Mechanisms of nanoparticle formation by ultra-short laser ablation of metals in liquid environment

International audienceLaser ablation in liquids is now commonly used to produce colloidal nanoparticles (NPs) that have found numerous applications in different areas. In the experiments, NPs of different materials can be rather easily produced by using laser systems with various pulse durations, shape, wavelengths, and fluence. Here, we focus our attention on metal (gold) nanoparticles produced by ultra-short laser pulses due to their unique plasmonic properties. To better understand the mechanisms of the NPs formation, we perform modeling of ultra-short laser interactions with gold target in the presence of a liquid (water). The model is similar to that presented in Ref. [1]. In the model, we vary laser fluence. Simulation results show that for smaller laser fluence, gold target is heated to moderate temperatures, so that metastable melted layer is then fragmented into particles (Fig. 1a). In addition, a shock wave, a void/bubble is formed in front of the target [2], so that the particles are ejected into the bubble. They rapidly reach the bubble's front, and all the ablated material can be found near the border of the bubble by ~ 3-4 ns. For larger laser fluences (Fig. 1b), meta-stable liquid decomposes in the vicinity of the critical point forming liquid-gas mixture. The enhanced ablation creates much higher pressure. The bubble is also formed but is rapidly stopped by the hot liquid-gas phase (at ~3 ns after the pulse). At that time, the ablated material is composed of two zone, liquid-gas and liquid, which are then compressed and pushed back. The bubble collapses as soon as at ~7.5 ns due to strong backpressure from the liquid. As a result of the interaction with the ablated material, the target is re-heated and is melted down to as deep as ~0.5 µm under initial target's surface. Thus, the obtained results demonstrate that in the presence of a liquid, the fragmentation of metastable material leads to the NPs ejection. At moderate fluences, liquid just limits material expansion. At larger fluences, liquid pushes the ablated material back enhancing target's temperature, so that more NPs can later originate from this region