Multi-spot ultrafast laser ablation at ambient pressure - A new window on coalescing shock wave interactions

During ultrafast laser ablation at ambient pressure, redeposition of nano-particulates occurs through backwards flux towards the end of the ablation process and is often viewed as undesirable. Here, on the contrary, we report on unique, highly symmetric redeposition patterns observed during ultrafast laser ablation of metals with closely spaced multi-spots in ambient gases. Spot symmetries were altered with a Spatial Light Modulator or beam splitting optics. At low fluence (relative to material ablation threshold), debris is highly confined within the spot patterns, while at higher fluence, jets of debris emanate along axes of symmetry reaching distances far exceeding the spot separations. These phenomena appear universal but depend on the spot proximity, substrate, ambient gas density and pulse energy. The jets, formed at the collision planes between plasma plumes, consist of agglomerated nanoparticle debris, lifted and accelerated by colliding supersonic Mach shocks whose early interactions are imprinted on the debris fields. Numerical simulation using computational fluid dynamics (CFD) of multi-spot ablation in ambient gas supports this view of the phenomena. These observations are relevant to an improved understanding of coalescing shock waves, induced air flows and re-deposition at ambient pressure.