Long-Time Feedback in the Formation of Self-Organized Nanostructures upon Multipulse Femtosecond Laser Ablation

The self-organization feedback in nanostructures (ripples) formation upon femtosecond laser ablation is investigated in detail, with particular emphasis on its dynamics. We study the influence of time separation between successive pulses on both the size and complexity of the nanostructures and on the size of the modified surface area. By varying the pulse separation between 1 ms and 1 s (rep. rate between 1 kHz and 1 Hz), we find that both modified area as well as pattern feature size and complexity decrease with increasing pulse-to-pulse delay, indicating that the coupling efficiency between laser and target increases with increasing repetition rate. Structure formation resulting from surface instability, induced by the laser impact, suggests that the laser-induced instability results in a transient increase of absorption probability, slowly decaying in time. The stronger this transient absorption, the better is the coupling for the succeeding pulse, thus resulting in a positive feedback. Experimental results indicate that the lifetime of the feedback can last up to one second and longer.In addition, a persisting modulated spatial modification of crystalline properties is observed well beyond the ablation spot, though no apparent influence on surface morphology is seen. Mapping the band-to-band photoluminescence from silicon indicates a dramatic increase of non-radiative recombination compared to unaffected material.