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Excitons in monolayer transition metal dichalcogenides (TMDCs), depending on the spin configuration of the conduction and valence band, can be either optically bright or dark. Even though spin-forbidden transitions are optically dark, these dark excitons have been observed in tungsten based TMDCs by adding in-plane magnetic field or via near-field coupling to surface plasmon polaritons [1,2]. However, in MoSe₂, the direct observation of dark excitons and the value of the bright-dark exciton splitting are still missing. We have performed low temperature magneto-photoluminescence (PL) and broadband reflection contrast measurements on BN encapsulated monolayer MoSe₂. The experiments reveal the dark exciton brightening and energy shift of bright and dark exciton branches by in-plane magnetic field. Both branches follow B² field dependence in magnetic fields up to about 14T as expected from a two-band model. By analyzing the field evolution of exciton branches, we extracted the value of zero-field bright-dark exciton splitting and excitons g-factors.
[1] Zhang, X.-X. et al. Nat. Nanotech. 12. 883-888 (2017).
[2] Zhou, Y. et al. Nat. Nanotech. 12. 865-860 (2017).