GRAPHENE OPTOELECTRONICS

Graphene is a gapless material that interacts with broadband radiation from far infrared to ultraviolet. The small heat capacitance and the weak electron-phonon coupling make the carbon atomic layer a promising platform for detecting TeraHertz photons — electromagnetic waves that lies between microwaves which can be efficiently manipulated by electronics, and infrared light that can be reliably maneuvered by optics. Detection of THz radiation, however, remains a significant challenge, despite its wide potential application for industrial scanning, security screening, and medical imaging. We are developing novel asymmetric devices based on graphene. These detectors are very fast and can be very sensitive. Our recent work has successfully improved the detector responsivity markedly by 2200 times.

VALLEY PSEUDOSPIN in TMDC SEMICONDUCTORS

Hexagonal (H) molybdenum (Mo) and tungsten (W) based transition metal dichalcogenide (TMDC) semiconductors possess an intrinsic valley quantum degree of freedom similar to charge and spin that have been developed for electronic and spintronic devices. Our lab works on understanding the valley polarization and coherence, as well as how to improve them. These studies highlight the importance of Coulomb exchange interactions in the system and lay the foundation for the robustness and usefulness of future valleytronic devices.

SYMMETRY and SYMMETRY BREAKING EFFECTS in QUANTUM MATERIALS

Optical spectroscopy has the advantage that it is highly sensitive to crystal symmetries. Exploring polarization resolved light-matter interaction provides useful insights into symmetry and symmetry breaking effects that are otherwise subtle to detect. We use this method to probe topologically nontrivial and/or highly correlated quantum materials. Examples include: distorted octahedral (T’) TMDC semimetals, MnBi2Te4,  La/GdTe3, silver ferrite delafossite. Reliable manipulation of these topologically robust quasiparticle excitations and highly correlated states may pave way for future topological quantum computing that can tackle problems impossible to solve by today’s classical computers.

TWISTRIONICS

When two different 2D crystals are stacked, or two identical 2D crystals are stacked at a small twist angle instead of being perfectly aligned, moiré patterns form in the 2D plane. Interaction between the two atomic layers and electronic states within can become highly nontrivial, leading to interesting phases such as insulating states in a presumably metallic system (Mott insulator), superconductivity with a very small density of charges, and topologically non-trivial many-body states. At UMass, we are developing a number of optical and optoelectronic techniques to explore and understand these moiré band physics.