Non-reciprocal topological photonics

The proposal and development of topological photonics have provided a new approach to fundamentally addressing the susceptibility of traditional photonic devices to defects or disorders, significantly enhancing the transmission efficiency and robustness of photonic devices. Among them, non-reciprocal topological photonics which break time-reversal symmetry and support chiral topological states are crucial branches of topological photonics. Their topological properties are characterized by non-zero Chern numbers in two dimensions or topological Chern vectors in three dimensions, exhibiting a rigorous and complete topological protection beyond that of reciprocal topological photonics. This review focuses on introducing the remarkable achievements of non-reciprocal topological photonics in exploring novel physical phenomena (chiral/antichiral edge/surface states, two-dimensional/three-dimensional photonic Chern insulators, magnetic Weyl photonics crystals, etc.) and constructing non-reciprocal robust topological photonic devices (unidirectional waveguides, broadband slow-light delay lines, arbitrarily shaped topological lasers, high-orbital-angular-momentum coherent light sources, etc.). Finally, the present status, potential challenges, and possible breakthroughs in the development of non-reciprocal topological photonics are discussed.