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

Abstract 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.