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

TL;DR

This study demonstrates control over spin and valley degrees of freedom in single-layer WS$_2$ by tuning optical excitation parameters, enabling selective excitation of free carriers with specific quantum properties.

Contribution

It introduces a method to manipulate spin and valley quantum numbers in WS$_2$ using optical excitation energy and polarization, providing direct spectroscopic evidence.

Findings

Optically generated free hole density can be doubled with circular polarization.

Varying photon energy tunes free carrier density in specific spin states.

Selective excitation of carriers with desired spin and valley characteristics is achieved.

Abstract

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 WS$_2$. 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.