Coherent and Incoherent Dynamics of Quasiparticles in Monolayer Molybdenum Diselenide

Monolayer Materials, especially single-layer graphite, called graphene, as the first synthesized and most prominent representative, have attracted significant research interest since its discovery in 2004. The efforts were rewarded with a Nobel prize in 2010 for the discovery of graphene, the same year in which the first monolayer transition metal dichalcogenide (ML-TMD) was found to have a direct bandgap. In contrast to graphene ML-TMDs have a direct bandgap in the visible or near-infrared spectral range, making them ideally suited for optoelectronic device applications. Explicit inversion symmetry breaking of the unit cell in ML-TMDs furthermore leads to a new interesting property, called valley pseudo-spin. Electrons excited within one valley are restricted to this valley due to momentum trapping. Investigating the valley pseudo-spin dynamics is of importance for both understanding of the fundamental physics as well as device applications since the valley pseudo-spin is a potential information carrier and has potential use for information storage or computing application. Additionally, the confinement to two dimensions leads to enhanced Coulomb interaction and increased dielectric screening between electron and hole. Interestingly, the two-dimensional screening effects were already studied before the first two-dimensional materials were synthesized on quasi-two-dimensional systems. The screening of the Coulomb interaction in turn leads to a significantly increased binding energy between electron and hole, such that the bound electron-hole state, so-called exciton, is stable up to room temperature and above. The same reasoning leads to an enhanced stability of charged excitons, so-called trions, which are the main focus of this dissertation. The optical response of ML-TMDs is therefore completely dominated by excitons and trions, requiring an in-depth understanding of these quasiparticles for device performance optimization. Time-resolved techniques can offer rich information compared to steady-state measurements. While steady-state measurements can resolve things such as the bandgap of a semiconductor or the fact that valley spin exists, time-resolved techniques allow the access of transients and reveal the lifetime of unstable or metastable states, which may be invisible in steady-state measurements. Coherent techniques are known for their ability of probing many-body effects and microscopic inhomogeneity. The technique used to investigate the coherent trion dynamics in this dissertation is two-dimensional coherent spectroscopy, a nonlinear coherent technique, that resolves the signal as a function of two time delays. Using two-dimensional spectroscopy, it is possible to measure the homogeneous linewidth, which is related to the coherence time, even in a strongly inhomogeneously broadened system. The measurement of the coherence time marks the first step in evaluating a material for possible quantum computation applications