  One of the remarkable capabilities of ultrashort polarized laser pulses is
the generation of laser-induced periodic surface structures (LIPSS). The origin
of this phenomenon is largely attributed to the interference of the incident
laser wave and surface electromagnetic wave that creates a periodic absorption
pattern. Although, commonly, LIPSS are produced by repetitive irradiation of
the same area by multiple laser pulses in the regime of surface melting and
resolidification, recent reports demonstrate the formation of LIPSS in the
single pulse irradiation regime at laser fluences well above the ablation
threshold. In this paper, we report results of a large-scale molecular dynamics
simulation aimed at providing insights into the mechanisms of single pulse
ablative LIPSS formation. The simulation performed for a Cr target reveals an
interplay of material removal and redistribution in the course of spatially
modulated ablation, leading to the transient formation of an elongated liquid
wall extending up to ~600 nm above the surface of the target at the locations
of the minima of the laser energy deposition. The upper part of the liquid wall
disintegrates into droplets while the base of the wall solidifies on the
timescale of ~2 ns, producing a ~100 nm-long frozen surface feature extending
above the level of the initial surface of the target. The properties of the
surface region of the target are modified by the presence of high densities of
dislocations and vacancies generated due to the rapid and highly nonequilibrium
nature of the melting and resolidification processes. The insights into the
LIPSS formation mechanisms may help in designing approaches for increasing the
processing speed and improving the quality of the laser-patterned periodic
surface structures.
