Impact of liquid layer thickness on the dynamics of nano- to sub-microsecond phenomena of nanosecond pulsed laser ablation in liquid

The dynamics of nanosecond pulsed laser ablation in liquid (ns-PLAL) are significantly altered in the immediate vicinity of a free boundary. To improve the control and efficiency of ns-PLAL, more knowledge is needed on the evolution of the plasma, shock waves, and cavitation bubbles that form when this technique is performed within a thin liquid layer. Here, we present time-resolved photoelastic observations of ns-PLAL using an epoxy resin target covered with a sub-mm liquid paraffin layer. The investigation was conducted on a nanosecond time scale. When the liquid layer thickness approximates the plasma size that is induced in bulk-liquid ablation, part of the plasma plume is formed outside the liquid. This plasma is more extensive than that produced in bulk-liquid ablation, but more compact than in air ablation. At greater liquid layer thicknesses, the plasma is entirely confined within the liquid. The layer surface is elevated due to the expansion of the cavitation bubble on the target. The laser-induced shock wave is reflected back-and-forth between the liquid-air boundary and target surface, causing the formation of a bulk cavitation zone in the liquid layer. This study also discusses the mechanism by which liquid layer thickness affects the strength of laser-induced stress.