Pump-Selective Spectral Shaping of the Ultrafast Response in Plasmonic Nanostars

Plasmonic nanostructures are, to date, well-known to offer unique possibilities for the tailoring of light-matter interactions at the nanoscale. Most recently, a new route to ultrafast all-optical modulation has been disclosed by combining the resonant features of plasmonic nanostructures with the giant third-order optical nonlinearity of noble metals regulated by highly energetic (hot) carriers. In this framework, a variety of nanostructures have been designed, with special attention to shapes featuring tips, where extreme and highly sensitive field enhancements (hot spots) can be attained. Here, we report on a broadband pump-probe spectroscopy analysis of an ensemble of spiky star-shaped nanoparticles, exploring both the perturbative and nonperturbative regimes of photoexcitation. The experiments are corroborated by semiclassical numerical simulations of the ultrafast optical response of the sample. We found that the peculiar hot spots supported by the star tips allow one to easily control the spectral shape of the transient optical signal, upon tuning of the pump wavelength. Our results elucidate the ultrafast response of hot electrons in star-shaped nanostructures and contribute to the understanding of the tip-mediated enhanced nonlinearities. This work paves the way to the development of ultrafast all-optical plasmonic modulators for pump-selective spectral shaping.