Multipulse feedback in self-organized ripples formation upon femtosecond laser ablation from silicon

The influence of positive feedback on self-organized nanostructure (ripples) formation is investigated for multipulse femtosecond laser ablation from silicon surface. We find an increase of the modified surface area and of complexity and feature size with accumulated dose, confirming the previously postulated feedback effect of dose accumulation. More interestingly, a variation of temporal pulse-to-pulse separation, at constant total incident irradiation dose, strongly affects the structure formation. Though the feedback becomes weaker with increasing time intervals between successive pulses, pulses do not act independently even for separations of up to one second. To account for this observation, a model of perturbation decay and outdiffusion from the excited volume is suggested and compared to the experimental results. Inspection by surface sensitive microscopy (AFM, SEM) and conventional and high-resolution transmission electron microscopy reveal complex structural modification upon the laser interaction: even well outside the irradiated area, the target surface exhibits fine ripple-like undulations, consisting of alternating crystalline and amorphous silicon. This is confirmed by photoluminescence studies on the band–band and the dislocation-related D1-line.