Ultrafast laser ablation giving unstructured surface roughness prior to the emergence of LIPSS

Since the first published observations of laser induced period surface structures (LIPSS) [1], such features have been produced by many research groups but aspects of the mechanism behind the emergence of these features remain unclear. Surface plasmons provide a periodic field which is understood to be involved in the formation of LIPSS as described in various publications [2-7]. However, when the incident laser energy is normal to the target surface LIPSS are formed despite there being no incoming vector component along the target surface, and hence no periodic energy structure to excite surface plasmons. A reasonable explanation is that if the surface is sufficiently rough, it will scatter the light and thereby provide periodic energy components parallel to the target surface. In this case spatial frequency components of the surface roughness could act as gratings to satisfy k vector matching to the plasmons [8]. LIPSS formation would therefore be expected to be strongly dependent on the initial surface roughness.