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 interestthe 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-of-plane 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.