Exploring 2D-LIPSS formation under circular polarization in ultrafast laser processing.

The formation of isotropic two-dimensional laser-induced periodic surface structures (2D-LIPSS) under circular polarization demonstrates a unique self-organization capability. In this study, we investigate LIPSS formation under both circularly and linearly polarized light, assessing the impact of polarization dynamics, from discrete rotation angles to ultrafast changes within a single optical cycle, on pattern formation. This phenomenon stems from the surface's ability to temporally integrate all polarization states within an optical cycle, leading to a circularly oriented response. Similarly to linear polarization, the interference between the topography-induced scattered field and the incident laser field guides the process, with feedback-driven topographical evolution sustaining structure growth. However, unlike linearly polarized light, which imposes unidirectional alignment, circular polarization promotes a more symmetric arrangement. Superposed on the hexagonal lattice of pillars with short-range order, near-field interactions generate radially oriented nanostructures that interconnect the pillars via concentric filaments. By investigating these two spatial scales separately, the respective formation mechanisms and their interplay can be clarified. Supported by electromagnetic simulations, these findings offer a comprehensive framework for understanding the mechanisms behind isotropic 2D-LIPSS formation.