Time-resolved plasma diagnostics and mass removal during single-pulse laser ablation

R. E. Russo, X. L. Mao, H. C. Liu, J. H. Yoo, S. S. Mao

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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.

Original languageBritish English
Pages (from-to)S887-S894
JournalApplied Physics A: Materials Science and Processing
Issue number7
StatePublished - 1999


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