Quasi-bound band in photonic crystal slabs

The bound state in the continuum (BIC) arises at the high symmetry points in the Brillouin zone of photonic crystals due to interference of counter-propagating modes. Its standing-wave field profile has a symmetry that prevents its coupling to far-field radiation and thus leads to a long lifetime. Prior experimental reports have only focused on these localized BIC modes thus limiting potential studies to wavevector-independent applications. In the present work, we measure wavevector-resolved transmission and photoluminescence of monolayer WSe₂ incorporated into a square-lattice photonic crystal slab made of Al disks and show that it supports a quasi-bound band along the Γ-X direction. Furthermore, we find that this band maintains a spectral width similar to that of a typical localized BIC through at least half of the Brillouin zone and terminates in a true symmetry-protected BIC at the Γ point. Our results demonstrate that states in the band arise from interference of partially counter-propagating modes, resulting in a partially standing and partially propagating field. Similar to BIC, the symmetry of the standing component suppresses the coupling to the radiating continuum and results in the extended lifetime. The spectrally narrow dispersing band merging into the BIC at its minimum can potentially facilitate the condensation of exciton-polaritons in the strong coupling regime.