EFRI NewLAW: Non-reciprocal, topologically protected propagation using atomically thin materials for nanoscale devices

Emory University, Atlanta GA

Investigators

Abstract

Reciprocity in optics can be described by the familiar observation: "If I can see you, you can see me." This phenomenon stems from the fact that laws of nature governing light and its interaction with matter on a microscopic level do not prefer any particular direction of time. In other words, if we were to hypothetically reverse the direction of time, the predictions of the laws of nature would remain unaltered. Of course, at a macroscopic scale, this is not the case as we know from experience that the arrow of time has a fixed direction. It turns out that an external magnetic field can break the symmetry of time even at the microscopic scale by picking a preferred direction of propagation, which can lead to a break down of reciprocity. In current optical devices, magnets are used to cause such a one-way propagation of light, which is crucial for optical communications and the Internet. One goal of this project is to achieve non-reciprocity of light without the use of magnets by using a new class of materials, which are atomically thin. Such materials feature an effective magnetic field due to their unique crystal structure and can be used in lieu of an applied external magnetic field for one-way propagation of light. This project aims to realize miniaturized optical devices and circuit elements based on these novel materials, which will allow for faster optical switches and defect-insensitive propagation on a reduced footprint. Such devices have the potential to transform the optical telecommunication industry. During the course of this project, the research team will provide science education and research experiences in cutting-edge technologies to middle school, undergraduate and graduate students, including those students from Historically Black Colleges and Universities, in an effort to enhance the science and engineering workforce of tomorrow. The goal of this project is to realize on-chip, non-reciprocal nanophotonic devices and circuit elements operating at optical frequencies and topologically protected edge states for photons. Such devices will feature novel functionalities such as reconfigurable one-way propagation and steering of light. Non-reciprocal propagation of energy and information in a time-independent and linear system requires broken time-reversal symmetry, which can be achieved by an external magnetic field. We will achieve non-reciprocal propagation at nanoscale in a magnetic-free way with the possibility of active control by external stimuli. Our approach will employ atomically thin materials such as transition metal dichalcogenides with unique electronic and optical properties to achieve these goals. In these materials, which break inversion symmetry, an effective magnetic field in the momentum-space called the Berry curvature is present. Although no net Berry curvature is present in these materials, electric control offers the possibility of spatially local time-reversal breaking and non-reciprocal propagation. In addition, we will rely on the strong light-matter interactions and non-linearity in these materials to increase non-reciprocity and also to realize topologically protected edge states of light. To this end, optical and plasmonic nano-cavities, which enhance light-matter interactions, will be exploited. This research project will advance our fundamental understanding of effective gauge-fields like Berry curvature in low-dimensional materials and how strong light-matter interactions can be exploited to achieve on-chip, reconfigurable non-reciprocity and topological states of light-matter.