Spin and valley control of free carriers in single-layer WS2

The semiconducting single-layer transition-metal dichalcogenides have been identified as ideal materials for accessing and manipulating spin-and valley-quantum numbers due to a set of favorable optical selection rules in these materials. Here, we apply time-and angle-resolved photoemission spectroscopy to directly probe optically excited free carriers in the electronic band structure of a high quality single layer (SL) of WS2 grown on Ag(111). We present a momentum-resolved analysis of the optically generated free hole density around the valence band maximum of SL WS2 for linearly and circularly polarized optical excitations. We observe that the excited free holes are valley polarized within the upper spin-split branch of the valence band, which implies that the photon energy and polarization of the excitation permit selective excitations of free electron-hole pairs with a given spin and within a single valley.