Robust Strong Coupling in Both Monolayer and Bilayer WS ₂ with Plasmonic Nanocavities Revealed by Scattering and Photoluminescence Spectroscopy

Strong coupling between excitons in two-dimensional transition metal dichalcogenides (TMDs) and plasmonic nanocavities offers a promising platform for advancing photonic and quantum technologies through enhanced light-matter interactions. Although plasmonic strong coupling has been widely studied, its manifestation between gold nanorods (GNs) and bilayer TMDs remains relatively unexplored. A critical challenge has been the scarcity of full anti-crossing behavior in photoluminescence (PL) spectroscopy, a defining signature of strong coupling. Here, we demonstrate unambiguous strong coupling between a single gold nanorod cavity and excitons in both monolayer and bilayer WS₂. This is confirmed by the direct observation of emission from both the upper and lower polariton branches in scattering and photoluminescence spectra. Combined experiments, supported by finite-difference time-domain (FDTD) simulations and dark-field spectroscopy, reveal a Rabi splitting of 190 meV in scattering for monolayer WS₂, with PL splitting of 95 meV under 532 nm excitation, respectively. For bilayer WS₂, we observe a Rabi splitting of 185 meV in scattering and a PL splitting of 75 meV under the same excitation conditions. These results indicate stronger plasmon-exciton coupling in the monolayer system. Furthermore, theoretical simulations using a coupled oscillator model accurately reproduce the energy separation and intensity ratios of the polaritonic peaks, showing excellent agreement with experimental scattering and PL data. The demonstration of substantial room-temperature Rabi splitting within a single plasmonic nanocavity opens new pathways for exploring nonlinear phenomena, electrical tuning, and quantum correlations, which are essential for developing practical quantum optoelectronic devices.