Surface instability on a metal target from multi-pulse KrF laser ablation

Abstract

Summary form only given, as follows. Aluminum targets were ablated by focusing a KrF excimer laser (248 nm, 40 ns, <1.2 J) down to a spot size of 0.05 cm/sup 2/ with a fluence of approximately 4.9 J/cm/sup 2/. After a few tens of pulses, surface irregularities (corrugations and pits) progressively emerge, with size 1-100 /spl mu/m which is much larger than the laser wavelength. After hundreds of laser pulses, large scale wavelike patterns, on the order of 30 /spl mu/m, are observed on the aluminum surface. We propose that these wave patterns are caused by the Kelvin-Helmholtz instability at the interface of the molten aluminum and the plasma plume. A parametric study is given in terms of the molten layer's thickness and of the spatial extent and kinetic energy density in the laser-produced plasma plume. Also included is an estimate of the cumulative growth in a multi-pulse laser ablation experiment. These estimates indicate that the Kelvin-Helmholtz instability is a viable mechanism for the formation of the large scale structures. Once formed, these large scale surface roughness causes multiple reflections of the laser light, and may increase the absorption coefficient over a pristine, flat surface by an order of magnitude.