  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.
