Experimental and theoretical analysis of photofragmentation of Au nanoparticles by picosecond laser radiation

This paper presents a novel model aimed at describing the basic phenomena that cause the fragmentation of a single Au nanoparticle (AuNP) by interaction with the second and the third harmonics of a Nd:YAG picosecond laser. In order to verify the model through a comparison with experimental results, we extended the single-AuNP fragmentation model to treat the macroscopic bleaching of a suspension of AuNPs. The sample of AuNPs is obtained by laser ablation in an aqueous solution of fifth generation ethylendiamine-core poly(amidoamine) (PAMAM-G5) with the fundamental wavelength of the same laser at 1064 nm. The dependence of photobleaching on the laser pulse energy at 355 and 532 nm is studied and hence compared with the theoretical model. In particular, we discuss the role of heating and the interplay among pure thermal processes, e.g., melting and evaporation, thermoionic emission, and photon-assisted ionization. We show that, although the AuNP temperature can overcome the evaporation threshold in our range of investigation, the experimental curves disagree with a process of fragmentation mainly driven by heating. Our results highlight the role of photon-assisted transitions in AuNP fragmentation and allow us to discuss different regimes of fragmentation at different fluences and intensities. Moreover, we show that PAMAM-G5 plays a crucial role in our experiments.