Relaxation processes and formation of thermal shock waves during laser ablation

One of possible approaches to description of laser-induced heating of transparent materials by femtosecond pulses is discussed. There are considered 1) traditional equation of heat transfer, 2) two-temperature model, 3) modified two-temperature model including relaxation terms and diffusion equation for electron plasma. It is discussed validity of those models for description of early stages of laser-induced heating (within few picoseconds after laser pulse has gone) when non-equilibrium laser-induced electron plasma co-exists with non-equilibrium phonon system, it is proposed to use modified two-temperature model for that early stage while two-temperature model is proposed to be applied for later moments of time when relaxation processes within both electron and phonon subsystems are close to finish and the systems are quasi-equilibrium. Formation of abrupt space variations of temperature similar to shock-wave front is discussed as one of important properties of solutions to modified two-temperature model. That process depends critically on value of linear absorption coefficient and temperature-induced variations of material parameters. Obtained results are discussed from the viewpoint of experimental data on morphology of ablation crates in transparent materials.