Effect of Laser Beam Spot Size on the Dynamics of Ultrashort Laser-Produced Plasma in Vacuum

Abstract

The hydrodynamic expansion features of a laser-produced plasma is governed by initial plume conditions and hence the spot size of the laser beam.1–​3 In this work, we investigated the expansion dynamics of a laser produced-plasma as a function of laser spot diameter in a vacuum environment (10–5 Torr) using multiple diagnostic tools. The diagnostic tools used were space- and time-resolved optical emission spectroscopy, 2-D fast-gated self-emission imaging and ion analysis using Faraday cup. The plume dynamics measured for varying spot sizes involves intriguing physics and hence demands detailed investigations, in view of a multitude of applications of laser ablation (LA) including elemental detection (LIBS, LA-LIF etc.), micromachining, laser-welding, nano-structuring, ion beam generation and pulsed laser deposition.

The plasma is produced on an aluminum target using ~ 120 fs pulses at ~ 800 nm, obtained from a regeneratively amplified Ti: Sapphire laser. The plume dynamics is studied for target positions before and after the geometric beam focus. The role of the ellipticity of the laser beam on the dynamics of the plume is also investigated. For a fixed laser energy, the ablation rate is found to increase with decreasing spot size, resulting in enhanced emission from various plume species. Plume expansion is found to be spherical for smaller spot size, while it is changed to cylindrical for larger laser spot size. For larger spot sizes, the time of flight (TOF) profile of the ions exhibits two peaks, corresponding to fast and slow ions respectively. Slow ions are emitted as a result of thermal vaporization while fast ions occur due to the time dependent ambipolar electric field. The role of spot size on changes in spectral intensity from excited atoms and ions, plume morphology, and temporal profiles of ion signals will be discussed in detail.