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 in terferograms. A picosecond ablation plasma was measured with an electron de nsity on the order of 10(20) cm(-3) originating from the breakdown of air. The longitudinal expansion of this plasma was suppressed due to the develop ment of a strong space- charge field. At post-pulse times, the lateral (rad ial) expansion of the plasma was found to follow the relation, r similar to t(1/2), consistent with the expansion from an instantaneous line source of energy. The electron number density and temperature were deduced by measuring spect roscopic emission-line broadening during the early phase (30-300 ns) of a m ass (atomic/ionic) plasma. These properties were measured as a function of the delay time and irradiance. Possible mechanisms such as inverse bremsstr ahlung and self-regulation were used to describe the data before an explosi on threshold of 20 GW/cm(2). The laser self-focusing and critical temperatu re are discussed to explain dramatic changes in these properties after the irradiance threshold. On the microsecond time scale, the surface explodes and large (> mu m) part icles are ejected. Mass removed from single-crystal silicon by high power ( 10(9)-10(11)W/cm(2)) single-pulse laser ablation is studied by measuring th e crater morphology. Time-resolved shadowgraph images show that the rapid i ncrease in the crater depth at the threshold corresponds to large-size drop lets leaving the surface; This rapid growth of the crater volume is attribu ted to explosive boiling.