Mechanism of the single-pulse ablative generation of laser induced periodic surface structures

TL;DR

This study uses large-scale molecular dynamics simulations to explore how single ultrashort laser pulses can create periodic surface structures on materials through ablation, revealing detailed mechanisms of formation.

Contribution

It provides new insights into the atomic-scale processes of single-pulse LIPSS formation, which were previously mainly understood in multi-pulse regimes.

Findings

Transient formation of elongated liquid walls during ablation

Solidification of the base creates nanoscale surface features

High defect densities modify surface properties

Abstract

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.