Unveiling nature and consequences of tungsten oxidation upon ultrafast laser irradiation

Despite ultrafast laser-induced topography modification becoming a recognized surface texturing technique in the recent years, comparatively little work has focused on the accompanying chemical alterations. This study aims to fill that gap by investigating the oxidation induced by ultrafast laser irradiation of metals, with a specific focus on tungsten, in different environments: ambient and high vacuum (10-7 mbar). Laser irradiating conditions were chosen to generate so-called High Spatial Frequency Laser Induced Periodic Surface Structures with a sub-100 nm period and sub-20 nm amplitude, as they are supposed to arise in a non-ablative regime. Contact angle measurements, Scanning Transmission electron microscopy cross-sectional images, and x-ray photoelectron spectroscopy analyses were used to investigate the surface chemistry of these structures and reveal significant structural differences between the laser-generated oxides and those accumulated over time from ambient exposure. To establish an oxidation mechanism during laser interaction with tungsten, Two Temperature Model and Molecular Dynamics simulations (TTM-MD) were conducted to determine the temperature evolution over time. The simulation results, complemented by oxygen diffusion data, provide a predictive insight into the development of a thin oxide layer induced by laser irradiation, a conclusion substantiated by the STEM images. These findings suggest that oxidation can occur mostly by solid-state diffusion while the surface is still in the process of cooling down to room temperature following ultrafast photoexcitation.