Robust bound states in the continuum in a dual waveguide system

Bound states in the continuum (BICs) provide a fascinating platform to route/manipulate waves with ultralow loss by patterning low-refractive-index materials on a high-refractive-index substrate. Principally, the phase of leaking channels can be manipulated via tuning the structural parameters to achieve destructive interference (i.e., the BIC condition), surprisingly leading to the total elimination of dissipation to the continuum of the substrate. Despite recent developments in BIC photonics, the BIC conditions can only be satisfied at specified geometric sizes for waveguides that dim their application prospects. Here, we propose a dual waveguide system that support BICs under arbitrary waveguide sizes by solely changing the intervals between the two waveguides. Our calculation results show that robust BICs in such architectures stem from the interaction (destructive interference) between leaking waves from the two waveguides. Furthermore, a cladding layer is introduced to improve the fabrication tolerance and reduce the sensitivity of the low-loss condition on the waveguide intervals of the presented dual waveguide system. The proposed approach offers an intriguing solution to establish a BIC concept and may be helpful to improve the potential of BIC photonic devices and circuits.