Effect of stationary femtosecond laser irradiation on substructures' formation on a mold stainless steel surface

Nano-scale and micro-scale laser-induced periodic surface structures (ripples), pores and splash-like structures with different formation thresholds fluence have been observed after stationary multiple linearly polarized femtosecond laser irradiations on a molding stainless steel. Nano-scale structures exhibit a formation threshold fluence lower than that of micro-scale ripples. When increasing both power density and number of pulses, nano-scale ripples are only sensitive to the latter, showing a 15% decrease of their spatial period in the range of used values. Micro-scale ripples respond to these two laser parameters by an increase followed by a decrease of their spatial period. The two types of ripples exhibit behavior differences that show that micro- and nano-scale ripples are the results of two distinct mechanisms.