Nanosecond interferometric studies of surface deformation induced by laser irradiation

Abstract

Transient surface morphology changes in dielectric materials induced by laser irradiation were investigated with time- resolved interferometry. Deformation images were acquired at various delay times after exposure to single pulses on fresh sample regions. Above the ablation threshold, we observe prompt ejection of material and the formation of a single unipolar compressional surface acoustic wave propagating away from the ablation crater. For calcite, no deformation - either transient or permanent - is discernable at laser fluences below the threshold for material ejection. Below- threshold behavior was investigated using a phosphate glass sample with substantial near IR absorption. KG3 exhibits the formation of a small bulge roughly the size of the laser spot that reaches its maximum amplitude by approximately 5 ns. At lower laser fluences, diffusion of thermal energy away from that region causes a much weaker and boarder bulge to appear on a slower time scale. At higher laser fluences, a pair of strong, unipolar rarefaction surface acoustic waves is launched, separating from the central region at roughly 17 and 22 ns. Details of the transient interferometry system will also be given.