Optically dark excitonic states mediated exciton and biexciton valley dynamics in monolayer WSe.

We present a theory to address the photoluminescence (PL) intensity and valley polarization (VP) dynamics in monolayer WSe, under the impact of excitonic dark states of both excitons and biexcitons. We find that the PL intensity of all excitonic channels including intravalley exciton (X), intravalley biexciton (XX) and intervalley biexciton (XX[Formula: see text]) in particular for the XX PL is enhanced by laser excitation fluence. In addition, our results indicate the anomalous temperature dependence of PL, i.e. increasing with temperature, as a result of favored phonon assisted dark-to-bright scatterings at high temperatures. Moreover, we observe that the PL is almost immune to intervalley scatterings, which trigger the exchange of excitonic states between the two valleys. As far as the valley polarization is concerned, we find that the VP of X shrinks as temperature increases, exhibiting opposite temperature response to PL, while the intravalley XX VP is found almost independent of temperature. In contrast to both X and XX, the intervalley XX[Formula: see text] VP identically vanishes, because of equal populations of excitons in the K and [Formula: see text] valleys bounded to form intervalley biexcitons. Notably, it is found that the X VP much more strongly depends on bright-dark scattering than that of XX, making dark state act as a robust reservoir for valley polarization against intervalley scatterings for X at strong bright-dark scatterings, but not for XX. Dark excitonic states enabled enhancement of VP benefits quantum technology for information processing based on the valley degree of freedom in valleytronic devices. Furthermore, the VP has strong dependence on intervalley scattering but maintains essentially constant with excitation fluence. Finally, the dependence of time evolution of PL and VP on temperature and excitation fluence is discussed.