Observation of bright exciton splitting in strained WSe2 monolayers

High-resolution magnetophotoluminescence measurements of strained ${\mathrm{WSe}}₂$ monolayers are reported. At low temperature, a splitting of a few meV in the neutral exciton peak is observed, which is attributed to the theoretically predicted in-plane ($xy$) anisotropy splitting. Measurements in high magnetic fields up to 30 T demonstrate that strain and intervalley electron-hole exchange mix the valley-exciton energy levels, leading to a relaxation of the valley-selective optical selection rules and to strongly modified exciton $g$ factors. Our observation of the bright exciton splitting provides crucial information about the dispersion of valley excitons and paves the way to develop valley functionality for photonic devices.