Femtosecond laser darkened copper surfaces: characterization for possible application as laser beam blocker

This study focuses on the optical and mechanical properties of copper surfaces darkened by femtosecond laser irradiation, with regard to their applicability as laser beam blockers. Copper surfaces were processed with a Ti-Sapphire laser (1 kHz repetition rate, ∼100–600 mJ/cm2 fluence), while scanning parameters were optimized for low reflectivity and processing speed. A total reflectivity below 6% was obtained for the 250–2500 nm wavelength range. SEM imaging and ion beam etching showed submicron period structures covered with high density nanoparticles, with a total depth of ∼3–4 μm. Numerical reflectivity simulations suggest that besides the structures, the formation of oxides also contributes to the observed darkening. Thermal and mechanical stability, chemical resistance and laser damage threshold were studied. Temperatures above 200°C caused surface degradation. Scratch tests showed that the nanostructured layers could withstand ∼60 MPa contact pressure. The surfaces could resist 48h treatment with various acids at 10−3M concentration. The laser damage threshold for the applied femtosecond laser showed that the surface could withstand the unfocused beam of most high-frequency femtosecond lasers. The results indicate that laser darkened copper can be a promising candidate for use in laser beam blockers.