Picosecond high-repetition-rate pulsed laser ablation of dielectrics: the effect of energy accumulation between pulses

We report experiments on the ablation of arsenic trisulphicle and silicon using high-repetition-rate (megahertz) trains of picosecond pulses. In the case of arsenic trisulphicle, the average single pulse fluence at ablation threshold is found to be > 100 times lower when pulses are delivered as a 76-MHz train compared with the case of a solitary pulse. For silicon, however, the threshold for a 4.1 -MHz train equals the value for a solitary pulse. A model of irradiation by high-repetition-rate pulse trains demonstrates that for arsenic trisulphide energy accumulates in the target surface from several hundred successive pulses, lowering the ablation threshold and causing a change from the laser-solid to laser-plasma mode as the surface temperature increases. 0 2005 Society of Photo-Optical Instrumentation Engineers.