Enhancement of valley polarization at high photoexcited densities in MoS2 monolayers

TL;DR

This study explores how valley polarization and coherence in MoS2 monolayers depend on temperature and laser power, revealing non-monotonic behavior and the impact of exciton interactions on valley dynamics.

Contribution

It provides new insights into the effects of excitation power and temperature on valley polarization and coherence, highlighting the role of exciton-exciton interactions in these processes.

Findings

Valley polarization peaks at 40 K due to motional narrowing.

Increasing laser power doubles valley polarization at 6 K.

High excitation power reduces valley coherence significantly.

Abstract

We have investigated the steady-sate valley polarization and valley coherence of encapsulated MoS2 monolayer as a function of the temperature and the power density with a continuous wave laser excitation. Both valley polarization and coherence exhibit a non-monotonic dependence on sample temperature, attaining a local maximum at T=40 K. This has been recently attributed to a motional narrowing effect: an enhancement of the valley relaxation time occurs when the scattering rate increases. At a fixed temperature of T=6 K, a two-fold increase of the steady-state valley polarization is achieved by increasing the laser excitation power, which we attribute to a local heating induced by the energy relaxation of photoexcited excitons outside the light cone and to an increase in the exciton-exciton scattering rate. In contrast, in the same power range only a moderate enhancement of valley coherence is observed. Further increasing the excitation power leads to a small reduction of valley polarization but a dramatic loss of valley coherence. Supported by spatial imaging of the excitonic luminescence and polarization, we attribute this behaviour to the detrimental role of exciton-exciton interactions on the pure dephasing rate.