Imaging Spin Dynamics in Monolayer WS2 by Time-Resolved Kerr Rotation Microscopy

TL;DR

This study uses time-resolved Kerr rotation microscopy to explore the origin and stability of long-lived spin and valley states in monolayer WS2, revealing correlations with exciton emissions and robustness against magnetic fields.

Contribution

It provides new insights into the mechanisms of spin and valley polarization in monolayer WS2, highlighting the role of dark trions and the stability of these states under magnetic fields.

Findings

Correlation between Kerr rotation and neutral exciton emission.

Evidence for long-lived spin/valley-polarized dark trions.

Spin/valley polarization remains stable up to 700 mT magnetic field.

Abstract

Monolayer transition metal dichalcogenides (TMD) have immense potential for future spintronic and valleytronic applications due to their two-dimensional nature and long spin/valley lifetimes. We investigate the origin of these long-lived states in n-type WS2 using time-resolved Kerr rotation microscopy and photoluminescence microscopy with ~1 micron spatial resolution. Comparing the spatial dependence of the Kerr rotation signal and the photoluminescence reveals a correlation with neutral exciton emission, which is likely due to the transfer of angular momentum to resident conduction electrons with long spin/valley lifetimes. In addition, we observe an unexpected anticorrelation between the Kerr rotation and trion emission, which provides evidence for the presence of long-lived spin/valley-polarized dark trions. We also find that the spin/valley polarization in WS2 is robust to magnetic fields up to 700 mT, indicative of spins and valleys that are stabilized with strong spin-orbit fields.