  Optical bound states in the continuum (BIC) allow to totally prevent a
photonic mode from radiating into free space along a given spatial direction.
Polariton excitations derived from the strong radiation-matter interaction of a
BIC with an excitonic resonance inherit an ultralong radiative lifetime and
significant nonlinearities due to their hybrid nature. However, maximizing the
light-matter interaction in these structures remains challenging, especially
with 2D semiconductors, thus preventing the observation of room temperature
nonlinearities of BIC polaritons. Here we show a strong light-matter
interaction enhancement at room temperature by coupling monolayer WS2 excitons
to a BIC, while optimizing for the electric field strength at the monolayer
position through Bloch surface wave confinement. By acting on the grating
geometry, the coupling with the active material is maximized in an open and
flexible architecture, allowing to achieve a 100 meV photonic bandgap with the
BIC in a local energy minimum and a record 70 meV Rabi splitting. Our novel
architecture provides large room temperature optical nonlinearities, thus
paving the way to tunable BIC-based polariton devices with
topologically-protected robustness to fabrication imperfections.
