Momentum-Space Topological Effects of Nonreciprocity

The connection between topology and nonreciprocity in photonic systems is reviewed. Topological properties such as Chern number, and momentum-space properties such as Berry phase and Berry connection, are used to explain back-scattering immune edge states and their topological protection. We consider several examples to illustrate the role of momentum-space topology on wave propagation, and discus recent magnet-less approaches.