  Formation of laser-induced periodic surface structures (LIPSS or ripples) was
studied on a metallic surface of polished copper by using irradiation with
multiple femtosecond laser pulses in different environmental conditions (air,
water, ethanol and methanol). Uniform LIPSS have been achieved by controlling
the peak fluence and the overlapping rate. Ripples in both orientations,
perpendicular and parallel to laser polarization, were observed in all liquids
simultaneously. The orientation of these ripples in the center of the ablated
line was changing with the incident light intensity. For low intensities the
orientation of the ripples is perpendicular to the laser polarization, whereas
for high intensities it turns parallel to it without considerable changes in
the period. Multi-directional LIPSS formation was also observed for moderate
peak fluence in liquid environments.
