Surface nano-structuring produced by spallation of metal irradiated by an ultrashort laser pulse

Abstract

It is known that under certain conditions the complex surface nano-structures are formed after irradiation on metals by ultrashort optical and X-ray laser pulses. In the paper the mechanism of formation and final geometry of such surface structures are discussed for the case of single pulse acting on a well-polished metal surface. The typical surface structures observed in our experiments and simulations are different from well-known ripples composing a regular pattern generated by excitation of surface plasmons. By contrast with the plasmon mechanism, the observed structures have spacial scales which are order of magnitude less than the used optical laser wavelengths. We demonstrate that such structures are formed after laser irradiation due to the thermomechanical spallation of ultrathin surface layer of melt, rather than the plasmon effects, which are found to be insignificant in given conditions of a single shot and initially smooth surface. Spallation is accompanied by a strong foaming of melt followed by breaking of the foam. After several nanoseconds the foam remnants freeze up with formation of complex nano-structures on a target surface. © Published under licence by IOP Publishing Ltd.