Observations of plasma formation during picosecond laser ablation of metals

We present experimental observations of plasma formation during single picosecond laser pulse (35 ps, 1064 nm) ablation of copper in air. We studied an early-stage air plasma which forms before the emergence of a material vapor plume. Using picosecond time-reserved shadowgraphs and interferograms, we measured the evolution of this air plasma that has an electron density on the order of 10(20) cm(-3). At post-pulse times, the longitudinal expansion of the plasma was found to be suppressed, while a power-law relation, r similar to t(1/2), was obtained for the temporal lateral expansion of the plasma radius. The results indicate that a large amount of the overall energy from the picosecond laser pulse is absorbed by the early-stage air plasma.