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{\textendash}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{\textendash}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.
}, year = {1999}, booktitle = {Applied Physics A: Materials Science \& Processing}, journal = {Applied Physics A: Materials Science \& Processing}, series = {Applied Physics A: Materials Science \& Processing}, volume = {69}, pages = {S887-S894}, month = {12/1999}, issn = {0947-8396 (print), 1432-0630 (online)}, doi = {10.1007/s003390051553}, language = {eng}, }