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The semiconducting single-layer transition metal dichalcogenides have beenidentified as ideal materials for accessing and manipulating spin- andvalley-quantum numbers due to a set of favorable optical selection rules inthese materials. Here, we apply time- and angle-resolved photoemissionspectroscopy to directly probe optically excited free carriers in theelectronic band structure of a high quality single layer of WS$_2$. We observethat the optically generated free hole density in a single valley can beincreased by a factor of 2 using a circularly polarized optical excitation.Moreover, we find that by varying the photon energy of the excitation we cantune the free carrier density in a given spin-split state around the valenceband maximum of the material. The control of the photon energy and polarizationof the excitation thus permits us to selectively excite free electron-holepairs with a given spin and within a single valley.

Spin and Valley Control of Free Carriers in Single-Layer WS$_2$