Initial cumulative effects in femtosecond pulsed laser-induced periodic surface structures on bulk metallic glasses

We investigate initial cumulative irradiation effects leading to variable surface topographies and nanoscale roughness, and triggering eventually the formation of laser-induced periodic surface structures (LIPSS) on Zr-based bulk metallic glasses (Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_25.5 (at%)). We discuss interconnected aspects related to electronic excitation and optical transients, potential variations in the cartography of thermally-driven chemical modifications and topographical features assisting the surface coupling of the electromagnetic field. The transient optical properties of Zr-based BMG surfaces upon ultrafast irradiation, measured by a two-angle time-resolved single-pump double-probe ellipsometry method, show a remarkable constancy up to the point of optical damage and rapid gas-phase transition beyond. In intermediate and low exposure conditions, in the vicinity of the damage domain, multi-pulse incubation effects determine the appearance of nanoscale surface structures. The aspects discussed here involve primarily the progression of nanoscale structuring with an increasing number of fs laser pulses starting from a rough surface and evolving towards ordered corrugation. We emphasize the role of initial roughness in determining light coupling and the generation of regular stationary patterns of scattered light, localized energy absorption and spatially-variant ablation or modulated temperature-driven factors for surface relief. From a material perspective, energy dispersive X-ray spectrometry (EDX) analysis shows potential selective vaporization of light elements, leading to gradual compositional changes and proving a spatially-modulated temperature pattern. A formation scenario is proposed involving interference between the incident laser and scattered light potentially mediated by localized surface plasmons. Finite-difference time-domain (FDTD) simulations are applied to validate the mechanism, showing that LIPSS appear intrinsically related to the surface superposition of electromagnetic waves.