Terahertz sensing and imaging is a ?grand challenge? that impacts medical imaging and national security. Tremendous progress has been made on materials and device fabrication of terahertz/infrared detection in the past decades. However, the conventional technologies only allow detection of single wave lengths with one active element in device, severely hampering the miniature and functionality of terahertz/infrared sensors. This EFRI team will harness topological insulator, a recent breakthrough in condensed matter physics. The team aims to engineer topological interface states residing at the boundary between a topological insulator and a conventional semiconductor. Such interface states are robust against imperfection due to topologically non-trivial electronic structure of topological insulator. Furthermore, topological interface states host rich spin-texture in momentum space, which opens the possibility of tuning their hybridization with electrical voltage (gating) and magnetic field. The team will address the challenges of materials synthesis, realization of optoelectronic and spintronic devices, and their characterizations. The device concepts envisioned in this project are potentially game-changing technologies, directly addressing the grand challenge and national need on the terahertz/infrared technological areas. Education and training of students is seamlessly integrated to the research activities, which enable them to learn fundamental material science, to master advanced microscopic techniques, and more importantly, to learn independent thinking. This project also integrates education and training of under-represented undergraduate students through various programs at Rutgers, RPI and John Hopkins, and of high school students through the Partner in Science program of Liberty Science Center.
Topological insulators are novel quantum states of matter, which host helical Dirac surface states within their bulk band gap. These robust topological surface states are immune to backscattering because of their spin-momentum locking, in contrast to the conventional 2-dimensional electron gases (2DEG) that reside at interfaces in conventional heterostructures. The objective of this project is to explore novel device applications by engineering the topological states at the interfaces of topological insulators with semiconductors. Preliminary theoretical work shows that these topological interface states can be qualitatively different from the surface states of the free surfaces in that they have completely different textures of spin-momentum locking. With the guidance of advanced first principle theory and modeling (Zhang+West), high quality thin films and heterostructures of topological insulators will be synthesized with state-of-the-art MBE (Oh), and characterized with transport (Oh), scanning tunneling microscopy and spectroscopy (Wu) and THz and optical spectroscopy (Armitage). The intimate feedback between theory, synthesis and characterization promises novel device physics and applications. The specific goals of the proposed work include two specific devices in the form of a tunable THz range photon detector and a source of fully polarized spin polarized current. These are both game changing technologies. Although many possible devices have been proposed for this class of materials, little has been actually realized thus far. With recent advances by these PIs and their areas of expertise, the time is ripe for a concerted push in these applications of topological insulators.
