Valley–layer locked waveguide states and large-area corner states in the bilayer gyromagnetic photonic crystal

We report the realization of a valley-layer-propagation-direction locking effect in a bilayer gyromagnetic photonic crystal. This effect originates from the synergy between out-of-plane reversed magnetizations-which break time-reversal symmetry in a layer-selective manner-and engineered in-plane inversion-symmetry-breaking interlayer coupling. Within a single topological waveguide, counter-propagating modes become spatially separated and locked to distinct layers: K-valley modes propagate leftward in the bottom layer, while K'-valley modes propagate rightward in the top layer. Furthermore, by designing tailored heterostructures, we demonstrate large-area topological corner states that can be simultaneously excited in both layers. These findings establish the bilayer platform as a new, to the best of our knowledge, paradigm for controlling topological states via the layer degree of freedom, exemplified by a layer-selective topological energy concentrator-a functionality fundamentally inaccessible in single-layer systems.