The dynamics of exciton formation in transition metal dichalcogenides is difficult to measure experimentally, since many momentum-indirect 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 WS2. 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 on its intrinsic timescale and observe the formation of a momentum-forbidden dark KΣ 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 with a fully microscopic theory, resolving the temporal and spectral dynamics of bright and dark excitons in WS2.
@article{arxiv.2012.11385,
title = {Momentum-resolved observation of exciton formation dynamics in monolayer WS$_2$},
author = {Robert Wallauer and Raul Perea-Causin and Lasse Münster and Sarah Zajusch and Samuel Brem and Jens Güdde and Katsumi Tanimura and Kaiqiang Lin and Rupert Huber and Ermin Malic and Ulrich Höfer},
journal= {arXiv preprint arXiv:2012.11385},
year = {2021}
}