From topologically protected coherent perfect reflection to bound states in the continuum

A mechanism for perfect reflection is proposed for the dielectric medium beyond the total internal reflection and band gaps. It arises from the coherence of multiple propagating modes, and can be determined by the topological vortex of a transmission coefficient with a nonzero winding number in parameter space. Based on the coherent perfect reflection, a generalized waveguide condition is derived analytically. In a photonic crystal slab, the modes that satisfy this condition are precisely bound states in the continuum. Our findings may have many potential applications in guided-wave optics.