Non-Linear Isolators: Fundamental Bounds and Optimal Designs

Non-linear non-reciprocity has recently attracted a lot of attention, because it allows the realization of non-reciprocal devices without any type of external biasing. However, all the non-linear non-reciprocal devices that have been proposed so far suffer from a trade-off between transmission contrast and maximum transmission. Here, we show that this trade-off is the result of time-reversal symmetry and can only be broken with appropriately designed multi-stable structures. Based on this general fact, we propose a generic design for a perfect non-linear isolator, i.e., an isolator which allows perfect transmission in one direction and zero transmission in the opposite one. A circuit implementation at microwave frequencies is provided and the role of parasitic effects, such as loss, on the device performance is discussed.