Topologically Protected Polaritonic Bound State in the Continuum
Bound states in the continuum (BICs) have emerged as powerful tools to realize ultra-high-Q resonances in nanophotonics. While previous implementations have primarily relied on dielectric metasurfaces, their optical confinement remains fundamentally limited by diffraction. In this work, we theoretically and numerically demonstrate and experimentally validate the existence of topologically protected phonon-polaritonic BICs in periodic arrays of cylindrical nanoresonators composed of isotopically enriched hexagonal boron nitride (h11BN), which support two restrahlen bands (lower (type-I) and upper (type II)), with the present work focusing on the lower Reststrahlen band (RB-1). Owing to the uniaxial anisotropy of hBN and the rotational symmetry of the structure, these systems support topologically symmetry-protected BICs at the Γ-point, where radiative losses are suppressed. The total quality factor is ultimately bounded by the intrinsic phonon damping of h11BN, enabling high-Q polaritonic modes with minimal radiation leakage. When cylindrical symmetry is broken via angular tilting of incident light away from normal incidence, these BICs transition into quasi-BICs (q-BICs), with strong field confinement and tunable radiation leakage. Their topological features enable robust control over mode lifetimes and confinement, paving the way towards scalable polaritonic platforms for mid-infrared optoelectronics, sensing and quantum nanophotonics.