Bypassing Thermalization Losses through Ultrafast Interfacial Charge Transfer in Plasmonic Nanocavities for Water Oxidation.

Spatially inhomogeneous plasmonic heterostructures concentrate light into nanoscale volumes and offer powerful routes for plasmon-mediated solar energy conversion. However, how hot carriers evolve and lose energy under such extreme nanophotonic confinement, particularly during interfacial transfer, remains largely unexplored. Here, we demonstrate that rationally engineered plasmonic nanocavities with intense nanoscopic field localization provide a unique platform to bypass these energy-loss channels by enabling nonthermal charge injection on a sub-40 fs time scale. Using polarization-resolved and time-resolved multiphoton photoemission spectroscopy, we reveal that the intense localized fields arising from plasmon-cavity mode coupling establish an accelerated and direct electron transfer channel across the interface. This behavior is accompanied by a distinctive inversion of the polarization dependence and by photon-energy-independent spectral features, confirming charge injection occurring prior to thermalization. Furthermore, the emergence of three-photon photoemission indicates a fundamental reconfiguration of the hot carrier generation and relaxation landscape resulting from the nanoscopic electric field within the plasmonic nanocavity. These insights establish a microscopic understanding of how interfacial charge transfer contributes to the enhanced water-oxidation activity in plasmonic nanocavity systems, offering a guiding framework for the development of advanced plasmonic photocatalysts and optoelectronic interfaces.