Laser-based techniques for nanoparticle synthesis have gained increasing attention as alternatives to conventional chemical routes, which often yield nanoparticles contaminated with surfactants and generate substantial chemical waste. Among them, laser ablation and microparticle fragmentation in liquids produce surfactant-free nanoparticles, align with Green Chemistry principles, and provide access to a broad library of nanomaterials. However, the dynamics of these processes and their connection to nanoparticle productivity and size distribution remain insufficiently understood. Furthermore, quantitative benchmarks for maximum achievable productivity under idealized conditions are lacking in the current state of the art.
This work systematically investigates the mechanisms that govern nanoparticle formation and establishes quantitative links between dynamics and final-state metrics. Single-pulse experiments allowed for precise control of laser parameters, while time-resolved pump–probe microscopy enabled direct observation of the process dynamics across timescales from picoseconds to milliseconds. The experiments revealed that redeposition of ablated material reduces ablation efficiency in liquids by a factor of four compared to ablation in air. Nonetheless, the fundamental power-specific productivity limit of 75 mg h^-1 W^-1, determined under idealized single-pulse conditions, exceeds current experimental records by a factor of four, highlighting substantial room for process improvement. This limit is largely independent of pulse duration, in contrast to ablation in air, where efficiency decreases with increasing pulse duration. Furthermore, it was found that for pulse durations below 10 ps, nonlinear absorption in the liquid layer reduces energy delivery to the target, while at durations beyond 1 ns, shielding by the cavitation bubble limits efficiency. This defines an optimal pulse duration window from 10 ps to 1 ns for laser ablation in liquids. Two distinct nanoparticle formation mechanisms predicted by simulations were experimentally validated: nucleation from vapor within the phase explosion regime yields primary nanoparticles below 10 nm, while spallation layer disintegration produces secondary nanoparticles up to several tens of nanometers. These two mechanisms also govern microparticle fragmentation, giving rise to similar size fractions.
In spherical microparticles, a third mechanism involving pressure focusing followed by fracture initiated in the microparticles core was identified, generating a third size fraction with diameters exceeding 100 nm. Under idealized single-pulse conditions, microparticle fragmentation achieved a power-specific productivity of 720 mg h^-1 W^-1, nearly an order of magnitude above the laser ablation in liquids limit, due to energy confinement and pressure focusing. The results define productivity limits for both laser ablation and microparticle fragmentation in liquids and identify key loss channels that constrain current performance. Based on these insights, distinct beam shaping strategies are proposed for the two techniques. In laser ablation in liquids, temporal pulse shaping such as pulse splitting or increasing the pulse duration can enhance productivity, with temporal pulse splitting also enabling narrower nanoparticle size distributions. In microparticle fragmentation in liquids, spatial beam shaping to achieve a uniform intensity profile can enhance productivity, while spatial beam splitting can be used to narrow the size distribution. These measures pave the way toward more efficient and scalable laser nanoparticle synthesis, with the potential to reach industrial-scale throughputs in the kilogram-per-hour range for applications in catalysis, clean energy and beyond.