Nonlinearity-Induced Nonreciprocity—Part I

Nonreciprocal electromagnetic devices allow signals to propagate asymmetrically along opposite directions, enabling essential functionalities for several microwave and optical technologies. Branching out from the conventional approach involving a dc magnetic bias, different avenues have been investigated over the past two decades to break reciprocity without the need for magneto-optical materials. In particular, nonlinearity-based approaches have recently attracted considerable attention, due to their simple implementation and bias-free operation, which make their fabrication and system integration particularly appealing. Nonetheless, these advantages are accompanied by limitations that need to be carefully assessed when using these devices for practical applications. In the first part of this work, we discuss the fundamental principles of nonlinearity-based nonreciprocal devices. We introduce a general model for two-port nonlinear resonators and show how to design a nonlinear system yielding highly nonreciprocal port-to-port transmission under continuous-wave excitation. We then analyze quantitatively the main limitations of these devices: the limited power and spectral bandwidth, the response under pulsed excitation, and the breakdown of nonreciprocity under simultaneous two-port excitation. In the second part, we will deepen our analysis by considering more complex scenarios involving coherent and incoherent two-port excitations and multiresonator systems offering more flexibility in the response.