Long valley lifetime of dark excitons in single-layer WSe2

TL;DR

This study investigates dark excitons in single-layer WSe2, revealing that while neutral dark excitons have short valley lifetimes, charged dark excitons exhibit long valley lifetimes exceeding several nanoseconds, promising for quantum information applications.

Contribution

The paper introduces a waveguide-based method to detect dark exciton emission and demonstrates long valley lifetimes in charged dark excitons in single-layer WSe2.

Findings

Dark neutral excitons have short valley lifetimes due to short-range exchange.

Charged dark excitons exhibit valley lifetimes exceeding several nanoseconds.

Waveguide-based detection enables observation of dark exciton emission.

Abstract

Single-layer transition metal dichalcogenides (TMDs) provide a promising material system to explore the electron's valley degree of freedom as a quantum information carrier. The valley degree of freedom in single-layer TMDs can be directly accessed by means of optical excitation. The rapid valley relaxation of optically excited electron-hole pairs (excitons) through the long-range electron-hole exchange interaction, however, has been a major roadblock. Theoretically such a valley relaxation does not occur for the recently discovered dark excitons, suggesting a potential route for long valley lifetimes. Here we investigate the valley dynamics of dark excitons in single-layer WSe2 by time-resolved photoluminescence spectroscopy. We develop a waveguide-based method to enable the detection of the dark exciton emission, which involves spin-forbidden optical transitions with an out-of-plane dipole moment. The valley degree of freedom of dark excitons is accessed through the valley-dependent Zeeman effect under an out-of-plane magnetic field. We find a short valley lifetime for the dark neutral exciton, likely due to the short-range electron-hole exchange, but long valley lifetimes exceeding several nanoseconds for dark charged excitons.