Effects of pulse duration on laser-induced crystallization of urea from 300 to 1200 fs: impact of cavitation bubbles on crystal nucleation

The dependence of pulse duration on crystal nucleation of urea induced by laser ablation of liquid was studied. The focused irradiation with laser pulses with all the duration conditions in this study (from 300 to 1200 fs) could induce the crystallization from the laser focus, which allowed for the systematic evaluation of probability of the laser-induced crystallization. We also carried out the microsecond-scale fast imaging of laser ablation of urea solutions to investigate the spatiotemporal dynamics of generation, expansion, collapse of cavitation bubbles induced by laser ablation, which can be one of the main triggers for crystal nucleation. As the result, we found that the laser-induced crystallization of urea is governed by the interplay of supersaturation increase by cavitation bubbles and supersaturation decrease by temperature rise. The findings provide the fundamental insight into the underlying mechanism of the laser-induced crystal nucleation and will also be useful for exploring optimized laser irradiation conditions for more efficient crystallization of organic materials in solutions.