One of the remarkable capabilities of ultrashort polarized laser pulses isthe generation of laser-induced periodic surface structures (LIPSS). The originof this phenomenon is largely attributed to the interference of the incidentlaser wave and surface electromagnetic wave that creates a periodic absorptionpattern. Although, commonly, LIPSS are produced by repetitive irradiation ofthe same area by multiple laser pulses in the regime of surface melting andresolidification, recent reports demonstrate the formation of LIPSS in thesingle pulse irradiation regime at laser fluences well above the ablationthreshold. In this paper, we report results of a large-scale molecular dynamicssimulation aimed at providing insights into the mechanisms of single pulseablative LIPSS formation. The simulation performed for a Cr target reveals aninterplay of material removal and redistribution in the course of spatiallymodulated ablation, leading to the transient formation of an elongated liquidwall extending up to ~600 nm above the surface of the target at the locationsof the minima of the laser energy deposition. The upper part of the liquid walldisintegrates into droplets while the base of the wall solidifies on thetimescale of ~2 ns, producing a ~100 nm-long frozen surface feature extendingabove the level of the initial surface of the target. The properties of thesurface region of the target are modified by the presence of high densities ofdislocations and vacancies generated due to the rapid and highly nonequilibriumnature of the melting and resolidification processes. The insights into theLIPSS formation mechanisms may help in designing approaches for increasing theprocessing speed and improving the quality of the laser-patterned periodicsurface structures.
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