Incubation during laser ablation with bursts of femtosecond pulses with picosecond delays

Abstract: We report on an experimental investigation of the incubation effect during irradiation of stainless steel with bursts of ultrashort laser pulses. A series of birefringent crystals was used to split the pristine 650-fs pulses into bursts of up to 32 sub-pulses with time separations of 1.5 ps and 3 ps, respectively. The number of selected bursts was varied between 50 and 1600. The threshold fluence was measured in case of Burst Mode (BM) processing depending on the burst features, i.e. the number of sub-pulses and their separation time, and on the number of bursts. We found as many values of threshold fluence as the combinations of the number of bursts and of sub-pulses constituting the bursts set to give the same total number of impinging sub-pulses. However, existing incubation models developed for Normal Pulse Mode (NPM) return, for a given number of impinging pulses, a constant value of threshold fluence. Therefore, a dependence of the incubation coefficient with the burst features was hypothesized and experimentally investigated. Numerical solutions of the Two Temperature Model (TTM) in case of irradiation with single bursts of up to 4 sub-pulses have been performed to interpret the experimental results

Short and long term surface chemistry and wetting behaviour of stainless steel with 1D and 2D periodic structures induced by bursts of femtosecond laser pulses

We investigate the short and long term wettability of laser textured stainless steel samples in order to better understand the interplay between surface topography and chemistry. Very different 1D and 2D periodic as well as non-periodic surface patterns were produced by exploiting the extreme flexibility of a setup consisting of five rotating birefringent crystals, which allows generating bursts of up to 32 femtosecond laser pulses with fixed intra-burst delay of 1.5 ps. The change of the surface morphology as a function of the pulse splitting, the burst polarization state and the fluence was systematically studied. The surface topography was characterized by SEM and AFM microscopy. The laser textured samples exhibited, initially, superhydrophilic behaviour which, during exposure to ambient air, turned into superhydrophobic with an exponential growth of the static contact angle. The dynamic contact angle measurements revealed a water adhesive character which was explained by XPS analyses of the surfaces that showed an increase of hydrocarbons and more oxidized metal species with the aging. The characteristic water adhesiveness and superhydrophobicity of laser textured surfaces can be exploited for no loss droplet reversible transportation or harvesting