Polaritonic Emission via Multiresonant Plasmonic Cavity Hybridization with a WSe2 Monolayer Inside a Gold Nanoparticle-over-Mirror (NPoM) Array

Abstract

Plasmonic nanocavities enable strong light–matter interaction at room temperature through intense mode confinement, yet intrinsic optical losses impose high coupling strength thresholds to attain the strong coupling regime. We successfully address this challenge by integrating a WSe2 monolayer inside the gap of a gold nanoparticle-over-mirror(NPoM) array. The nanoparticle array excites a propagating surface plasmon polariton (SPP) at the mirror/gap interface that hybridizes with the localized surface plasmon (LSP) mode in the nanogap. While the binary LSP-WSe2 exciton interaction is relatively weak, the strong LSP-SPP coupling introduces a hybrid plasmonic mode near the exciton resonance, yielding an effective coupling constant of 2geff ≈ 218 meV, that elevates the Rabi splitting above the system dissipation threshold (145 meV). Mode dispersion and coupling are modeled numerically and studied experimentally using far-field reflectivity and near-field photoemission. Momentum-resolved hyperspectral photoluminescence measurements show anticrossing in the emission spectrum, indicating that polaritonic states decay predominantly through the radiative channels arising from the hybrid modes.