Dynamics of an air breakdown plasma on a solid surface during picosecond laser ablation

Using picosecond time-resolved shadowgrams and interferograms, we measured the lateral expansion of an early stage ablation plasma induced by a 1064 n m, 35 ps laser pulse on a copper target. The plasma was found to have an el ectron density on the order of 10(20) cm(-3) near the target surface. Prior to the expanding material vapor plume, this high density plasma originates from the breakdown of air, assisted by laser-induced electron emission fro m the target surface. The longitudinal expansion of the plasma was suppress ed due to the development of a strong space-charge region. At postpulse tim es, the relation r similar to t(1/2) was found for the temporal lateral exp ansion of the radius of the plasma. Measurements of energy absorption by th e plasma provide an interpretation for the experimentally measured reductio n in ablation efficiency as the laser fluence increases beyond approximatel y 100 J/cm(2). (C) 2000 American Institute of Physics. [S0003-6951(00)01801 -5].