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 so lids. Prior to the formation of the air plasma, a shock wave was establishe d by the first laser pulse interaction with the solid target. The second pi cosecond laser pulse interacted with the shock wave and induced the air pla sma at the interface between the shock wave front and the ambient air. Afte r the end of the second picosecond ablation laser pulse, the temporal expan sion of the air plasma was found to follow the relation r similar tot(2/5). Results from this air plasma suggest that the shock wave induced by the fi rst laser pulse only absorbs approximately 5% of the energy of the second p icosecond laser pulse. Nevertheless, the expansion of the air plasma toward s the target results in an enlarged heat-affected zone at the target surfac e. (C) 2001 American Institute of Physics.