Computational investigation of short pulse laser melting of metal targets

Although melting is a common and well studied phenomenon, the nature of the structural instability of a superheated crystal and the microscopic mechanisms of melting are still subjects of active scientific discussions. Short pulse laser irradiation combined with new optical and electron diffraction time-resolved probe techniques has a promise of providing new insights into the mechanisms and kinetics of ultrafast phase transformations. At the extremely high levels of superheating realized in short pulse laser melting, however, the applicability of macroscopic kinetic approaches based on classical thermodynamics is questionable and should be verified by a detailed microscopic analysis of the involved processes. The goal of this dissertation is to perform a systematic computational investigation of the kinetics and microscopic mechanisms of short pulse laser melting. Simulation results have direct implications on interpretation of the results of ultrafast pump-probe experiments as well as on applications that involve short pulse laser melting, such as surface micro- and nano-patterning below the ablation threshold, laser drilling and cutting, and pulsed laser texturing.; To enable a realistic computational study of laser-induced processes in metal targets, a new atomistic-continuum model is developed. The model combines the classical molecular dynamics method for simulation of non-equilibrium processes of lattice superheating and fast phase transformation with a continuum description of laser light absorption by conduction band electrons, energy transfer to the lattice due to the electron-phonon coupling, and fast electron heat conduction.; The combined model is applied for computational investigation of the kinetics and microscopic mechanisms of laser melting of Ni and Au films and bulk targets irradiated by short, from 200 fs to 150 ps, laser pulses. The interplay of two competing processes, the propagation of the liquid-crystal interfaces (melting fronts) from the external surfaces and homogeneous nucleation and growth of liquid regions inside the crystal, is found to be responsible for the melting process. The relative contributions of the homogeneous and heterogeneous melting mechanisms are defined by the laser fluence, pulse duration, and the strength of the electron-phonon coupling in the target material.; Under conditions of the inertial stress confinement, realized in the case of short (tau ≤ 10 ps) laser pulses and strong electron-phonon coupling (Ni targets), the dynamics of the relaxation of the laser-induced pressure has a profound effect on the temperature distribution in the irradiated targets as well as on the kinetics of both homogeneous and heterogeneous melting processes. (Abstract shortened by UMI.)...