Untersuchung der ultraschnellen Dynamik dunkler Exzitonen in WS2 mittels zeitaufgelöster Impulsmikroskopie

This thesis investigates the ultrafast electron dynamics in two-dimensional WS_2, which serves as a model system for studying the unique optical properties of single-layer transition-metal dichalcogenides (TMDC). In these materials, strong spin-orbit coupling and Coulomb interactions result in a variety of excitons, which are distributed over the whole Brillouin zone. While bright excitons can be investigated with optical experiments, dark excitons are difficult to access experimentally. Especially dark momentum-forbidden excitons, where electron and hole are located in different areas in the Brillouin zone, can not be accessed with optical standard methods that rely on a direct interband transition. The ambition of this work is to investigate the excitonic landscape and the associated electron dynamics. With the construction of a new experimental setup for time-resolved momentum microscopy, electron scattering processes were directly imaged in the two-dimensional momentum space. This allowed the observation of the subsequent exciton formation This method combines two-photon photoemission (2PPE) with an energy- and momentum-resolving electron analyzer. By using extreme ultraviolet probe (XUV) pulses and ultrashort tuneable pump pulses, the scattering processes within the first Brillouin zone can be investigated on an ultrafast timescale. One part of the experiments investigated the formation process of dark excitons in a monolayer of WS_2. With the separation of the different excitonic species in energy and momentum, the scattering processes were followed on an ultrafast timescale. The comparison of the experimental data with a microscopic theory shows the strong influence of electron-phonon coupling on the formation process of the dark excitons at the Σ-Point. Furthermore, it was shown how the exciton dynamics can be manipulated by varying the photon energy. A new optical pumping scheme allows excitation with well-defined circularly polarized light. The most elegant way to obtain a circular polarization at the sample surface is by excitation under normal incidence. For this purpose, a small mirror is located close to the optical axis of the momentum microscope, enabling the excitation under an incident angle close to 0°. The resulting pulse front mismatch from thedifferent angles of incidence of the pump and probe and the resulting degradationof the temporal resolution were compensated by tilting the pulse front of the pump beam. Using this technique, the temporal resolution of the collinear incidence could be maintained. The new optical setup enabled the preparation of a well-defined initial state with a single spin polarization, which corresponds to a selective excitation into only one of the two distinct K-valleys. Starting from this well-defined initial state, the formation of momentum- and spin-forbidden excitons and their scattering rate through selected channels, which are otherwise not accessible, could be observed. These experiments contributed to a deeper understanding of electron scattering and exciton formation processes in TMDCs. They provide a basis for a detailed study of the charge transfer across the interface in TMDC-heterostructures and other TMDC-interfaces. The implementation of a cryostat into the new experimental setup will make it possible to perform temperature-dependent measurements in order to study so far less investigated scattering processes such as exchange interaction.