Spin/Valley pumping of resident electrons in WSe2 and WS2 monolayers

TL;DR

This paper demonstrates highly efficient optical spin and valley polarization of resident electrons in n-doped WSe2 and WS2 monolayers using continuous wave laser excitation, revealing large dynamic electron polarization effects.

Contribution

It introduces a novel method for optical pumping of resident electrons in monolayer transition metal dichalcogenides, achieving significant polarization effects with continuous wave laser.

Findings

Negative trion lines show opposite circular polarization sign.

Photoluminescence of triplet trion is over four times stronger with circular excitation.

Large dynamic polarization of resident electrons is achieved using circular light.

Abstract

Monolayers of transition metal dichalcogenides are ideal materials to control both spin and valley degrees of freedom either electrically or optically. Nevertheless, optical excitation mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here we demonstrate a very efficient spin/valley optical pumping of resident electrons in n-doped WSe2 and WS2 monolayers. We observe that, using a continuous wave laser and appropriate doping and excitation densities, negative trion doublet lines exhibit circular polarization of opposite sign and the photoluminescence intensity of the triplet trion is more than four times larger with circular excitation than with linear excitation. We interpret our results as a consequence of a large dynamic polarization of resident electrons using circular light.