Bound Topological Edge State in the Continuum for All-Dielectric Photonic Crystals

The canonical way to sustain photonic topological edge states relies on the interface formed by topologically trivial and nontrivial photonic crystals that share a common band gap to prevent the coupling of edge states from the radiation continuum of free space. Herein, we report a scheme to achieve bound topological edge states in the continuum at the air edge of a single topologically nontrivial photonic crystal utilizing the physics of bound states in the continuum. These localized states, though existing in the continuum, exhibit topological features inherited from the nontrivial topology of bulk photonic bands, such as pseudospin-momentum locking unidirectional propagation. Microwave experiments are provided to further validate the theoretical predictions. Our work not only has practical importance but also opens an avenue where the nontrivial topology of bulk photonic bands could be exploited to enrich the physics of bound states in the continuum.