Surface damage of crystalline silicon by low fluence femtosecond laser pulses

Crystalline silicon kept at atmospheric pressure was irradiated with 775 nm multiple laser pulses of 150 fs duration at repetition rate 250 Hz. The laser pulses were circularly polarized, with a peak laser fluence of 0.03 J/cm². We observed surface damage at a much lower fluence and lower number of pulses compared to that reported in the literature. Surface damage, cracks and pits formation were observed. The evolution of the surface damage as a function of the number of laser pulses was recorded. The observations were in contrast to the findings in the literature that the silicon surface became structured when irradiated by multiple pulses.