Research Interests: We are interested in probing and controlling light-matter interactions and quantum phenomena in novel low dimensional materials. Our current projects focus on emerging optical phenomena in low-dimensional quantum materials, including one-dimensional (1D) carbon nanotubes and two-dimensional (2D) graphene and transition metal dichalcoginides (MX2). Our previous studies have revealed remarkable electronic, optical, and plasmonic behaviors in these novel low-dimensional materials, including Luttinger liquid plasmon in carbon nanotubes, bandgap engineering and tunable plasmonics in graphene, and light-matter interactions in monolayer MX2. With further improvement in both experimental techniques and sample qualities, we aim at exploring more exotic quantum phenomena ranging from quantum valley Hall insulator to “relativistic” thermal waves in these low-dimensional materials.
Experimental Techniques: Our group primarily uses optical spectroscopy to discover and understand emerging phenomena in low-dimensional quantum materials. Exploiting the advances in photonic technology, we develop and employ novel optical spectroscopy that simultaneously achieves ultrabroad energy tunability, exquisite polarization control, nanometer spatial resolution, and femtosecond temporal resolution. This is achieved by combining tunable femtosecond lasers covering the whole electromagnetic spectrum with innovative near-field plasmonics probes and polarization control. Such light spectroscopy, complemented by materials development and electrical transport, allows us to explore dynamic quantum phenomena with unprecedented selectivity and sensitivity, and at their natural length and time scales.
Two-dimensional materials: The successful isolation and manipulation of atomically-thin sheets of 2D crystals has ushered in a new era of basic scientific research and technological innovation. Atomically thin 2D layers can exhibit completely different properties than their three dimensional counterparts. In addition, different 2D materials can now be grown separately and then simply stacked together to form a new class of materials — the van der Waals heterostructures– with unprecedented flexibility and control. This enables the design and creation of functional 2D materials that combine extremely different properties, something that was simply not possible before. In our group, we are interested in different 2D materials and their heterostructures.
- Graphene: Monolayer graphene features massless Dirac electrons with long coherence lengths. Bilayer graphene exhibits a continuously tunable bandgap with massive Dirac electrons and quantum valley Hall states. Our research focuses tunable optical properties and novel Dirac electron physics in graphene.
- Transition metal dichalcogenides (TMDC): Monolayer TMDC materials like MoS2 and WSe2 are direct bandgap semiconductors with unique electronic, spin, and valley properties. Our research focuses on exciton physics, ultrafast dynamics, and valley physics in these 2D materials.
- Two-dimensional covalent organic frameworks: Covalent organic framework materials are two-dimensional organic polymers that combine the design of functional molecules with 2D lattice structures. Our research focuses on exploring how novel physical properties emerge and may be tuned by the underlying inter- and intra-molecular interactions.
- Two-dimensional heterostructures: Two-dimensional heterostructures allow realization of new material combinations (e.g., CDW/superconductor/topological insulator/magnetic heterostructures) with emerging quantum properties that were simply not possible in the past, and unprecedented control of individual layers may be achieved through electrostatic gating, mechanical strain, and inter-layer coupling. We are interested in exploring a wide range of physics in vdW heterostructures, ranging from field-induced 2D magnetism and semiconductor heterojunctions to exciton Bose-Einstein condensates and topological phases.
One-dimensional materials: Electrons confined in one-dimension can exhibit fascinating new phenomena due to the greatly enhanced electron-electron interactions and the constraint in the phase space. For example, Coulomb interactions qualitatively change the electron behavior in 1D and lead to the Luttinger liquid behavior, which exhibit anomalous correlation function and spatially separated collective charge and spin excitations. Carbon nanotubes, with diameters around 1nm and lengths of microns to millimeters, provide ideal experimental realizations of Luttinger liquid. In metallic nanotubes, the massless Dirac electrons exhibit linear energy dispersion up to 1eV. Consequently, the Luttinger liquid behavior is rigorous in metallic nanotubes with a temperature up to thousands of Kevin. In semiconducting nanotubes, the electron dispersion is hyperbolic and the Fermi energy can be tuned across a large range with electrostatic gating. We explore such Luttinger liquid physics in 1D carbon nanotubes as well as graphene nanoribbons.