  Photonic bound states in the continuum (BICs) are a standout nanophotonic
platform for strong light-matter coupling with transition metal dichalcogenides
(TMDCs), but have so far mostly been employed as all-dielectric metasurfaces
with adjacent TMDC layers, incurring limitations related to strain, mode
overlap, and material integration. In this work, we experimentally demonstrate
for the first time asymmetry-dependent BIC resonances in 2D arrays of
monolithic metasurfaces composed solely of the nanostructured bulk TMDC WS$_2$
with BIC modes exhibiting sharp and tailored linewidths, ideal for selectively
enhancing light-matter interactions. Geometrical variation enables the tuning
of the BIC resonances across the exciton resonance in bulk WS$_2$, revealing
the strong-coupling regime with an anti-crossing pattern and a Rabi splitting
of 116 meV. The precise control over the radiative loss channel provided by the
BIC concept is harnessed to tailor the Rabi splitting via a geometrical
asymmetry parameter of the metasurface. Crucially, the coupling strength itself
can be controlled and is shown to be independent of material-intrinsic losses.
Our BIC-driven monolithic metasurface platform can readily incorporate other
TMDCs or excitonic materials to deliver previously unavailable fundamental
insights and practical device concepts for polaritonic applications.
