Direct observation of ultrafast dark exciton formation in monolayer WS$_2$

The exciton landscape in transition metal dichalcogenides is crucial for their optical properties and difficult to measure experimentally, since many exciton states are not accessible to optical interband spectroscopy. Here, we combine a tuneable pump, high-harmonic probe laser source with a 3D momentum imaging technique to map photoemitted electrons from monolayer WS$_2$. This provides momentum-, energy- and time-resolved access to excited states on an ultrafast timescale. The high temporal resolution of the setup allows us to trace the early-stage exciton dynamics and the formation of a momentum-forbidden dark K$\Sigma$ exciton a few tens of femtoseconds after optical excitation. By tuning the excitation energy we manipulate the temporal evolution of the coherent excitonic polarization and observe its influence on the dark exciton formation. The experimental results are in excellent agreement to a fully microscopic theory, resolving the temporal and spectral dynamics of bright and dark excitons in WS$_2$.