Efficient valley polarization of charged excitons and resident carriers in MoS2 monolayers by optical pumping

Abstract

We investigate with polarized microphotoluminescence the optical pumping of the valley degree of freedom in charge-tunable MoS2 monolayers encapsulated with hexagonal boron nitride at cryogenic temperatures. We report a large steady state valley polarization of the different excitonic complexes following circularly-polarized laser excitation 25 meV above the neutral exciton transition. For the first time in this material we reveal efficient valley pumping of positively-charged trions, which were so far elusive in non-encapsulated monolayers due to defect and laser-induced large electron doping. We find that negatively-charged trions present a polarization of 70 % which is unusually large for non-resonant excitation. We attribute this large valley polarization to the particular band structure of MoS2, where an optically dark exciton ground state coexists with a bright conduction band ordering in the single-particle picture, leading to a supression of the valley relaxation for negatively-charged trions. In addition, we demonstrate that circular excitation induces a dynamical polarization of resident electrons and holes, as recently shown in tungsten-based monolayers. This manifest itself as a variation in the intensity of different excitonic complexes under circular and linear excitation.

References in corpus (23)

• Atomically thin MoS2: A new direct-gap semiconductor
• Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides
• Control of valley polarization in monolayer MoS2 by optical helicity
• Valley polarization in MoS2 monolayers by optical pumping
• The Valley Hall Effect in MoS2 Transistors
• Symmetry-dependent phonon renormalization in monolayer MoS2 transistor
• Excitonic linewidth approaching the homogeneous limit in MoS2 based van der Waals heterostructures : accessing spin-valley dynamics
• Cleaning Interfaces in Layered Materials Heterostructures
• Exciton Radiative Lifetime in Transition Metal Dichalcogenide Monolayers
• Robust optical emission polarization in MoS2 monolayers through selective valley excitation
• Valley dynamics probed through charged and neutral exciton emission in monolayer WSe2
• Electrical suppression of all nonradiative recombination pathways in monolayer semiconductors
• Charged excitons in monolayer WSe${}_2$: experiment and theory
• Exciton Valley Dynamics probed by Kerr Rotation in WSe2 Monolayers
• Fine Structure and Lifetime of Dark Excitons in Transition Metal Dichalcogenide Monolayers
• Measurement of the Spin-Forbidden Dark Excitons in MoS2 and MoSe2 monolayers
• Exciton diffusion in WSe2 monolayers embedded in a van der Waals heterostructure
• Long-Lived Valley Polarization of Intra-Valley Trions in Monolayer WSe2
• Measurement of Conduction and Valence Bands g-factors in a Transition Metal Dichalcogenide Monolayer
• Encapsulation Narrows Excitonic Homogeneous Linewidth of Exfoliated MoSe${}_2$ Monolayer
• Imaging charge and spin diffusion of minority carriers in GaAs
• Controlling exciton many-body states by the electric-field effect in monolayer MoS${}_2$
• Exposing the trion's fine structure by controlling the carrier concentration in hBN-encapsulated MoS${}_2$