In this work, the effect of laser fluence on Au nanoparticles synthesized via laser ablation in liquids is studied for 1064 nm irradiation with 25 ps pulses. Particle size and polydispersity is found to display a negative trend with fluences up to ∼14 J cm(-2). Erratic size tendencies are observed at low fluences, i.e. slightly above the ablation threshold. This overall behavior is reconciled with recent computational studies and to fluctuations in ablation due to surface morphology. The effectiveness of the commonly used surfactant sodium dodecyl sulfate (SDS) is shown to diminish at higher fluence due to pyrolysis. In addition, shadowgraph imaging of the cavitation bubble is shown as a useful technique for determining the ablation threshold. Our findings are in good agreement with threshold values determined by traditional methods and are comparable to computational values, when differences in pulse duration are taken into account.

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Titanium dioxide is one of the most important materials today in terms of green technology. In this work, we synthesis ultra-small titanium dioxide nanoparticles (NPs) via a two step process involving infrared laser ablation of a bulk titanium target in DDI water and subsequent irradiation of the colloidal solution with visible light. The as-prepared NPs contain defect states related to oxygen vacancies which lead to visible light sensitization as observed by photodegradation of methylene blue. Irradiation of the colloidal T iO 2 solution, with a 532 nm picosecond laser, lead to fragmentation and ultimate formation of ultra-small (< 3 nm) anatase particles. Shadowgraph was utilized to capture shockwave and cavitation bubble propagation during both the ablation and fragmentation processes. High-frequency ripples within the primary shockwave are identified as coming from laser induced stress-wave reflections within the metal target. A blueshift of the bandgap, for the ultra-small NPs, is explained by quantum confinement effects and rationalized using the Brus model.

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We elucidate nonisotropic interactions present during laser ablation of ridged surface features under linearlypolarized light and their effect on nanoparticle synthesis via the laser ablation in liquid method. A relationship between the relative polarization/ridge orientation and ablation processes was established by measuring cavitation bubble kinetics, examining the morphology of the ablated surface, and evaluating nanoparticle size distribution. Anisotropic behavior was observed, and this is attributed to selective heating of linear surface structures as confirmed with rigorous coupledwave analysis. When the incident electric field is aligned parallel to the linear ridges on the target surface, the resulting nanoparticle distributions are much more monodisperse. This phenomenon has a strong influence in the polydispersity and batchtobatch reproducibility in laser ablation in liquid.

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