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

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.

Upconversion of Light into Bright Intravalley Excitons via Dark Intervalley Excitons in hBN-Encapsulated WSe2 Monolayers

“… 45 Actually, the exciton states at the Λ-valleys play a significant role as intermediate states in exciton formation and relaxation, since the conduction band minima at these valleys are relatively close (about 35 meV) to the K -valley. 46 , 47 The exciton–phonon scattering in the phonon-assisted UPC mechanism may occur between the K - and Λ-valleys, which also fits to the aforementioned theoretical model.…”

Colloquium: Nonthermal pathways to ultrafast control in quantum materials

“…Time-and angle-resolved photoemission spectroscopy (tr-ARPES) measures changes in the band structure and single-particle spectral function of solids with momentum resolution (Bovensiepen and Kirchmann, 2012;Gedik and Vishik, 2017;Haight et al, 1988;Lv et al, 2019;Nicholson et al, 2018;Petek and Ogawa, 1997;Smallwood et al, 2016;Wegkamp et al, 2014;Zhou et al, 2018). This technique has been used, for example, to study decoherence effects in the excitation process (Höfer et al, 1997;Ogawa et al, 1997;Reutzel et al, 2019), to shed light on the physics of high-temperature superconductors (Avigo et al, 2013;Parham et al, 2017;Smallwood et al, 2012;Yang et al, 2014Yang et al, , 2019, to track the melting and recovery of charge-density wave orders (Hellmann et al, 2010(Hellmann et al, , 2012Perfetti et al, 2006;Rettig et al, 2016;Rohwer et al, 2011;Schmitt et al, 2008;Zong et al, 2019b), to directly probe excitonic states (Cui et al, 2014;Madéo et al, 2020), to measure the relaxation dynamics of photocurrents (Güdde and Höfer, 2021;Reimann et al, 2018) and the coupling between electronic and lattice degrees of freedom (Gerber et al, 2017;Kemper et al, 2017;Na et al, 2019). tr-ARPES also permits the detection of transiently populated topological states (Belopolski et al, 2017;Sobota et al, 2012…”

Efficient phonon cascades in WSe2 monolayers

“…3d. The conduction band minima in these valleys are relatively close (~35 meV) to K, and play a crucial role in exciton formation and relaxation [62][63][64][65] . In this case, h remain in K (or K 0 ), but e scatter to any of the 6 available Λ valleys, and then scatter between these Λ valleys, before going back to K (K 0 ).…”