Simultaneously Enhanced Emission and Valley Polarization of Dark Excitons of Monolayer WSe₂ Using the Dual Effect of Strain and Magnetic Proximity

Monolayer WSe₂ exhibits an interesting dark exciton ground state and remarkable valley properties due to broken inversion symmetry and spin-orbit coupling; such features mean WSe₂ holds significant promise for novel valleytronic devices. However, the exploitation of dark excitons in valleytronic devices is still in its infancy as it remains a challenge to achieve insensitive emission or/and high valley polarization of dark excitons. In this work, using a patterned yttrium iron garnet (YIG) substrate, we investigated the effects of magnetic proximity and strain on the optical properties of monolayer WSe₂. It is found that with just strain the emission from dark excitons can be enhanced but with nearly zero valley polarization. The magnetic exchange interaction between YIG and single-layer WSe₂ can enhance the valley polarization of exciton emissions of monolayer WSe₂, but the emission from dark excitons is very low. Interestingly, under the dual effect of strain and YIG magnetic proximity, the emission and valley polarization of dark excitons can be simultaneously and significantly enhanced. The intensity differences in emission excited by left and right circularly polarized light of excited dark excitons, both dark exciton (X^D) and dark trion (X^DT), are enhanced by more than a factor of 15 compared with only magnetic proximity effects. Moreover, the emission and the valley polarization of intervalley excitons were also enhanced, and valley polarization of intervalley excitons reaches 18%.