Time-resolved plasma diagnostics and mass removal during single-pulse laser ablation
Laser ablation processes occurring over several orders of magnitude in time were investigated by using time-resolved spectroscopy, shadowgraphs and interferograms. A picosecond ablation plasma was measured with an electron density on the order of 1020 cm-3 originating from the breakdown of air. The longitudinal expansion of this plasma was suppressed due to the development of a strong space-charge field. At post-pulse times, the lateral (radial) expansion of the plasma was found to follow the relation, r∼t1/2, consistent with the expansion from an instantaneous line source of energy.
The electron number density and temperature were deduced by measuring spectroscopic emission-line broadening during the early phase (30–300 ns) of a mass (atomic/ionic) plasma. These properties were measured as a function of the delay time and irradiance. Possible mechanisms such as inverse bremsstrahlung and self-regulation were used to describe the data before an explosion threshold of 20 GW/cm2. The laser self-focusing and critical temperature are discussed to explain dramatic changes in these properties after the irradiance threshold.
On the microsecond time scale, the surface explodes and large (>μm) particles are ejected. Mass removed from single-crystal silicon by high power (109–1011 W/cm2) single-pulse laser ablation is studied by measuring the crater morphology. Time-resolved shadowgraph images show that the rapid increase in the crater depth at the threshold corresponds to large-size droplets leaving the surface. This rapid growth of the crater volume is attributed to explosive boiling.