Anomalous laser-induced periodic surface structures

The authors report the results of a study into the formation of periodic damage patterns on nominally smooth Ge surfaces using s-polarised laser light from a KrF excimer laser working at 249 nm. Dependent upon the incident fluence and to a lesser extent the angle of incidence, two types of pattern are found. At low fluences, patterns orientated parallel to the polarisation are found, whilst at higher fluences anomalous patterns that are neither parallel nor perpendicular to the polarisation are observed. By invoking a new formation mechanism for the anomalous patterns, both types of pattern are shown to be consistent with a previously developed theory.