Enhanced Strong Coupling in the Hybrid Dielectric-Metallic Nanoresonator and WS2 Monolayer

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Exciton polaritons in metallic nanocavities and transition-metal dichalcogenide monolayers has led to striking discoveries, ranging from Bose-Einstein condensation to slowing light. Although plasmonic nanocavities offer small mode volumes, the intrinsic losses of plasmonic nanocavities remain an open challenge in exciton-polariton devices. Consequently, dielectric nanocavities are used as an alternative candidate due to their low intrinsic losses. However, large mode volumes are a central bottleneck in dielectric nanocavities. Here, we theoretically propose to use a hybrid dielectric-metallic nanocavity to enhance light-matter interactions between the excitons of the ${\mathrm{WS}}_{2}$ monolayer and the hybrid nanocavity. Such a hybrid nanoresonator inherits the advantages of both dielectric and metallic nanocavities, including ultrasmall mode volume, ultralow losses, and reasonably-high-Q factor. It is demonstrated that the thickness and material of the central gap film together with the thickness of the metallic substrate play vital roles in governing the coupling strength between 1L-${\mathrm{WS}}_{2}$ excitons and the cavity. After optimizing the geometry and material parameters, the Rabi splitting is increased to 113 meV, almost twice that in dielectric metasystems. The significant improvement can be attributed to the greatly enhanced near field and the ultrasmall mode volume. Furthermore, we show that Rabi splitting can be further boosted to 151 meV by increasing the number of layers of ${\mathrm{WS}}_{2}$ and h-$\mathrm{BN}$ film in the nanocavity.