BIC slow light waveguides based on interband coupling

Harnessing bound states in the continuum (BICs) for guiding light in leaky environments has unlocked new possibilities in photonic integrated circuits. BIC confinement enables low-loss waveguiding of leaky transverse-magnetic (TM) modes in etchless waveguides based on dielectric wires loaded on plane slabs. We have recently reported BIC slow light waveguides by introducing one-dimensional photonic crystals into such etchless waveguides. However, they were restricted to a high-symmetry point ($X$ point), limiting their applicability. In this Letter, we propose and numerically demonstrate BIC slow light waveguides at off-high-symmetry points by exploiting Friedrich-Wintgen BICs, arising from the interband coupling of two guided modes sharing a radiation continuum. We identified a systematic approach for tuning the loss minimum position in momentum space and simultaneously achieved a high group index over $100$ and a low propagation loss of less than $5 \times 10^{-2}~\mathrm{dB/cm}$ at an off-high-symmetry point. Our findings pave the way for advanced control of light-matter interactions in non-Hermitian photonic systems.