Thesis defense of Karl Jakob Schiller

Quantum materials often host emergent phases and excitations that arise from interactions between electrons and other quasiparticles. Capturing the ultrafast dynamics of these states requires experimental access to the electronic structure and its evolution in energy, momentum, and time. Time-resolved momentum microscopy provides this access by combining pump–probe photoemission with parallel multidimensional detection, but its performance is fundamentally limited by the trade-off between temporal and energy resolution. In this thesis, a femtosecond high-harmonic-generationbased extreme-ultraviolet beamline is presented that operates in two modes optimized either for energy or for temporal resolution. This dual-mode concept mitigates the time–bandwidth trade-off and enables flexible optimization of spectral and temporal performance. Combined with a momentum microscope for multidimensional detection, the instrument achieves an energy resolution better than 110 meV and a temporal resolution below 50 fs in the respective modes. Using this setup, lightdriven dynamics in layered semiconductors and heterostructures are investigated. In bulk WS2, carrier relaxation across separate valleys of the Brillouin zone is tracked. In a WS2/WSe2 heterobilayer, resonant excitation reveals signatures consistent with the formation and decay of excitons. Finally, core-level spectroscopy combined with theoretical modeling identifies chromium interstitials and bromine vacancies as sources of intrinsic n-type doping in the van der Waals antiferromagnet CrSBr, while sub-band-gap excitation reveals field-driven photoemission effects.