Directly visualizing the momentum forbidden dark excitons and their dynamics in atomically thin semiconductors

Madéo, Julien
Man, Michael K.L.
Sahoo, Chakradhar
Campbell, Marshall
Pareek, Vivek
Wong, E Laine
Al Mahboob, Abdullah
Chan, Nicholas S.
Karmakar, Arka
Mariserla, Bala Murali Krishna
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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.