Near-unity fueling light into a single plasmonic nanocavity.

Plasmonic nanocavities have emerged as a powerful platform for extreme light confinement, enabling transformative applications in single-molecule Raman spectroscopy, ultra-sensitive sensing, strong light-matter interactions, etc. By harnessing localized surface plasmons, these nanostructures support unprecedented field enhancement, exceeding 1,000-fold in the sub-nanometer gap. However, a fundamental trade-off exists between deep sub-wavelength field localization and its efficient coupling to free-space light, limiting their practical performance. Here, we show that by balancing the electric and magnetic resonance, more than 55 % of a focused Gaussian beam can be fueled into a nanocube-on-mirror nanocavity. With few concentric gratings, the coupling efficiency can even go up to >95 % at optimal conditions. This design can work at both visible and telecommunication wavelengths and show robust tolerance to fabrication imperfections. Our work indicates that the long-standing conflict between deep field localization and efficient external coupling in plasmonic systems can be resolved by multiscale structure design, promising the use of a single metal nanoparticle for advanced nanophotonic or optoelectronic devices.