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 nm, 35 ps laser pulse on a copper target. The plasma was found to have an electron density on the order of 10²⁰ 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 from the target surface. The longitudinal expansion of the plasma was suppressed due to the development of a strong space-charge region. At postpulse times, the relation r∼t^1/2 was found for the temporal lateral expansion of the radius of the plasma. Measurements of energy absorption by the plasma provide an interpretation for the experimentally measured reduction in ablation efficiency as the laser fluence increases beyond approximately 100 J/cm².