Bound states in the continuum and high-Q resonances supported by a dielectric ridge on a slab waveguide

We investigate the diffraction of guided modes of a dielectric slab waveguide on a simple integrated structure consisting of a single dielectric ridge on the surface of the waveguide. Numerical simulations based on aperiodic rigorous coupled-wave analysis demonstrate the existence of sharp resonant features and bound states in the continuum (BICs) in the reflectance and the transmittance spectra occurring at oblique incidence of a TE-polarized guided mode on the ridge. Using the effective index method, we explain the resonances by the excitation of the cross-polarized modes of the ridge. The formation of the BICs is confirmed using a theoretical model based on the coupled-wave theory. The model suggests that the BICs occur due to coupling of quasi-TE and quasi-TM modes of the structure. Simple analytical expressions for the angle of incidence and the ridge width predicting the location of the BICs are obtained. The existence of high-Q resonances and BICs makes the considered integrated structure promising for filtering, sensing, transformation of optical signals, and enhancing nonlinear light-matter interactions.