, both subsystems relax intorespective new thermodynamic equilibria within a characteristic thermalizationtime, which is different for each one of them. Moreover, they equilibrate theirtemperatures by electron phonon coupling. The relaxation of the electronicnonequilibrium and its effect on the electron phonon coupling was subject to anumber of studies and it is comparably well understood, while thenonequilibrium within the phononic subsystem is usually neglected and itsinfluence of the nonequilibrium phonons on other relaxation processes isunclear. Our calculations show significant differences in the energy transferrate between the electrons and the phonons depending whether a nonequilibriumdistribution is assumed for the phonons or not. Here, we present a model tostudy the relaxation of the nonequilibrium phonon subsystem. Collisions betweenphonons are described within the frame of Boltzmann integrals. From this, anenergy-dependent relaxation time can be extracted and inserted into arelaxation-time approach. Within the frame of this model, we study thethermalization of a phonon distribution induced by ultrafast laser irradiation.We show, that the thermalization time of such a distribution is of the order ofsome hundreds of picoseconds. Moreover, we discuss the energy transfer betweenFermi-distributed electrons and nonequilibrium phonons and compare this to theenergy transfer for equilibrium distributions in both subsystems.

Relaxation of a nonequilibrium phonon distribution induced by femtosecond laser irradiation