Evidence for phase-explosion and generation of large particles during highpower nanosecond laser ablation of silicon

The craters resulting from high-irradiance (1x10(9)-1x10(11) W/cm(2)) singl e-pulse laser ablation of single-crystal silicon show a dramatic increase i n volume at a threshold irradiance of 2.2x10(10) W/CM2. Time-resolved shado wgraph images show ejection of large particulates from the sample above thi s threshold irradiance, with a time delay similar to 300 ns. A numerical mo del was used to estimate the thickness of a superheated layer near the crit ical state. Considering the transformation of liquid metal into liquid diel ectric near the critical state (i.e., induced transparency), the calculated thickness of the superheated layer at a delay time of 200-300 ns agreed wi th the measured crater depths. This agreement suggests that induced transpa rency promotes the formation of a deep superheated layer, and explosive boi ling within this layer leads to particulate ejection from the sample. (C) 2 000 American Institute of Physics. [S0003-6951(00)03206-X].