Inherently high valley polarizations of momentum-forbidden dark excitons in transition-metal dichalcogenide monolayers

TL;DR

This study theoretically demonstrates that finite-momentum dark excitons in WSe₂ monolayers are inherently highly valley-polarized and immune to exchange-induced depolarization, with implications for valleytronic and photonic applications.

Contribution

The paper provides a symmetry-based theoretical analysis showing that inter-valley dark excitons in WSe₂ monolayers are naturally highly valley-polarized and resistant to depolarization effects.

Findings

Finite-momentum dark excitons are immune to exchange-induced valley depolarization.

High valley polarization of dark excitons is transferable to optical polarization in phonon-assisted processes.

Analysis explains observed brightness and polarization in tungsten-based TMD monolayers.

Abstract

High degree of valley polarization of optically active excitons in transition-metal dichalcogenide monolayers (TMD-MLs) is vital in valley-based photonic applications but known to be likely spoiled by the intrinsic electron-hole exchange interactions. In this study, we present a theoretical investigation of the valley and optical properties of finite-momentum dark excitons in WSe$_2$-MLs by solving the density-functional-theory(DFT)-based Bethe-Salpeter equation (BSE) under the guidance of symmetry analysis. %We reveal that, in general, finite-momentum excitons are actually well immune from the exchange-induced valley depolarization, except for those with specific exciton momenta coincident with the $3\sigma_v$ and $3C_2'$ symmetries in the $D_{3h}$ point group of TMD-MLs. We reveal that, unlike the bright exciton inevitably subjected to electron-hole exchange interaction, inter-valley finite-momentum dark excitons in WSe$_2$-MLs are well immune from the exchange-induced valley depolarization and inherently highly valley-polarized under the enforcement of the crystal symmetries. More importantly, the superior valley polarizations of the inter-valley dark excitons in WSe$_2$-MLs are shown almost fully transferable to the optical polarization in the phonon-assisted photo-luminescences because of the native suppression of exchange-induced depolarization in the second-order optical processes. The analysis of phonon-assisted photo-luminescences accounts for the recently observed brightness, high degree of optical polarization and long lifetime of the inter-valley dark exciton states in tungsten-based TMD-MLs.