Understanding and Controlling the Optical Properties of Quasi Two-Dimensional Materials

In this thesis, I discuss the understanding and control of the optical properties of quasi-two-dimensional materials, an emerging field since the discovery of graphene. This thesis not only aims to understand and predict the distinct optical properties of quasi-two-dimensional materials from theoretical and numerical approaches, but also incorporates and quantitatively explains relevant experimental data when available. This thesis is organized as follows:In the first chapter, I give a brief background overview on 1) research on excited states in general, 2) first-principles GW-BSE method that calculates the electron quasiparticle bands and exciton properties, and 3) recent progress on the optical properties of two-dimensional semiconductors and light-matter interactions in these materials.In the second chapter, I review the valley physics in transition metal dichalcogenide monolayers, which builds the foundation of the more advanced topics that we discuss in the next chapters.In the third chapter, I present several studies on the unusual optical properties of transition metal dichalcogenide monolayers arising from the novel exciton physics, including strongly-bound non-hydrogenic exciton series, light-like exciton dispersion, and magnetic brightening of the dark states. These results show the distinct optical properties of two-dimensional semiconductors compared with those in other dimensions.In the fourth chapter, I demonstrate some consequences of topological effects on optical transitions in two-dimensional semiconductors, which leads to a new set of optical selection rules dictated by the winding number of interband optical matrix elements. The new selection rules go beyond the selection rules for conventional semiconductors which have been used for over 6 decades, and explains the experimental results on the photo-current spectroscopy of gapped bilayer graphene.In the last chapter, I present materials engineering aspects of two-dimensional materials via van der Waals interfacial engineering. We show that by changing the interlayer stacking configurations and by applying out-of-plane electric fields, the electronic and optical properties of van der Waals layers can be rationally engineered and controlled