Thermo-mechanical coupling induced Near-threshold double pulses femtosecond laser ablation enhancement in titanium

Near-threshold femtosecond laser surface treatment of titanium produces functional nanostructures, which has received much attention in applications and scientific researches. However, researches on the mechanism of near-threshold temporally-shaped femtosecond laser ablation of titanium are scarce, because the difficulty of investigating the modification mechanism of surface properties by sub-pulses below the ablation threshold. In this study, the double-pulse femtosecond laser ablation with various pulse separation was investigated. We found that the maximum ablation area appeared around 1 ps pulse separation at different fluences, and the ablation enhancement weakened with the increase of the fluence. The ultrafast pump-probe microscopy technique and ultrafast thermoelasticity model were used to investigate the mechanism. The result of pump-probe microscopy technique indicated that the enhancement of the area was caused by the decrease of transient reflectivity in the early stages which facilitated the subsequent sub-pulse energy absorption. The theoretical results indicate that the maximum ablation enhancement occurred at 1 ps, which is due to the combination of thermal and me-chanical effects. The maximum of long axis diameter of periodic structure also appeared at 1 ps in multi-point processing. These findings have significant implications for the modulation of titanium surface nanostructures and providing theoretical support for processing.