Graphene Thermoelectric THz Direct and Heterodyne Detectors

University Of Massachusetts Amherst, Amherst MA

Investigators

Abstract

Detection of terahertz or THz radiation, the electromagnetic waves with frequencies in-between that of microwaves and infrared light, is useful for a wide range of applications, including investigating the formation and evolution of stars and galaxies in the universe, analyzing the thickness of coatings on pills and tablets in the pharmaceutical industry, distinguishing cancer cells from healthy tissues, spotting manufacturing flaws for non-destructive quality-control analysis, identifying concealed objects under clothing, and sniffing out explosives and illegal drugs remotely. A long-standing objective of THz technology research has been to develop a multi-pixel THz "camera" that can produce images of the THz radiation from an object, similar to the commonly-used digital cameras that take pictures with visible light. Because of the limited sensitivity and speed for typical room temperature devices, THz imaging presently requires the use of a high power, coherent, THz illumination source. Meticulously engineered THz detectors can reach high sensitivity and fast speed. However such devices typically need to be cooled down to cryogenic temperatures. Here graphene, a single atomic sheet of carbon atoms, will be used to make compact and high performance THz detectors operating at room temperature. Specifically graphene will be processed into a thermocouple that can rapidly and efficiently sense the heating effects due to THz radiation absorption. Fully optimized devices from the project are expected to reach a performance equivalent to existing systems that have to be cooled down with liquid helium, paving way for developing a compact room temperature THz camera that can record thermal images of the environment without the need for an intense THz light source. This research project will provide a unique inter-disciplinary scientific education and training program in two dimensional materials, nano science, technology and engineering, optics, THz instrumentation and condensed matter physics to graduate, undergraduate and high school students from diverse socio-economic backgrounds and under-represented communities. Recent intense electrical and optical studies of graphene have pushed the material to the forefront of THz research due to the atomically thin crystal's high mobility, weak electron-phonon coupling, tunable broadband optical response and minute specific heat. The proposed research seeks to take advantage of these unique properties and fabricate high quality graphene-boron nitride atomic stacks to detect THz radiation through a thermoelectric mechanism: THz radiation heats up electrons in graphene while keeping the lattice in thermal equilibrium with the environment; the diffusion of hot electrons creates a temperature gradient which, in a device with broken mirror symmetry, generates a thermoelectric voltage signal. This detection mechanism can effectively circumvent performance roll off at high THz frequencies, commonly encountered in gallium arsenide Schottky barrier diode and semiconductor plasmon detectors, and provide very fast response due to graphene?s small electron heat capacity. The THz detectors will be coupled to the incoming radiation through an integrated circuit antenna and a silicon lens. The project will develop: 1. graphene thermoelectric direct detectors with high responsivity, fast speed and low noise equivalent power; 2. graphene thermoelectric heterodyne detectors reaching high sensitivity with low local oscillator power requirement and room temperature operation. The potential for employing the heterodyne detectors in future THz array imagers will be evaluated. In addition to potential applications, the results are expected to provide key information to elucidate the underlying mechanism of the relevant physical processes, including the speed of the thermoelectric process, generation and relaxation of hot electrons, as well as the impacts of the charge density and impedance profile on the thermoelectric voltage.