All-Optical Valley Polarization Switch via Controlling Spin-Triplet and Spin-Singlet Interlayer Exciton Emission in WS 2 /WSe 2 Heterostructure.

Although selective singlet and triplet interlayer exciton (IX) emission of transition metal dichalcogenides (TMD) heterostructures can be achieved by applying an electric or magnetic field, the device structure is complex and a low temperature is usually required. Here, we demonstrate a simple all-optical approach to selectively enhance the emission of singlet and triplet IX by selectively coupling singlet or triplet IX of a WS 2 /WSe 2 heterostructure to a SiO 2 microsphere cavity. Angle-resolved photoluminescene reveals that the transition dipole of triplet IX is almost along the out-of-plane direction, while singlet IX only has 69% out-of-plane dipole moment contribution. Since the out-of-plane dipole presents a higher Purcell factor within the cavity, we can simultaneously enhance the emission intensity of IX and control the emissive IX species at room temperature in an all-optical route. Importantly, we demonstrate an all-optical valley polarization switch with a record high on/off ratio of 35.