Asymmetric Transmission and Isolation in Nonlinear Devices: Why They Are Different

TL;DR

This paper clarifies the fundamental differences between nonlinear asymmetric devices and true isolators, emphasizing the role of material loss and the limitations of nonlinear devices as microwave isolators.

Contribution

It introduces a mushroom-structure nonlinear device as an example and discusses the limitations of nonlinear devices in achieving true isolation.

Findings

Nonlinear devices can exhibit asymmetric transmission but are not true isolators.

Material loss is essential for nonlinear devices to function as isolators.

Lossless nonlinear devices may be time-reversal invariant and not truly nonreciprocal.

Abstract

Here, we highlight the fundamental differences between nonlinear two-port devices with strongly asymmetric transmission responses and "isolators". We use a mushroom-structure loaded with nonlinear elements as a guiding example. The mushroom-metamaterial can be operated in a regime where it behaves as a nearly ideal electromagnetic diode, such that the individual excitations of the two ports lead to strongly asymmetric responses. We point out the limitations of using this type of nonlinear devices as microwave isolators. In particular, we underline the crucial importance of material loss to attain the isolator functionality and that in the lossless regime nonreciprocal devices may be time-reversal invariant.