Evolution of nano-ripples on stainless steel irradiated by picosecond laser pulses

The characteristics of laser-induced periodic surface structures are investigated after laser irradiation with a pulse duration (10 ps) under a certain laser fluence (0.27 J/cm²) and pulse number (N = 50-2000). In the experiments, at the lowest number of pulses, ultrafine ripples are observed with sub-200-nm spatial periods and an orientation parallel to laser polarization direction. With the increasing pulse number, a chain of events that has been observed consists of ultrafine ripples followed by a transformation towards regular ripples with an orientation perpendicular to laser polarization direction and holes finally breaking regular ripples. Although the generation of ultrafine and regular ripples upon laser irradiation is a universal phenomenon, the origin behind the transition is discussed here to help understand the exact physical mechanism and their occurrence with their dependence on the irradiation parameters.