Seeded-Surface Plasmonic Nanocavity with Enhanced Field Localization for Exciton–Plasmon Coupling

Achieving strong light–matter interaction in monolayer transition metal dichalcogenides (TMDCs) at room temperature requires plasmonic nanocavities with extreme field confinement and minimized mode volume. Although conventional nanoparticle-on-mirror (NPoM) cavities support strong gap-mode plasmon confinement, further compression of the electromagnetic field is desirable for enhancing exciton–plasmon interactions with smaller excitonic ensembles. Here, we developed a seeded-surface NPoM (SS-NPoM) nanocavity consisting of a 50 nm Au nanocube decorated with 10 nm Au nanospheres, separated from an Au mirror by a 10 nm Al2O3 spacer and integrated with monolayer WS2. The surface nanoseeds introduce additional nanoscale protrusions that redistribute and compress the gap-plasmon field into highly localized subhotspots. Dark-field scattering measurements revealed a systematic detuning-dependent anticrossing with mode splitting of 106 meV. Because this value is comparable to the stringent loss-based threshold, the system is conservatively classified as operating in an intermediate-coupling regime close to the onset of strong coupling. Quasinormal-mode calculations show that surface seeding can reduce the optical mode volume by 1 order of magnitude, corresponding to an estimated minimum of ∼14 coupled excitons in the most strongly confined configuration. Experimentally, the area-corrected photoluminescence enhancement reaches ∼690, compared with ∼220 for conventional NPoM cavities. These results demonstrate that seeded-surface engineering provides an effective strategy for constructing highly localized plasmonic nanocavities and offers a promising platform for room-temperature TMDC polaritonics and efficient nanoscale light sources.