Interband Transitions Are more Efficient than Plasmonic Excitation in the Ultrafast Melting of Electromagnetically-Coupled Au Nanoparticles

We investigated the effects of ultrafast laser excitation of Au nanoparticles (NPs)
having strong interparticle electromagnetic coupling, by irradiating the NPs either
at interband or plasmon-resonance wavelengths (13-100 J/m
2
fluence regime). We
observed that interband excitation is significantly more efficient than plasmonic exci-
tation in reshaping, coalescing and ultimately sublimating the NPs, despite the light-
absorption cross-section of interband excitation being almost half that of plasmonic
irradiation. We ascribed this to the different localization of radiation-induced heat
sources in the strongly-coupled NPs in the two cases. Interband excitation induces
homogeneous heat generation in Au, and so the conventional NP heating pathway is
followed, eventually leading to overall melting, coalescence and ablation of Au. Plas-
monic irradiation, on the other hand, promotes strong localization of the heat sources
within small energetic hot spots, a fact that we suggest may lead to non-thermal effects
that melt and reshape the NPs only on the local scale, leaving the system otherwise
relatively unscathed.