Achieving Nonreciprocity in Microwave Electronics at the Materials and Device Level
Nonreciprocity is “hot” again for two reasons. First, it was discovered in the last decade in condensed matter physics in the form of strange states in certain special crystals (=topological insulators) in which electrons traveling in one direction can only possess a certain spin, and the opposite spin electron states must necessarily move in the other direction. This mysterious effect is due to materials that possess a high spin-orbit interaction, meaning due to the special atomic arrangement, an effective magnetic field is generated internally upon flow of current. This one-way transport of electrons in a crystal is indeed new, but the concept has been in use in microwave electronics for more than half a century. For example, the ubiquitous circulators in microwave Tr/Rx systems ensure the high-power microwave output of a power amplifier of the Tr module does not burn out the LNA in the Rx module by ensuring a one-way roundabout traffic of the microwave signal. The second reason why nonreciprocity is exciting again is that it has raised the possibility of applying ideas of topologically constrained nonreciprocity to microwave electronics. Circuit-level implementations of such ideas have already been realized for achieving simultaneous transmit and receive (STAR) at low-frequency RF using silicon CMOS. However, ideas of nonreciprocity have not yet been applied at the device and materials level. As a matter of fact, even the ubiquitous microwave circulator achieves its nonreciprocity using magnets, which is partly a circuit implementation. We will discuss a few possible implementations of nonreciprocity at the materials and device level for microwave electronics.