Initiation of an early-stage plasma during picosecond laser ablation of solids

Picosecond time-resolved images of plasma initiation were recorded during pulsed-laser ablation of metal targets in an air atmosphere. An early-stage plasma was observed to form before the release of a material vapor plume. Close to the target surface, interferometry measurements indicate that the early-stage plasma has an electron number density on the order of 10²⁰ cm^-3. The longitudinal expansion of the ionization front for this plasma has a velocity 10⁹ cm/s, during the laser pulse. In contrast, a material-vapor plume forms approximately 200 ps after the laser pulse, and it moves away from the target at 10⁶ cm/s. The experimental observations of the early-stage plasma were simulated by using a theoretical model based on a two-fluids description of laser plasmas. The results indicate that the initiation of the plasma is due to air breakdown assisted by electron emission from the target.