Observation of dark doublets induced by spin-flip processes in WSe₂

Transition metal dichalcogenides (TMDs) are promising sources of single-photon emitters (SPEs), which arise from defect-induced brightening of forbidden transitions. These SPEs, known as localized intervalley defect excitons, appear as doublets with an energy difference driven by electron-hole exchange interactions. Typically, spin forbidden transitions do not manifest in flat two-dimensional materials; however, the engineering of curved materials gives rise to novel phenomena. In this study, we engineered curved WSe₂ monolayers, enabling the direct identification of dark doublets associated with spin-flip transitions of intravalley defect excitons. We present a comprehensive first characterization of these dark doublets revealing an intriguing linear polarization with a 45° phase difference compared to the bright doublet emission. Additionally, a new fine structure splitting emerges from exchange interactions, coupling bright and dark intravalley transitions. This effect can be attributed to the nanopillar induced curvature, which tilts the magnetic moment away from the z-axis, leading to a mixing of in-plane and out-of-plane magnetic field effects. This mixing manifests in a spin-flip effect even in the out-of-plane configuration. Also, an unusually large in-plane g-factor of 4.5 suggests this mixed configuration. This discovery provides critical insights into the coupling mechanisms between dark and bright excitonic states, opening new avenues for exploiting exciton behavior in nanostructured materials.