Nonreciprocity Based on Nonlinear Resonances

Breaking reciprocity is fundamentally important for the protection of sources from incoming signals and to route signals traveling in opposite directions asymmetrically. Nonreciprocal devices are usually realized through certain types of magnetic materials, such as ferrites, under biasing with a static magnetic field, but recently, there has also been interest in magnetless approaches, including spatiotemporal modulation and nonlinear effects. Here, we provide an overview of nonreciprocal devices realized through nonlinearities, which have the advantage of not requiring any sort of bias and therefore, being fully passive. We explain the operation principle of such devices focusing on the most common type, nonlinear isolators based on nonlinear Fano resonators. We show that, despite being able to achieve high isolation, these structures are subject to a tradeoff between forward transmission and isolation power range stemming from time-reversal symmetry. Next, we show how this tradeoff can be overcome in structures comprising multiple nonlinear resonators and provide an example at microwave frequencies. Finally, we discuss the limitations imposed by thermodynamics on the operation of nonlinear isolators when they are simultaneously excited from both sides, and we envision practical operations in which nonlinear isolators can be fundamentally useful.