Resolving the momentum degree of freedom of excitons - electron-hole pairs bound by the Coulomb attraction in a photoexcited semiconductor, has remained a largely elusive goal for decades. In atomically thin semiconductors, such a capability could probe the momentum forbidden dark excitons, which critically impact proposed opto-electronic technologies, but are not directly accessible via optical techniques. Here, we probe the momentum-state of excitons in a WSe2 monolayer by photoemitting their constituent electrons, and resolving them in time, momentum and energy. We obtain a direct visual of the momentum forbidden dark excitons, and study their properties, including their near-degeneracy with bright excitons and their formation pathways in the energy-momentum landscape. These dark excitons dominate the excited state distribution - a surprising finding that highlights their importance in atomically thin semiconductors.
@article{arxiv.2005.00241,
title = {Directly visualizing the momentum forbidden dark excitons and their dynamics in atomically thin semiconductors},
author = {Julien Madéo and Michael K. L. Man and Chakradhar Sahoo and Marshall Campbell and Vivek Pareek and E Laine Wong and Abdullah Al Mahboob and Nicholas S. Chan and Arka Karmakar and Bala Murali Krishna Mariserla and Xiaoqin Li and Tony F. Heinz and Ting Cao and Keshav M. Dani},
journal= {arXiv preprint arXiv:2005.00241},
year = {2020}
}