Single‐Valley Chiral Landau Levels and Photonic Valley Demultiplexing in Topological Multimode Systems

Chiral Landau levels (LLs) associated with two valleys have been proposed and realized in 2D photonic Dirac cone systems by introducing an inhomogeneous effective mass; however, chiral LLs with single‐valley polarization have not yet been reported. Therefore, here we report the experimental realization of single‐valley‐polarized chiral Landau levels (LLs) in gyromagnetic photonic crystals (PCs) through inhomogeneous effective mass engineering, which enables unidirectional transport with tunable valley polarization via magnetic field or pseudo‐vector potential regulation. Building on this, we propose a topological multimode valley demultiplexer based on an AB n C photonic heterostructure, by integrating two graded gyromagnetic PCs to separate dual‐valley signals into pure single‐valley states. The heterostructure exhibits distinct topological channels: a one‐way bulk state (K' valley) and a chiral edge state (K valley). Field modulation further enables reconfigurable valley multiplexing and dual‐functional dual‐use devices. Our results validate multimode coexistence and advance valley photonic technologies for integrated applications.