# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1903.12586