Dynamic Polarization of Resident Electrons in WSe2 and WS2 Monolayers

We demonstrate very efficient spin-valley pumping of resident electrons in both WSe2 and WS2 mono- layers using circularly polarized light [1]. We use the degree of circular polarization of the photoluminescence (PL) associated with negative trions as probes of the polarization of resident electrons : both the intervalley triplet trion and the intravalley singlet trion which consist in the binding of a photo-generated electron-hole pair with a resident electron from the opposite and same valley respectively. In a charge adjustable WSe2 monolayer [2], we measure a very large positive PL circular polarization ~ 90% for the triplet trion and a negative polarization −40% for the singlet trion. This valley pumping also manifests as a large increase of the triplet trion PL intensity with circular excitation as compared to linear excitation. Interestingly, these results demonstrate that circularly polarized excitation photo-generates electron-hole pairs in one valley and dynamically polarize resident electrons in the opposite valley.  The spatial variation of the resident electron spin/valley polarization will also be discussed.

[1] C. Robert et al, Nature Com. 12, 5455 (2021)

[2] M. Yang et al, Arxiv 2110.00887 (2021)