Microscopic Theory of Exciton Dynamics in Two-Dimensional Materials

Abstract: Transition Metal Dichalcogenides (TMDs) present a giant leap forward towards the realization of nanometer-sized quantum devices. As a direct consequence of their truly two-dimensional character, TMDs exhibit a strong Coulomb-interaction, leading to the formation of stable electron-hole pairs, so-called excitons. These quasi-particles have a large impact on optical properties as well as charge-transport characteristics in TMDs. Therefore, a microscopic understanding of excitonic degrees of freedom and their interactions with other particles becomes crucial for a technological application of TMDs in a new class of optoelectronic devices. The aim of this thesis is to investigate the many-particle processes governing the ultrafast dynamics of excitons in TMD mono- and bilayers. Based on the density matrix formalism we develop equations describing an interacting system of electrons, phonons and photons, and numerically simulate the dynamics of excitons in TMDs. First, we provide a detailed picture of exciton-light and exciton-phonon interactions with special focus on the impact of momentum-dark exciton states. In particular, we develop and apply quantitative models for the i) broadening of excitonic resonances in linear absorption spectra, ii) formation of side peaks in photoluminescence spectra resulting from phonon-assisted recombination of momentum-dark excitons and iii) dynamical simulations of the formation of bound excitons out of a free electron-hole gas. Then, we investigate how the exciton-light interaction is modified when two TMD monolayers are vertically stacked into homo- and hetero-bilayers. Here we focus on the modification of optical spectra in bilayer systems by controlling the stacking angle. In particular, we iv) show how the interlayer hybridization of momentum-dark excitons can be controlled through the stacking angle and v) investigate how the localization phase of moir\'e excitons can be tuned. Our theoretical models have allowed us to predict experimentally accessible excitonic characteristics, which have been demonstrated in several joint experiment-theory collaborations including linear absorption, photoluminescence and ultrafast pump-probe experiments.