Scan speed and fluence effects in femtosecond laser induced micro/nano-structures on the surface of fused silica

The influence of scan speed and fluence on the formation of micro/nano-structures were explored under 800 nm, 35 femtosecond (fs) laser irradiation. Surface morphology, roughness, and laser-induced defects of prepared samples were characterized. The antireflection property and wettability of laser treated samples were also tested and compared with that of original fused silica. Results showed that the laser irradiated structures included melt zone, nanofibers, laser-induced periodic surface structures (LIPSS), and fluffy structures. At a fix scan speed, high-spatial-frequency LIPSS (HFSL) were observed in the low fluence region. A transition from HSFL to LSFL (low-spatial-frequency LIPSS) occurs when a critical fluence threshold is exceeded. Otherwise, the scan speed significantly influences the surface roughness and should be controlled carefully during the preparation process. The material structural modification in laser-irradiated fused silica surface was verified by the Raman spectra. The concentration of oxygen deficient centers (ODCs), and non-bridging oxygen hole centers (NBOHCs) increased after femtosecond laser irradiation by analyzing the photoluminescence. The laser irradiated surface has a better antireflection property in comparison with that of original fused silica, which may be due to the scattering and absorption of micro/nano structures. After laser treatment, the micro/nano structured fused silica presented super-hydrophilic property. The contact angle of water droplet decreases can be explained by Wenzel's model.