Chemical etching mechanisms and crater morphologies pre-irradiated by temporally decreasing pulse trains of femtosecond laser

We report the influence of temporally decreasing pulse trains on femtosecond laser-induced chemical etching (FLICE) of fused silica. A systematic comparison of the unshaped pulse and decreasing pulse trains of femtosecond laser for FLICE was conducted, and the differences were interpreted using a plasma model. The results revealed that the decreasing pulse trains not only affected the etching efficiency but also affected the morphology of the etched crater. When an etched crater was pre-irradiated by decreasing pulse trains, it presented a funnel-like shape at the early stage of the etching process, which contrasted with the one pre-irradiated by unshaped pulse. At the later stage of the etching process, the funnel-like shape gradually disappeared, and the crater increased in size. Compared with the unshaped pulse under the same processing conditions, the decreasing pulse trains enhanced the etched crater volume by approximately 18 times. Theoretical calculations based on the plasma model indicated that the free-electron density generated using the unshaped pulse was much higher than that generated by the decreasing pulse trains in skin layer of the sample during the first few hundred femtoseconds. The high free-electron density increased the reflectivity in skin layer of the sample; thus, the tail part of the incident pulse was strongly reflected. Consequently, the laser energy deposition into the fused silica sample decreased, eventually led to a low etching efficiency.