Initial Cumulative Effects in Femtosecond Pulsed Laser-induced Periodic Surface Structures on Bulk Metallic Glasses

Abstract

We investigate initial cumulative irradiation effects leading to variable surface topo graphies and nanoscale roughness , and triggering eventually the formation of laser -induce d periodic surface structures (LIPSS) on Zr -based bulk metallic glasses ( Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 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 topo graphical 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 t he vicinity of the damage domain , mult i- pulse incubation effect s determine t he 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 emp hasize 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 .