Optically dark excitonic states mediated exciton and biexciton valley dynamics in monolayer WSe₂

We present a theory to address the photoluminescence (PL) intensity and valley polarization (VP) dynamics in monolayer WSe₂, 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_b), intravalley biexciton (XX_k,k) and intervalley biexciton (XX) in particular for the XX_k,k 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_b shrinks as temperature increases, exhibiting opposite temperature response to PL, while the intravalley XX_k,k VP is found almost independent of temperature. In contrast to both X_b and XX_k,k, the intervalley XX VP identically vanishes, because of equal populations of excitons in the K and valleys bounded to form intervalley biexcitons. Notably, it is found that the X_b VP much more strongly depends on bright–dark scattering than that of XX_k,k, making dark state act as a robust reservoir for valley polarization against intervalley scatterings for X_b at strong bright–dark scatterings, but not for XX_k,k. 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. (paper)