Spatiotemporally Modulated Gyrators with Extended Bandwidth through Dual-Sideband Operation

Abstract

One of the major challenges in the design of nonreciprocal devices is the application of magnetic biasing. This problem has motivated a flurry of work for the development of magnetless nonreciprocal devices, with spatiotemporal modulation being one of the most promising approaches to this end. The most fundamental nonreciprocal component is the gyrator, which can be the building block of other nonreciprocal components, like ciculators and isolators. Spatiotemporally modulated (STM) gyrators are typically based on a pair of frequency converters with different modulation phases, connected through a linear time-invariant network. Such gyrators are usually designed to operate with a single modulation sideband. In this paper we argue that this approach is suboptimal and show how adding just one more sideband can increase the bandwidth by $40\mathrm{%}$ with a lower modulation frequency and the same modulation amplitude. We present an implementation of the proposed approach based on a double-balanced circuit with Wheatstone bridges of varactors, which provides large isolation between the input signal and the sidebands. We provide a theoretical and an experimental analysis of the circuit with excellent agreement to each other. In addition to introducing a topology of STM gyrators with improved bandwidth compared with existing designs, our results highlight a potential connection between the bandwidth of STM devices and their harmonic order, which might also find applications in other STM systems, like isolators, circulators, and photonic topological insulators.