Optical isolator i.e., devices that allow photons to travel in one direction, but prohibit reverse propagation play a crucial role to route optical signals in optical fiber networks and to provide stability in laser operation. Commercially available isolators today are exclusively based on magnetically-biased garnets or ferromagnetic materials. However, because of the weak character of magneto-optical effects, they are typically based on bulky optical components, they are based on expensive materials, and they are impossible to integrate in a nanophotonic platform due to lattice mismatch with conventional substrates. The objective of this effort consists in introducing new theoretical concepts and design principles, as well as experimentally realizing integrated nanophotonic devices that can isolate without requiring magnetic effects. Our approach is centered on the nanophotonic analog to the Zeeman effect, the physical mechanism based on which conventional isolation is realized with magnetic materials: we will be able to induce a strong isolation on-chip by biasing with angular-momentum suitably designed "meta-atoms" in the form of spatio-temporally modulated nanoring resonators. The findings of this project are expected to attract significant interest from the nanophotonics industry, since monolithic integration of non-reciprocal nanodevices can dramatically minimize the cost and footprint of these essential devices. More broadly, the proposed research combines a plethora of exciting topics in electrical engineering, that can directly involve undergraduate and graduate students in some of the most important fields of electrical engineering such as nanophotonics, metamaterials, integrated electronics, nanofabrication, and modeling, opening unique opportunities to inspire the next generation of scientists and researchers, with special attention to under-represented minorities. The proposed research introduces disruptive concepts for nanophotonics, allowing the realization of magnetic-free optical components that can break Lorentz reciprocity without requiring magnetic bias or special ferromagnetic response. Being fully based on components and materials that are already available in conventional nanophotonic boards, such as dielectric waveguides and semiconductor junctions, the newly proposed non-reciprocal components can be directly integrated into conventional nanophotonic systems. In addition, the proposed structures will optimally benefit from recent advances in the quickly growing fields of silicon photonics, nano-optics and electronics. At completion of this effort, we will have demonstrated non-reciprocal optical components based on angular-momentum biasing, achieved with suitable spatio-temporal modulation of resonant nanorings based on electric radio-frequency signals. The careful combination of a strong resonance and of a precise form of spatiotemporal modulation in the azimuthal direction will be able to drastically enhance the otherwise weak electro-optical effects responsible for spatio-temporal modulation, thus leading to giant non-reciprocity within a footprint comparable or smaller than the wavelength. Our additional investigations of angular spatio-temporal modulation in plasmonics and graphene-based platforms will set the basis for a new technology platform able to process and control light in novel ways at a deeply subwavelength scale.
