Two-dimensional crystals form a unique class of materials that is atomically thin in one dimension, but with length scales much larger than the size of an atom in the other two dimensions. Recent years have seen rapid progress in both the understanding of the fundamental properties and the emerging real-life applications of these materials. The two-dimensional crystals include: graphene – a semimetal, monolayer transition metal dichalcogenide – a semiconductor and h-BN – an insulator. All of them feature two inequivalent valleys in their Brillouin zone, with different sizes of band gap.

In this talk, I will present two recent works that are focused on the distinct optical and electronic properties of monolayer transition metal dichalcogenides. In a magneto-optic study, we demonstrate the valley splitting and polarization of monolayer MoSe2 by valley-resolved magneto-PL spectroscopy. We understand the observed effects by the lowering of time-reversal symmetry through the interaction of the valleys with an external magnetic field up to 10 T. The ability to control the energy splitting of the valleys and further produce valley polarization has important implications to potential valleytronic applications. In the nonlinear-optic study, we detect the broken inversion symmetry and determine the crystallographic orientation in the monolayers by optical second-harmonic generation. In addition to the results for monolayers, we describe the behavior of few-layer materials, where the symmetry properties change layer by layer.

报告人简介：Dr. Yilei Li received his PhD degree in physics from Columbia University in 2014, and his BEng degree (with distinction) in Electrical Engineering from Hong Kong university of Science and Technology in 2010. His current research interest is focused on the light-matter interactions in two-dimensional crystals.