Generation of subsurface voids and a nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target

Structural transformations in a shallow surface region of a bulk Ag (001) target irradiated by a femtosecond laser pulse are investigated in large-scale atomistic simulations and experiments. The simulations reveal a complex interplay of fast laser melting, rapid resolidification, and dynamic relaxation of laser-induced stresses that leads to the formation of a subsurface porous region covered by a nanocrystalline surface layer. The generation of the porous region is consistent with the experimental observation of surface "swelling" occurring at laser fluences below the spallation/ablation threshold and may be related to the incubation effect in multipulse laser ablation of metals. The nanocrystalline layer is produced by massive nucleation of crystallites triggered by a deep undercooling of the melted surface region experiencing fast quenching at a rate on the order of 10 super(11) K/s. The predicted surface structure features random crystallographic orientation of nanograins and a high density of stacking faults, twins, and nanoscale twinned structural elements with fivefold symmetry, which suggests high hardness and possible enhancement of catalytic activity of the surface.