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Most electronic and photonic structures follow a fundamental characteristic called Lorentz reciprocity, which require waves to travel the same way for both forward and reverse directions. Lorentz reciprocity, because it transmits signals in both directions, can inject noise into circuits and damage devices such as lasers. Scientists know that breaking reciprocity and symmetry by manipulating waves could result in more stable, scalable, and broadband non-reciprocal circuits and devices.
A typical method to breaking Lorentz reciprocity in radar microwave transmitters is by using an external magnetic field, which require greater power consumption. Nonmagnetic alternatives can do the same job, albeit with far less efficiency. For instance, a single nonlinear resonator – a common feature of nonlinear isolators – must sacrifice transmission efficiency to transmit over a broad bandwidth.
"We theoretically show, and then experimentally demonstrate using a microwave circuit, that the combination of one Fano and one Lorentzian nonlinear resonator, and a suitable delay line between them, can provide unitary transmission, infinite isolation, broad bandwidth and broad isolation intensity range," wrote Alù and co-authors Dimitrios L. Sounas and Jason Soric in Nature Electronics.
"We also show that a larger number of resonators can be used to further increase the isolation intensity range without diminishing the other metrics of the device," they added.
The device works at microwave frequencies, but can be applied to optical applications, as well.