Optical bound states in the continuum (BICs) provide a way to engineer resonant response in planar photonic structures with ultimately high-quality factors. Here, we employ optical BIC in a photonic crystal slab (PCS) to realize the regime of strong coupling between light and excitons in a monolayer MoSe2 semiconductor. We experimentally demonstrate BIC-like behavior of the lower polariton branch, with strong suppression of radiation into far-field at Γ-point of the Brillouin zone. With the decrease of the in-plane Bloch wavevector, polariton lifetime rapidly increases, while the group velocity decays. These effects balance each other granting a nearly constant polariton propagation length of less than 10 μm over a broad range of k-vectors. This result suggests the possibilities for the development of ultracompact BIC-based planar polaritonic devices for sensing, lasing, and nonlinear optics.

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