Bound States\nin the Continuum and Long-Range Coupling\nof Polaritons in Hexagonal Boron Nitride Nanoresonators

Bound states in the continuum (BICs) garnered significant\ninterest\nfor their potential to create new types of nanophotonic devices. Most\nprior demonstrations were based on arrays of dielectric resonators,\nwhich cannot be miniaturized beyond the diffraction limit, reducing\nthe applicability of BICs for advanced functions. Here, we demonstrate\nBICs and quasi-BICs based on high-quality factor phonon-polariton\nresonances in isotopically pure h<sup>11</sup>BN and how these states\ncan be supported by periodic arrays of nanoresonators with sizes much\nsmaller than the wavelength. We theoretically illustrate how BICs\nemerge from the band structure of the arrays and verify both numerically\nand experimentally the presence of these states and enhanced quality\nfactors. Furthermore, we identify and characterize simultaneously\nquasi-BICs and bright states. Our method can be generalized to create\na large number of optical states and to tune their coupling with the\nenvironment, paving the way to miniaturized nanophotonic devices with\nmore advanced functions.