Photonic axion insulator with non-coplanar chiral hinge transport

Axion insulators represent a unique class of magnetic topological phases, linking the two-dimensional quantum anomalous Hall effect to the magnetic higher-order phase of three-dimensional topological insulators. Within axion insulators, axion electrodynamics exhibits novel topological magneto-electric phenomena such as quantized Faraday and Kerr rotation and half-integer surface Hall response. However, among them, the chiral hinge state with non-reciprocal hinge transport as their essential hallmark has yet to be experimentally observed since it was predicted theoretically. Here we report the first photonic axion insulator based on a three-dimensional antiferromagnetic-like structure in microwave bands. Such an artificial magnetic lattice consists of bilayer square-lattice arrays of ferrites imposed with equal but opposite embedded magnets, simultaneously with inversion-symmetric interlayer couplings. By probing all twelve hinges and detecting all eight vertices of the photonic axion insulator, we directly map out the non-coplanar chiral hinge states and observe the non-reciprocal robust hinge transport. The different performances between odd- and even-layer axion insulators are also investigated. These results enrich the family of topological photonics and the controllable dimension of electromagnetic waves, opening up a photonic way to study rich magnetic topological phases that have already been proposed but are challenging to implement in solid-state materials.