Dynamics of femtosecond laser-induced cluster emission from silicon
Résumé
Ultrashort laser pulses have opened unique opportunities for studying
mechanisms of laser-matter interactions by analyzing particles emitted
from the irradiated surfaces. A number of recent experiments indicate
that efficient emission of clusters can occur under certain conditions of
fs-laser ablation/desorption of semiconductors. Analysis of cluster emission
could provide a considerable insight into the complex interplay of
thermal and ultrafast, non-thermal processes in the laser-irradiated targets.
Here we report time-resolved pump-probe measurements of the yield of
singly-charged positive cluster ions (n = 2-11) from a silicon surface and
theoretical modeling of the laser-induced excitation of the Si target for
the experimental regimes. A 80-fs Ti:sapphire laser pulse was split into
two subthreshold pulses with a variable time delay and cluster emission
was monitored after each pair of pulses. When temporal separation between
the pulses is around few hundred femtoseconds, the emission is
found to be stronger than that with a single laser pulse of the same total
energy. We demonstrate also that there is significant incubation in silicon
in respect of cluster yield which is maximized after 30-40 pulses applied
to the same spot. Temporal and spatial behavior of electron and ion temperatures
in the Si target as well as surface charge density were simulated
as a function of pump-probe delay. Both modeling results and observations
imply a new mechanism of cluster emission involved. We suggest
that the first pulse induces the lattice destabilization and surface
structural changes while the second pulse, applied to the unstable surface,
acts as a trigger resulting in efficient emission of clusters. The role
of surface defects and premelting in cluster formation process will be
discussed.