Breaking of the electronic-optical dynamics correspondence in ultrafast plasmonics: a universal mechanism beyond the perturbative regime.

Understanding the transient optical response of plasmonic nanostructures is of crucial relevance for a variety of fields and applications, from light harvesting to nonlinear nanophotonics. Accurate predictions of the dynamics of ultrafast processes taking place at the nanoscale following excitation with electromagnetic radiation are critical to this aim. So far, a strict correspondence has been assumed between the temporal evolution of the optical signal (retrieved via interaction with a low-intensity probe pulse, in the framework of transient absorption spectroscopy) and the dynamics of the population of photoexcited hot-electrons, described in terms of their temperature. We report on a study testing this hypothesis by investigating the transient optical response of Au nanoparticles in a non-perturbative excitation regime, both theoretically and experimentally. Our results indicate that the correspondence does not hold under such a regime; moreover, the main mechanism presiding over its breaking is universal in plasmonics, being related to the nonlinear smearing of the Fermi-Dirac distribution, describing the occupation probability of the hot electrons in any metallic lattice.