Influence of preformed shock wave on the development of picosecond laser ablation plasma

Using picosecond time-resolved shadowgrams, we measured the development of an expanding air plasma during double picosecond pulse laser ablation of solids. Prior to the formation of the air plasma, a shock wave was established by the first laser pulse interaction with the solid target. The second picosecond laser pulse interacted with the shock wave and induced the air plasma at the interface between the shock wave front and the ambient air. After the end of the second picosecond ablation laser pulse, the temporal expansion of the air plasma was found to follow the relation r~t^2/5. Results from this air plasma suggest that the shock wave induced by the first laser pulse only absorbs approximately 5% of the energy of the second picosecond laser pulse. Nevertheless, the expansion of the air plasma towards the target results in an enlarged heat-affected zone at the target surface.