Research

EFRI NewLAW: Magnetic Field Free Magneto-optics and Chiral Plasmonics with Dirac Materials

This project explores a new frontier in magneto-optics and magnetic-field-free non-reciprocal light transport based on Dirac materials such as transition-metal dichalcogenide monolayers, where the breaking of inversion symmetry and large spin-orbit coupling lead to valley-spin locking. The intrinsic Berry curvature of these materials further acts as an effective magnetic field in momentum space, which under a valley imbalance can give rise to chiral plasmon modes that enable non-reciprocal light propagation at mid infrared and terahertz frequencies.

Valley polarization in these gapped Dirac materials will be induced through three approaches: 1) doping with transition-metal impurities; 2) proximity to magnetic materials with out-of-plane magnetic anisotropy; and 3) electrical spin injection. Electromagnetic modeling and calculations of the Berry curvature, magneto optical effects, and chiral plasmons in selected monolayers and heterostructures will provide guidance for material synthesis by molecular beam epitaxy and chemical vapor deposition, and atomic scale characterization with spin-resolved scanning tunneling microscopy/spectroscopy, angle-resolved photoemission spectroscopy, and polarization selective photoluminescence, as well as far-field optical characterization and near-field scanning optical microscopy imaging. Through an integrated experimental-theoretical approach, this project aims to demonstrate wave guiding chiral plasmons in the mid-infrared to terahertz range to enable magnetic-field-free optical devices such as non-reciprocal Faraday isolators and tunable optical circulators.

Tailoring the properties of heterostructures from monolayers: Epitaxial growth and doping

Heterostructures of conventional semiconductors, which provide enhanced electrical and/or optical characteristics well beyond that of each individual constituent material, have been the key enabling element in modern telecommunication, energy efficient displays, and energy harvesting technologies. This project explores a new approach to synthesize heterostructures made of sheets of “two-dimensional materials” in which atoms within a sheet form strong bonds, but interactions between the layers are very weak. Atomic-resolution imaging and calculations are carried out to understand how this characteristic anisotropic bonding facilitates growth and assembling of the single atomic layers like Lego blocks, thus allowing the design and synthesis of new materials with tailored properties and functionalities beyond the limits of materials that currently exist in nature. This project provides both graduate and undergraduate students training in this interdisciplinary field of materials synthesis and characterization at the atomic scale, as well as electronic structure calculations. The open source distribution of electronic structure codes continues to provide the scientific community the benefits of our team’s code development efforts. An ongoing Research Experience for Teachers outreach program brings cutting-edge research on two-dimensional materials to high school students to inspire their interest in science and engineering.

Exploring boundary states in Dirac materials: Graphene and topological insulators

Fueled by the discovery of graphene in 2004, symmetry-protected topological phases of matter have emerged as a major focus of basic research in condensed matter physics and materials science. Distinct from the classic dichotomy of metals and semiconductors, these materials, now referring to as Dirac materials, exhibit gaped bulk states characterized by non-trivial topological invariants, and conducting boundary states described by effective Dirac equations for massless fermions instead of the Schrödinger equation that is suitable for conventional materials such as semiconductors. The symmetry protected boundary states are robust against perturbations, thus offering exciting opportunities in fundamental physics and potential applications in energy conversion and energy efficient computation.

This project aims to gain an atomic scale understanding of boundary states in two prototypical Dirac materials – graphene and three-dimensional topological insulators, integrating the investigators’ strengths in material synthesis by molecular beam epitaxy and chemical vapor deposition, atomic scale imaging/spectroscopy characterization, and calculations. This integrated approach enables the controlled growth and characterization of Dirac materials at the atomic scale, necessary steps towards tailoring their electronic and magnetic properties for applications in energy conversion and energy efficient computation.