Combined atomistic-continuum modeling of short-pulse laser melting and disintegration of metal films

The kinetics and microscopic mechanisms of laser melting and disintegration of thin Ni and Au films irradiated by a short, from 200 fs to 150 ps, laser pulse are investigated in a coupled atomistic-continuum computational model. The model provides a detailed atomic-level description of fast nonequilibrium processes of laser melting and film disintegration and, at the same time, ensures an adequate description of the laser light absorption by the conduction band electrons, the energy transfer to the lattice due to the electron-phonon coupling, and the fast electron heat conduction in metals. The interplay of two competing processes, the propagation of the liquid-crystal interfaces (melting fronts) from the external surfaces of the film and homogeneous nucleation and growth of liquid regions inside the crystal, is found to be responsible for melting of metal films irradiated by laser pulses at fluences close to the melting threshold. 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. At high laser fluences, significantly exceeding the threshold for the melting onset, a collapse of the crystal structure overheated above the limit of crystal stability takes place simultaneously in the whole overheated region within \ensuremath{\sim}2 ps, skipping the intermediate liquid-crystal coexistence stage. Under conditions of the inertial stress confinement, realized in the case of short $\ensuremath{\tau}<~10\mathrm{ps}$ laser pulses and strong electron-phonon coupling (Ni films), the dynamics of the relaxation of the laser-induced pressure has a profound effect on the temperature distribution in the irradiated films as well as on both homogeneous and heterogeneous melting processes. Anisotropic lattice distortions and stress gradients associated with the relaxation of the laser-induced pressure destabilize the crystal lattice, reduce the overheating required for the initiation of homogeneous melting down to $T\ensuremath{\approx}{1.05T}_{m},$ and expand the range of pulse durations for which homogeneous melting is observed in 50 nm Ni films up to \ensuremath{\sim}150 ps. High tensile stresses generated in the middle of an irradiated film can also lead to the mechanical disintegration of the film.