Ultrabroadband Excitation of Hot Carriers in Plasmonic Nanorods Revealed by Two‐Dimensional Electronic Spectroscopy

Plasmonic nanostructures photoexcited with ultrashort light pulses exhibit a strong nonlinear optical response driven by nonequilibrium ‘hot’ carriers. Studying the spectro-temporal evolution of such nonlinearities to extract information on hot electron dynamics has attracted significant interest, given the unparalleled opportunities unlocked by these high-energy carriers in fields ranging from photocatalysis to optical communications. However, in typical samples of size-dispersed nanoparticles, effects such as inhomogeneous broadening and pump-pulse-induced selectivity can distort the system response, hindering accurate characterizations. This study dissects the ultrafast response of polydisperse gold nanorods employing two-dimensional electronic spectroscopy (2DES), a powerful technique offering a unique combination of temporal and spectral resolution. The ultrabroadband pulses cover both the transverse and longitudinal nanorod resonances, enabling an accurate analysis of their distinct behavior. By complementing experiments with a quantitative model of hot-carrier-mediated nonlinearities that incorporates sample polydispersity, the broadband excitation, and the nanorods’ resonant absorption, the work provides a comprehensive understanding of the underlying mechanisms and identifies fingerprints of electron–electron scattering in the 2DES maps. Performed on a simple yet prototypical system, this analysis advances the study of plasmonic hot carriers and supports further applications of 2DES to explore ultrafast mechanisms in more advanced hybrid plasmon-based systems, e.g. strongly-coupled complexes.