Laser-Induced Thermal Processes: Heat Transfer, Generation of Stresses, Melting and Solidification, Vaporization, and Phase Explosion.

The vast field of laser-enabled material synthesis, manufacturing, and processing to a large degree relies on the ability to induce and control a range of thermal processes triggered by the laser energy deposition as well as subsequent transport processes involving electrons and phonons. This chapter provides a review of the fundamental mechanisms, thermodynamic driving forces, and kinetics of thermal processes involved in laser-material interactions, with a particular focus on the far-from-equilibrium conditions characteristic of laser processing with short and ultrashort pulses. The peculiarities of the energy redistribution under conditions of electron-phonon nonequilibrium produced by an ultrashort laser excitation are discussed first and followed by analysis of the effect of dimensionality of the heat transfer at different stages of laser-materials interactions. The generation of strong thermoelastic stresses, which may lead to photomechanical spallation, generation of crystal defects, and activation of surface processes are then outlined, along with the implications of laser-induced stresses for practical applications. The discussion of laser-induced phase transformations starts from a brief review of experimental and computational results revealing the conditions leading to transition between the heterogeneous to homogeneous melting mechanisms. The implications of rapid melting and resolidification on microstructure and surface morphology of laser-processed surfaces are considered, and the conditions leading to chemical homogenization, amorphization, and generation of extreme densities of crystal defects are elaborated. The vaporization, which may take the form of evaporation from the surface or an explosive decomposition of superheated liquid (phase explosion), is discussed as the main process responsible for the material removal from the target, that is, for laser ablation. The mechanisms responsible for the generation of nanoparticles in the course of the phase explosion and through the condensation in the ablation plume are also considered and related to the particle size distributions.