Wavelength-dependent laser fragmentation of gold microparticles in water

Nanosecond laser fragmentation in liquids of micrometer-scale feedstocks is poorly understood, particularly regarding how wavelength steers photothermal versus photomechanical pathways. We investigate the fragmentation of Au microparticles (10–20 μm) in water using 5.9 ± 0.7 ns Nd:YAG pulses at 1064, 532, and 355 nm (up to 30 000 pulses). Colloid evolution was tracked in situ by UV–Vis–NIR extinction and two pulsed photoacoustic (PA) regimes: (i) focused, high-fluence fragmentation pulses and (ii) unfocused, low-fluence thermoelastic pulses mainly sensitive to absorption. With Mie-based absorption/scattering efficiencies and electron microscopy, the results indicate wavelength-dependent regimes. At 1064 nm, fragmentation appears progressive and tends to self-limit as optical coupling decreases with shrinking size, yielding a colloid dominated by primary nanoparticles (mean ∼10 nm, mostly 50 nm) with limited aggregation. At 355 nm, strong interband absorption in a thin near-surface layer can promote transient thermal gradients and thermomechanical stresses; together with cavitation transients in the liquid, these effects may contribute to abundant irregular submicrometer fragments (hundreds of nm) alongside a smaller nanoparticle population (mean ∼5 nm). At 532 nm, plasmon-resonant coupling may initially generate both submicrometer fragments and nanoparticles, while continued irradiation can further break down larger species and, at longer exposure, aggregation/coalescence and partial sedimentation may also become relevant, producing nonmonotonic extinction and the lowest primary particle index. Overall, combined extinction–PA monitoring helps distinguish absorption-driven evolution from scattering-dominated contributions.