Robust Strong Coupling of Monolayer WS 2 in Plasmonic Nanocavities via Scattering and Photoluminescence Spectroscopy

Strong coupling (SC) between plasmonic nanocavities and excitons in two-dimensional transition-metal dichalcogenides (2D-TMDs) has promoted fundamental studies in quantum electrodynamics and applications in photonic quantum technologies. Although previous SC research with 2D-TMD predominantly characterized cavity polaritons through scattering spectroscopy, the observation of the complete anticrossing behavior in photoluminescence (PL) spectroscopy has been less frequently reported and is crucial for ascertaining the underlying physics. In this study, we robustly demonstrate an unambiguous SC between a single gold-nanorod cavity and monolayer WS2 excitons. This was achieved by observing complete upper and lower polariton branch emissions via both scattering and PL spectroscopy. The sharp tips of the plasmonic nanocavity of the nanorods give rise to a large single exciton coupling strength up to 14.9 meV. We estimate that the number of excitons in the strongly coupled entangled state range from 8.7 to 17.3. Correlated scattering and PL spectra measurements on a single coupled system confirm the presence of strong plasmon-exciton interactions. Further theoretical simulations using a coupled-oscillator model show excellent agreement with the measured scattering and PL spectral data, effectively capturing the energy separation and intensity ratio of the polaritonic peaks. The high yield of SC structures achieved presents an opportunity to explore their nonlinear, electrical, and quantum correlation properties, which may be sufficient for practical quantum optoelectronic devices.