Efficient energy exchange between plasmon and cavity modes via Rabi-analogue splitting in a hybrid plasmonic nanocavity.

Plasmonic analogues of Rabi-splitting have been extensively studied in various metallic nanosystems hybridized with semiconductor quantum dots, nanocrystals and organic molecules, with a focus on the splitting energy gap where surface plasmon polaritons (SPPs) strongly couple with excitons. Similar strong coupling also occurs for individual metallic nanoparticles locating inside a photonic microcavity or nearby a waveguide due to the strong interaction between localized surface plasmons and photonic modes in the near-infrared wavelength range. In this work we study experimentally and theoretically the strong coupling between propagating SPPs and the Fabry-Perot (F-P) cavity mode in a metallic nanoparticle array-nanocavity hybrid system in the visible spectral range. The strong modal hybridization created giant modal anti-crossing which can be considered as the classical phenomenon of Rabi splitting i.e. a Rabi-analogue. In addition to the observation of a giant Rabi-analogue splitting energy of 148 meV at the strong coupling regime, we also reveal highly-efficient energy exchange between SPP and F-P modes at the low frequency dispersion branch through detailed numerical near-field studies and experimental phase delay analysis. The observed efficient mode conversion in the investigated plasmonic nanocavity is useful for designing novel nanophotonic devices, in which conventional photonic components need to be integrated with miniaturized plasmonic devices or vice versa.