Thermal and dynamic effects of laser irradiation of thin metal films

The problem of irradiation of a thin gold film deposited on a glass substrate by a narrowly focused single femtosecond laser pulse is considered. Different surface structures can emerge depending on amount of radiation energy absorbed by an irradiated surface.The most important thermal driver for the formation of surface structures is the lateral electron heat flow in the film. This effect consists from three stages: (1) the distribution of the absorbed in the skin layer of laser energy from the frontal boundary of the film to the rear boundary to equalize the temperature; (2) lateral transfer of energy along the film from the center to the edges; (3) cooling and recrystallization of the heated region of the light spot. A model for the study of the effect is presented based on the two-temperature equations of S. I. Anisimov and coauthors and the semi-empirical wide-range equation of state of metal. The model takes into account Gaussian pulse absorption, electron thermal conductivity and electron-ion relaxation in the metal. If the invested energy is large enough, the shock-wave effect on the formation of holes in the film becomes possible. It includes following stages: (1) generation of a shock wave in the glass due to the transfer of energy from the metal; (2) spherization of the formed shock wave, i.e. transition from one-dimensional to two-dimensional propagation mode; (3) transverse propagation of the shock wave in the substrate along the boundary with the film; (4) accumulation of momentum of a film in direction of vacuum. Pressure behind the shock pushes material of a film away from the substrate. When material of a film accumulates enough momentum (and thus velocity in direction to vacuum) it loses connection with substrate. This leads to formation of a hole. Layers of backing material at the same time acting on film as the pistons. A hydrodynamic model for the study of holes formation based on the equations of hydrodynamics of the ideal Euler medium is presented.