Passive Nonlinear Optical Isolators Bypassing Dynamic Reciprocity

Magnetic-free optical isolators are critical components for the realization of integrated optical systems. The underlying physics of passive nonlinear optical isolators is not solely about breaking the Lorentz reciprocity without requiring any external bias. Indeed, one major obstacle to the operation of self-Kerr-type nonlinear optical isolators is the so-called dynamic reciprocity, of which a serious outcome is that a backward signal cannot be isolated in the presence of a forward strong signal. In this work, we advocate the concept of velocity-selective nonlinearity to bypass such dynamic reciprocity. Using a proof-of-principle platform with warm rubidium atoms in an asymmetric cavity, we experimentally achieve the isolation of a backward signal in the presence of a strong forward signal, with experimental observations in agreement with our theoretical simulations. This work has thus made one essential step towards functioning Kerr-type passive optical isolators.