Effect of laser parameters on plasma shielding in single and double pulse configurations during the ablation of an aluminium target

The process of nanosecond-laser ablation in air is a complex phenomenon, where the drilling process is a plasma mediated mechanism strongly affected by the shielding of the trailing part of the laser pulse. The laser pulse parameters as well as the target and ambient gas properties affect the dynamics of the process, modifying the importance and the initiation times of plasma shielding and laser-supported wave mechanisms. In this paper, the laser-target coupling of a Nd : YAG laser pulse in air at 1.06 μm impinging on an aluminium target was investigated by spectroscopic analysis of plasma emission and by inspection of craters with video-confocal microscopy. The effects of laser pulse width and energy were investigated both in the single pulse configuration and in the orthogonal pre-pulse double pulse configuration. While in the single pulse case, a strong plasma shielding is observed at irradiances higher than (7-9) x 108 W cm-2, which is thought to coincide with the onset of laser-supported wave mechanism, such a change of regime is not observed in the double pulse case. A physical interpretation of the data is proposed.