Femtosecond laser-induced fabrication and performance study of scattering protrusions on silicon dioxide surfaces

Abstract This study systematically investigates the fabrication mechanism and process controllability of femtosecond (fs) laser–induced protrusive structures on silicon dioxide (SiO₂) surfaces using a 515 nm fs laser. To elucidate the coupling between laser energy deposition and thermal accumulation, an extended two-temperature model applicable to ultrashort-pulse excitation of dielectric materials is employed to analyze the transient thermal response of SiO₂ under fs laser irradiation. In this framework, protrusion formation is interpreted as a consequence of localized thermal softening and volumetric expansion when the lattice temperature exceeds a critical threshold. Experimental results demonstrate that the protrusion morphology can be effectively tuned by adjusting key laser parameters, including average power, pulse repetition frequency, and scan speed, enabling controllable protrusion widths and heights from the submicrometer to micrometer scale. Within the sub-ablation regime, the simulated thermal field distributions exhibit good qualitative agreement with the experimentally observed protrusion patterns. In addition, fs-laser-processed SiO₂ surfaces show a pronounced enhancement in apparent reflectivity. For SiO₂ layers with a thickness of 3 μ m, the reflectance in the visible range reaches up to 11.7%, corresponding to an approximately fourfold increase compared with untreated regions, while thicker SiO₂ layers (5 μ m) exhibit reflectance values of up to ∼31%, representing an enhancement of up to ∼18×. The improved optical visibility and compatibility with conventional photolithographic processes indicate that the proposed approach holds considerable potential for applications in micro-optical devices, lithographic alignment markers, and surface-functionalized dielectric microstructures.