Greatly enhanced exciton-photon coupling in a WS₂-Si₃N₄ nanohole array-Ag film heterostructure

Two-dimensional transition metal dichalcogenides (TMD) monolayers are recognized as a promising platform for realizing strong coupling due to their exceptionally large binding energies of excitons. Various types of optical resonances, including DBR-based Fabry-Perot resonances, plasmonic resonances, Mie resonances, and guided mode resonances, have been proposed to facilitate strong coupling between excitons in TMD monolayers and optical nanocavities. However, there has been limited research on the strong coupling between multiple resonances and excitons in TMD monolayers. In this study, we propose a heterostructure composed of a WS₂-Si₃N₄ nanohole array and an Ag film to achieve robust strong coupling among surface transverse waveguide modes, surface lattice resonances (or guided mode resonances), and excitons in a WS₂ monolayer. Such a hybrid heterostructure inherits both advantages of surface lattice resonances in a Si₃N₄ photonic crystal slab with the surface plasmon-like modes at the dielectric-Ag interface, resulting in low-loss optical resonances and excellent field confinement. Consequently, the light-matter interaction among the surface transverse waveguide mode, surface lattice resonances, and excitons in the WS₂ monolayer is significantly enhanced. Utilizing angle-resolved scattering spectra measurements and numerical simulations, we observe substantial Rabi splitting and shifts in resonant peaks, which are indicative of hybrid mode coupling. The experimental results closely align with the simulations, thereby confirming the hybrid coupling of the surface transverse waveguide mode, surface lattice resonances, and excitons. Notably, we show that Rabi splitting, an indicator of the coupling strength, is significantly increased to 318 meV, thereby entering the strong coupling regime within a three-oscillator framework. These results not only deepen the understanding of hybrid mode interactions within dielectric photonic structures but also promote advancements in high-performance optoelectronic applications.