Dynamics of femtosecond laser-induced cluster emission from silicon

International audienceUltrashort laser pulses have opened unique opportunities for studyingmechanisms of laser-matter interactions by analyzing particles emittedfrom the irradiated surfaces. A number of recent experiments indicatethat efficient emission of clusters can occur under certain conditions offs-laser ablation/desorption of semiconductors. Analysis of cluster emissioncould provide a considerable insight into the complex interplay ofthermal and ultrafast, non-thermal processes in the laser-irradiated targets.Here we report time-resolved pump-probe measurements of the yield ofsingly-charged positive cluster ions (n = 2-11) from a silicon surface andtheoretical modeling of the laser-induced excitation of the Si target forthe experimental regimes. A 80-fs Ti:sapphire laser pulse was split intotwo subthreshold pulses with a variable time delay and cluster emissionwas monitored after each pair of pulses. When temporal separation betweenthe pulses is around few hundred femtoseconds, the emission isfound to be stronger than that with a single laser pulse of the same totalenergy. We demonstrate also that there is significant incubation in siliconin respect of cluster yield which is maximized after 30-40 pulses appliedto the same spot. Temporal and spatial behavior of electron and ion temperaturesin the Si target as well as surface charge density were simulatedas a function of pump-probe delay. Both modeling results and observationsimply a new mechanism of cluster emission involved. We suggestthat the first pulse induces the lattice destabilization and surfacestructural changes while the second pulse, applied to the unstable surface,acts as a trigger resulting in efficient emission of clusters. The roleof surface defects and premelting in cluster formation process will bediscussed