Ultrafast investigation of room temperature valley polarization in“optical bilayer”WS_(2)

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”WS_(2)formed by transferring monolayer WS_(2)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”WS_(2)resemble actual bilayer WS_(2),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”WS_(2)provide a particularly simple method to enhance valley control at room temperature.