Pulsed Laser Ablation of Zinc in Tetrahydrofuran: Bypassing the Cavitation Bubble

We applied a high-power (25 W) picosecond-pulsed laser system in combination with fast scanner optics for pulsed laser ablation in liquids in order to generate zinc/zinc oxide nanoparticles in tetrahydrofuran with optimized efficiency. Systematic variation of repetition rate and interpulse distance of subsequent laser pulses strongly affects the ablation efficiency. Shielding of subsequent laser pulses by induced cavitation bubbles could be minimized by these parameters. The analysis of experimental data results in a time constant of 55 mu s concerning the cavitation bubble decay and a nonspherical shape with a lateral elongation of 120 mu m after 100 mu s. Regarding these parameters allows temporal and spatial bypassing of the cavitation bubble to enhance ablation efficiency and nanoparticle productivity. Furthermore, there is a nonlinear dependency of ablation efficiency on interpulse distance even if an effect coupled by cavitation bubbles can be excluded. We interpret this as a competition between two ablation mechanisms including thermal vaporization and phase explosion. For that purpose, we assume a transient preheating of the target by previous pulse, which leads to less efficient heat conduction that favors phase explosion instead of thermal vaporization. Calculations of ID-heat conduction and analysis of generated nanoparticles support that interpretation. We were able to model the shape of the cavitation bubble and the experimental data by an adopted fit function.