Effect of laser beam size on the dynamics of ultrashort laser-produced aluminum plasma in vacuum

Abstract

In laser-produced plasma experiments, the diameter of the irradiating laser beam on the target surface is a major parameter that influences the ablation mechanisms, plasma emission intensity, charged particle ejection, and plume morphology. In this work, the expansion dynamics of an ultrashort laser-produced aluminum plasma is investigated as a function of the laser beam size on the target, using a combination of diagnostic tools, viz., optical emission spectroscopy, fast gated time-resolved imaging, and ion current measurements. A Ti:sapphire laser delivering 100 fs, 6 mJ pulses at 800 nm is used for producing plasma from a pure Al target placed in vacuum (10 -5 Torr) at different positions with respect to the geometrical focus of the beam. Optical emission spectroscopic analysis of the plasma shows that higher emission intensities and ion populations are obtained for smaller beam sizes. Time-resolved Intensified Charge Coupled Device (ICCD) imaging of the expanding plasma shows a spherical morphology for plumes produced by smaller beam sizes and a cylindrical morphology for those produced by larger beam sizes. Temporal profiles of ion emission measured using a Faraday cup are in agreement with ICCD data, featuring a dual peak structure for larger beam sizes indicating distinct slow and fast ionic species, arising from changes in the ablation mechanism for varying laser fluences. Plume expansion is modelled by free expansion for the fast species and by shock wave propagation for the slow species. Ion flux and velocities are relatively high for smaller beam sizes. These studies can be of potential importance for laser processing applications, including laser welding, drilling, and micromachining. © 2019 Author(s).