Selective Broadband Topological Slow Light in Multimode Waveguides

Abstract Topological protection provides an appealing chance to suppress the backscattering loss and wave localization. The emerging broadband topological slow‐light waveguides are promising for many on‐chip applications. However, topological slow‐light waveguides so far are limited to single mode, multiple slow‐light modes are still unexplored which can promote the transmission capacity and bring in novel functionalities. Here, through Brillouin zone winding induced by side‐coupled resonator arrays, topological one‐way multimode waveguides with group velocity selectivity resulting from mode parity matching are demonstrated. One‐way modes with a large group velocity contrast and dual slow light modes are respectively obtained. The conversion between two modes with distinct group velocity due to the perturbation of mirror symmetry is also observed. Furthermore, in line perturbed one‐way waveguides, near‐perfect mode fidelity, and backscattering immunity against strong asymmetric disorders is simultaneously achieved. The results reveal exciting opportunities toward the exquisite group velocity manipulation for slow‐light applications.