Nonreciprocal Photonics Without Magneto-Optics

Optical isolators, which break the Lorentz reciprocity, are fundamental building blocks for regulating the flow of light. Here, we provide a brief perspective on the developments in nonmagnetic approaches to constructing optical isolators. We show that nonlinear optical isolators are fundamentally constrained by dynamic reciprocity and cannot completely reproduce functionalities of standard magneto-optical devices. We also show that complete optical isolation is achievable with dynamic modulation of refractive index. Moreover, the reciprocity-breaking in modulated systems is closely related to the concept of an effective gauge potential for photons. The use of such gauge potential points to a route toward novel nonreciprocal topological photonics physics as well as new capabilities for controlling the propagation of electromagnetic waves.