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 of WS2. We observe that the optically generated free hole density in a single valley can be increased by a factor of 2 using a circularly polarized optical excitation. Moreover, we find that by varying the photon energy of the excitation we can tune the free carrier density in a given spin-split state around the valence band maximum of the material. The control of the photon energy and polarization of the excitation thus permits us to selectively excite free electron-hole pairs with a given spin and within a single valley.
@article{arxiv.1608.06023,
title = {Spin and Valley Control of Free Carriers in Single-Layer WS$_2$},
author = {Søren Ulstrup and Antonija Grubišić Čabo and Deepnarayan Biswas and Jonathon M. Riley and Maciej Dendzik and Charlotte E. Sanders and Marco Bianchi and Cephise Cacho and Dan Matselyukh and Richard T. Chapman and Emma Springate and Phil D. C. King and Jill A. Miwa and Philip Hofmann},
journal= {arXiv preprint arXiv:1608.06023},
year = {2017}
}