Hydrodynamic phenomena during laser irradiation: a finite difference approach

We have developed a numerical method to compute the physical behavior of metals and semiconductors, such as Au and Si, under the influence of laser irradiation at low fluences. Electronic effects and evaporation of single atoms are neglected. In the model employed, the thermal reaction of the solid-melt system is calculated by solving the two-dimensional heat conduction equation, where the phase transition solid-liquid is taken into account. Accelerations acting on the melt caused by the thermal expansion of the solid and the phase transition at the interface are also computed. All this information is then parametrically considered in our melt dynamics solver, which makes use of the so-called MAC (Marker and Cell) method. This technique describes the flow of an incompressible fluid, the boundary of which is partially confined and partially free. Our results for laser irradiation of Au and Si show the same tendency as the experimentally observed data: Au, in contrast to Si, tends to from droplets on the surface. Those experimental findings are consequently consistent with the dominance of hydrodynamic effects under low-fluence laser irradiation.