Abstract
Chiral edge modes in topological photonic insulators are inherently immune to backscattering and disorder due to topological protection, enabling these photonic systems to serve as highly promising platforms for robust light transport in compact integrated photonic circuits. Leveraging the interference among multiple unidirectional chiral edge modes with distinct dispersions, we demonstrate magnetically tunable multimode interference in gyromagnetic topological photonic insulators, enabling controllable power splitting ratio via magnetic field strength or incident frequency adjustments. Furthermore, the backscattering-immune property of chiral edge modes makes them exceptionally well-suited for slow-light waveguides, which significantly enhance light-matter interactions but typically suffer from disorder-induced attenuation. By engineering the dispersion of the chiral edge modes to wind multiple times around the Brillouin zone through coupling with corner states in a second-order topological insulator array, we theoretically achieve broadband, uniform topological slow light within the mid-gap region.