Large area laser-induced periodic surface structures on steel by bursts of femtosecond pulses with picosecond delays

Bursts of linearly polarised femtosecond laser pulses with variable intra-burst delays on the picosecond timescale and different number of sub-pulses were used to produce laser-induced periodic surface structures (LIPSS) on stainless steel surfaces. The influence on the LIPSS morphology of the number sub-pulses, from 2 to 32, and the time separation between them, from 1.5 ps to 24 ps, was systematically investigated and compared to the case of unsplit pulses. The spatial periods and depths of the LIPSS produced by different irradiation conditions were derived by scanning electron and atomic force microscopies revealing that, in case of bursts with only two sub-pulses, an increase of the intra-burst delay produces nanoripples with higher spatial separation but shallower depth which is ascribed to a shielding effect. Whereas, increasing the number of sub-pulses a slight increase of the LIPSS spatial period has been observed