Ripple Formation by Femtosecond Laser Pulses for Enhanced Absorptance of Stainless Steel

Formation of laser induced periodic surface structures (LIPSS), also called ripples, is a widelyknown phenomenon which occurs when multiple laser pulses with the fluence near the material ablation threshold are absorbed by the target material. Ripple formation depends on material properties as well as experimental conditions such as laser wavelength, pulse duration, fluence, number of pulses, polarization etc. Optical properties of the ripple-textured metal surface are changed and this can be employed to blacken the metals to enhance light absorption. In our work, ripples were formed on a stainless steel surface using femtosecond Yb:KGW laser (pulse duration 300 fs, pulse repetition rate 100 kHz) operating at 515 nm wavelength. The ripple period varied between 240-400 nm depending on the experimental conditions. Specular reflection measurements at 800 nm wavelength revealed, that the absortance of the ripple-textured surface increased exponentially with increasing laser irradiation dose (pulse number multiplied by laser fluence). The energy dispersive X-ray chemical analysis (EDS) showed changes in surface chemical composition, therefore both effects contributed to changes in surface absorbtance. The results were applied to control local reflectivity of metallic surfaces.