Nonreciprocity and Topological Propagation in Time-Modulated Metamaterials
In this talk, we provide an overview of our recent efforts in the realization of magnet-free nonreciprocal and topological devices and metamaterials based on spatio-temporal modulation. Lorentz reciprocity is a fundamental principle in electromagnetics, stipulating the fact that signal transmission between two points in space is the same for both transmission directions. Breaking this symmetry is important in various practical instances, for instance for the protection of lasers from spurious back-reflections and the design of circulators for full-duplex communication systems. Nonreciprocity is conventionally achieved through magnetic biasing, but the challenges related to integration on-chip have recently raised a lot of interest in developing magnet-free nonreciprocal devices. In this context, our group has introduced the concept of momentum biasing, according to which strong nonreciprocal response as in conventional nonreciprocal devices can be obtained if the static magnetic biasing is replaced with momentum biasing effectively generated in a nanophotonic or radio-wave circuit through spatiotemporal modulation. In this talk, we will present our recent progress in this direction and show how spatio-temporal modulation can largely break reciprocity and realize magnet-less isolators, circulators and metamaterials with metrics satisfying the requirements of practical applications, spanning from full-duplex systems, RFIDs and radars, amenable to different implementations, including PCB, integrated circuit and MEMS, and spanning towards optical frequencies, optomechanics and nanophotonics, creating exciting opportunities for fundamental and applied research. We will also discuss how these nonreciprocal elements may be used as building blocks of metamaterials and nanomaterials with largely unusual propagation properties, including highly robust one-way propagation along their edges, stemming from the topological properties of their band diagram. We will discuss how these findings may have relevant implications for broadband nonreciprocal and slow-light devices, relevant for a broad range of applications of interest to the microwave and nano-optics community and beyond.