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

Enhancement of valley polarization at high photoexcited densities in MoS2 monolayers