Dark excitons and the elusive valley polarization in transition metal dichalcogenides

Abstract

A rate equation model for the dark and bright excitons kinetics is proposed which explains the wide variation in the observed degree of circular polarization of the PL emission in different TMDs monolayers. Our work suggests that the dark exciton states play an important, and previously unsuspected role in determining the degree of polarization of the PL emission. A dark exciton ground state provides a robust reservoir for valley polarization, which tries to maintain a Boltzmann distribution of the bright exciton states in the same valley via the intra valley bright dark exciton scattering mechanism. The dependence of the degree of circular polarization on the detuning energy of the excitation in MoSe${}_2$ suggests that the electron-hole exchange interaction dominates over two LA phonon emission mechanism for inter valley scattering in TMDs.

arXiv commenttext without changes, the misspelling of one of the names corrected

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• Two-dimensional semiconductors in the regime of strong light-matter coupling
• Hundredfold Enhancement of Light Emission via Defect Control in Monolayer Transition-Metal Dichalcogenides
• Enhanced Light-Matter Interaction in Two-Dimensional Transition Metal Dichalcogenides
• Nanophotonics with 2D Transition Metal Dichalcogenides
• Moiré Intralayer Excitons in a MoSe${}_2$/MoS${}_2$ Heterostructure
• Probing the inter-layer exciton physics in a MoS${}_2$/MoSe${}_2$/MoS${}_2$ van der Waals heterostructure
• Optical valley Hall effect for highly valley-coherent exciton-polaritons in an atomically thin semiconductor
• Imaging Spin Dynamics in Monolayer WS2 by Time-Resolved Kerr Rotation Microscopy
• Defect healing and charge transfer mediated valley polarization in MoS${}_2$/MoSe${}_2$/MoS${}_2$ trilayer van der Waals heterostructures
• Long valley lifetime of free carriers in monolayer WSe2
• Observation of exciton-exciton interaction mediated valley depolarization in monolayer MoSe${}_2$
• Excitonic structure of the optical conductivity in MoS${}_2$ monolayers
• Resonance profiles of valley polarization in single-layer MoS${}_2$ and MoSe${}_2$
• Valley polarized relaxation and upconversion luminescence from Tamm-Plasmon Trion-Polaritons with a MoSe2 monolayer
• Observation of macroscopic valley-polarized monolayer exciton-polaritons at room temperature
• Intervalley Scattering of Interlayer Excitons in a MoS${}_2$/MoSe${}_2$/MoS${}_2$ Heterostructure in High Magnetic Field
• Momentum-Resolved Exciton Coupling and Valley Polarization Dynamics in Monolayer WS${}_2$
• Exciton valley depolarization in monolayer transition-metal dichalcogenides
• Brightening of spin- and momentum-dark excitons in transition metal dichalcogenides
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• Valley Degree of Freedom in Two-Dimensional van der Waals Materials
• Spin accumulation and dynamics in inversion-symmetric van der Waals crystals
• Exploration of the bright and dark exciton landscape and fine structure of MoS${}_2$ (using G${}_0$W${}_0$-BSE)
• Long-lived spin polarization in n-doped MoSe${}_2$ monolayers
• Impact of indirect transitions on valley polarization in WS${}_2$ and WSe${}_2$
• Incoherent phonon population and exciton-exciton annihilation in monolayer WS2 revealed by time-resolved spontaneous Raman scattering
• Intrinsic edge excitons in two-dimensional MoS${}_2$
• Magnetically tunable exciton valley coherence in monolayer WS${}_2$ mediated by the electron-hole exchange and exciton-phonon interactions
• Third-order Optical Nonlinearity in Two-dimensional Transition Metal Dichalcogenides
• A steady state approach for studying valley relaxation using optical vortex beam
• Investigation of indirect excitons in bulk $2H$-MoS${}_2$ using transmission electron energy-loss spectroscopy
• Probing spin dynamics of 2D excitons with twisted light
• Laser-field detuning assisted optimization of exciton valley dynamics in monolayer WSe${}_2$: Geometric quantum speed limit
• Optical Measurement of Pseudo-Spin Texture of the Exciton Fine-Structure in Monolayer WSe2 within the Light Cone
• Ellipticity-Controlled Bright-Dark Coherence Transition in Monolayer WSe2