Fine Structure and Lifetime of Dark Excitons in Transition Metal Dichalcogenide Monolayers

TL;DR

This study investigates dark excitons in WSe2 monolayers, revealing their energy splitting, optical properties, and a lifetime significantly longer than bright excitons, advancing understanding of exciton dynamics in 2D materials.

Contribution

First experimental determination of dark exciton lifetime and energy splitting in transition metal dichalcogenide monolayers using magneto-optical spectroscopy.

Findings

Dark exciton energy splitting is 0.6 meV.

Dark exciton lifetime is approximately 110 ps.

Dark excitons have a lifetime two orders of magnitude longer than bright excitons.

Abstract

The intricate interplay between optically dark and bright excitons governs the light-matter interaction in transition metal dichalcogenide monolayers. We have performed a detailed investigation of the "spin-forbidden" dark excitons in WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field Bz. In agreement with the theoretical predictions deduced from group theory analysis, magneto-photoluminescence experiments reveal a zero field splitting $\delta=0.6 \pm 0.1$ meV between two dark exciton states. The low energy state being strictly dipole forbidden (perfectly dark) at Bz=0 while the upper state is partially coupled to light with z polarization ("grey" exciton). The first determination of the dark neutral exciton lifetime $\tau_D$ in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure $\tau_D \sim 110 \pm 10$ ps for the grey exciton state, i.e. two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at T=12 K.