Ab-initio MD simulations of laser-induced pressure waves tuning nonthermal melting in silicon thin films

Intense ultrafast-laser pulses can induce tremendous structural changes in materials. In particular, at the surface the deposited energy can cause several modifications like ripple formation, droplets, holes, or super-hydrophilic behavior. Most of those effects are initiated by laser-induced nonthermal conditions and their ensuing ultrafast processes like nonthermal melting. But how does the free surface affect the microscopic pathways of those ultrafast phenomena, in particular nonthermal melting. Here, we performed ab-initio molecular dynamics simulations of laser-excited thin silicon films, starting from an electronic temperature around the bulk laser-melting threshold above which irreversible changes to the material are induced. Our findings indicate that the broken symmetry at the surface has a big influence on laser-induced ultrafast processes and therefore on the reconstruction effects. In more detail, we observe that an out-of-plane pressure wave is induced by the excitation, which reflects back and forth from the surfaces. For moderate levels of excitation, this causes a breathing mode perpendicular to the film/surface, whereas for higher levels of excitations transient melting below the surface takes place, which is healed by the incoming pressure wave.