Journal
SCIENCE
Volume 370, Issue 6521, Pages 1199-1203Publisher
AMER ASSOC ADVANCEMENT SCIENCE
DOI: 10.1126/science.aba1029
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Funding
- JSPS KAKENHI [JP17K04995]
- Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University
- National Science Foundation (NSF) through the University of Washington Materials Research Science and Engineering Center [DMR-1719797]
- AMOS program, Chemical Sciences, Geosciences, and Biosciences Division, Basic Energy Sciences, U.S. Department of Energy [DE-AC02-76-SF00515]
- Micron Foundation
- GLAM postdoctoral fellowship at Stanford
- NSF [ACI-1053575]
- NSF through the Center for Dynamics and Control of Materials (NSF MRSEC) [DMR-1720595]
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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.
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