Tunable Couplings of Photons with Bright and Dark Excitons in Monolayer Semiconductors on Plasmonic-Nanosphere-on-Mirror Cavities

Tunable exciton-photon couplings are of great importance for cavity quantum electrodynamics (QED), polariton chemistry, optical computing, and bosonic lasing. In this regard, two-dimensional (2D) excitons in transition metal dichalcogenides (TMDs) have attracted tremendous interest-the huge oscillator strengths endow sensitive responses to optical modes, and their excitonic properties can be actively tuned by external stimuli. However, tunable coupling with spin-forbidden dark excitons in monolayer TMDs has rarely been demonstrated, and how the bright/dark-exciton-photon couplings coexist in one system is still a mystery. Herein, we utilize waveguide and antenna modes in a gold nanosphere-on-mirror cavity to match the in-plane and out-ofplane dipole moments of bright and dark excitons in monolayer WS2, respectively, and simultaneously. Strong bright-exciton-photon couplings with a coupling strength of up to 60 meV are observed at room temperature. We show that the waveguide mode can be tuned in wavelengths by controlling the spacer thickness and demonstrate the anti-crossing feature of polaritons. Next, by increasing the excitation angle, we dynamically enlarge the relative contribution from the antenna mode coupled with dark excitons and suppress the waveguide mode coupled with bright states. Moreover, a tunable bright exciton linewidth is achieved as a result of suppressed homogeneous broadening caused by the dominant dark exciton decays under strong dark-exciton-photon couplings. This is not only proof of tunable couplings with dark excitons but also an insightful finding that reveals the novel interactions between bright- and dark-exciton-photon hybrids in a single optical cavity, opening new possibilities in tunable QED.