Millisecond laser processing of sapphire assisted by femtosecond laser-induced air filament

High-energy continuous wave (CW) lasers are mostly used in laser damage applications, but efficient laser ablation of transparent materials is challenging due to low optical absorption. Considering the potential of femtosecond (fs) laser-induced air filament for high-peak laser transmission over long distances, femtosecond (fs) laser-induced air filaments are combined with a millisecond (ms) laser to form an fs-ms CPL, enhancing the efficiency of sapphire ablation through synchronized spatial-temporal focusing. Experimental results show that ablation efficiency increases with the ms peak power and duty ratio. Excessive thermal stress leads to fragmentation of the sapphire when the ms duty ratio is over 30% at the peak power of 800 W, or when the peak power is over 500 W at a duty ratio of 100%. Also, the mechanism of high-efficiency damage is revealed through in-situ high-speed imaging. According to it, the ablation process went through 4 stages within 1.5 ms: defect-creating, melting and ablation, spattering, and fragmentation. Finally, the equivalent ablation efficiency of the fs-ms CPL is as high as 1.73×10⁷ µm³/J, about 28 times higher compared to the fs laser only. The CPL damage method explored in this paper can provide theoretical guidance for efficient laser damage of transparent materials. (Figure presented.)