Ultrafast investigation of room temperature valley polarization in "optical bilayer" WS2

Monolayer transition metal dichalcogenides (TMDCs) have become a promising platform in valleytronics due to possessing the regulatable valley degrees of freedom. While, as a result of the rapid intervalley scattering, it is difficult to measure the PL valley polarization of monolayer TMDCs at room temperature, which limits their application in valleytronics devices. Here, we report a room temperature photoluminescence (PL) valley polarization up to 3.73% in an "optical bilayer" WS2 formed by transferring monolayer WS2 onto flat Ag film. Furthermore, in the transient absorption (TA) measurements, a remarkably long valley depolarization lifetime is found. Thus, we demonstrate the valley properties of such "optical bilayer" WS2 resemble actual bilayer WS2, in which the robust valley polarization can be attributed to the phonons depletion effect and the blocked interlayer hopping processes. These peculiar valley features in "optical bilayer" WS2 provide a particularly simple method to enhance valley control at room temperature.