Directly visualizing the momentum-forbidden dark excitons and their dynamics in atomically thin semiconductors
Julien Madéo, Michael K. L. Man, Chakradhar Sahoo, Marshall Campbell, Vivek Pareek, E. Laine Wong, Abdullah Al-Mahboob, Nicholas S. Chan, Arka Karmakar, Bala Murali Krishna Mariserla, Xiaoqin Li, Tony F. Heinz, Ting Cao, Keshav M. Dani
Abstract
Resolving momentum degrees of freedom of excitons, which are electron-hole pairs bound by the Coulomb attraction in a photoexcited semiconductor, has remained an elusive goal for decades. In atomically thin semiconductors, such a capability could probe the momentum-forbidden dark excitons, which critically affect proposed opto-electronic technologies but are not directly accessible using optical techniques. Here, we probed the momentum state of excitons in a tungsten diselenide monolayer by photoemitting their constituent electrons and resolving them in time, momentum, and energy. We obtained a direct visual of the momentum-forbidden dark excitons and studied their properties, including their near degeneracy with bright excitons and their formation pathways in the energy-momentum landscape. These dark excitons dominated the excited-state distribution, a surprising finding that highlights their importance in atomically thin semiconductors.