Chiral hinge-surface transport across dimensions in three-dimensional magneto-optical topological materials

Chiral boundary states with perfect conducting channels are essential characteristics of magnetic topological materials. Prominent examples include the one-dimensional (1D) chiral edge and 2D chiral surface states found in 2D and 3D quantum Hall materials under magnetic fields, respectively. However, these boundary states are restricted to specific fixed dimensions, so they hardly facilitate cross-dimensional energy and information transport. Here, we fabricate a unique 3D photonic antiferromagnetic topological insulator with net zero magnetization that can simultaneously support different-dimensional hinge states and unpaired surface Dirac cones on neighboring facets. Owing to the chiral anomaly present in a finite-size sample, the gapless surface Dirac cone, neighbored by facets with surface Dirac masses of opposite signs, is further converted, exhibiting 2D planar one-way propagation. In conjunction with 1D hinge states, we experimentally observe a closed chiral loop for nonreciprocal hinge-surface transport across dimensions in topological photonics, similar to that theoretically proposed in 3D quantum anomalous Hall materials. Our findings enrich the chiral boundary features of 3D magnetic topological insulators and offer a topological strategy for exploring ideal cross-dimensional devices.