Novel Excited States in 2D van der Waals Structures and Moiré Superlattices

Recent experiments revealed spectroscopic signatures of novel excited states and intriguing pump-probe responses in 2D van der Waals structures. The nature of many of these phenomena remains to be fully understood. Here, we present results on the photophysics of these systems based on an ab initio interacting Green's function approach. We show that there is a rich diversity of excitons in transition metal dichalcogenide (TMD) moiré superlattices, including unforeseen novel intralayer charge-transfer moiré excitons. In pump-probe calculations, we discovered a self-driven exciton-Floquet effect in the time-resolved ARPES of 2D materials, wherein prominent satellite bands and renormalization of the quasiparticle bands are induced by excitons, analogously to the optical Floquet effect driven by photons. We demonstrated a new exciton mechanism (direct coupling of intralayer with interlayer excitons) in the ultrafast optical response of TMD heterobilayers. Moreover, we showed that strong excitonic physics in 2D materials can greatly enhances their nonlinear optical responses (e.g., shift currents and SHG). This has led to the discovery of a striking phenomenon of formation of light-induced shift current vortex crystals in TMD moiré systems – i.e., 2D periodic arrays of moiré-scale current vortices and associated magnetic fields with remarkable intensity under laboratory laser setup. Our studies are made possible with the development of two new methods that allow for the ab initio calculations of excitonic physics and photo responses of systems with thousands of atoms in the unit cell and in the time domain.