Investigation of plume dynamics during picosecond laser ablation of H13 steel using high-speed digital holography

Abstract

Ablation of H13 tool steel using pulse packets with repetition rates of 400 and 1000 kHz and pulse energies of 75 and 44μJ, respectively, is investigated. A drop in ablation efficiency (defined here as the depth per pulse or μ m / μ J) is shown to occur when using pulse energies of Epulse>44μJ, accompanied by a marked difference in crater morphology. A pulsed digital holographic system is applied to image the resulting plumes, showing a persistent plume in both cases. Holographic data are used to calculate the plume absorption and subsequently the fraction of pulse energy arriving at the surface after traversing the plume for different pulse arrival times. A significant proportion of the pulse energy is shown to be absorbed in the plume for Epulse>44μJ for pulse arrival times corresponding to > 1 MHz pulse repetition rate, shifting the interaction to a vapour-dominated ablation regime, an energetically costlier ablation mechanism.