Fine structure and lifetime of dark excitons in transition metal dichalcogenide monolayers

The intricate interplay between optically dark and bright excitons governs the light-matter interaction in transition metal dichalcogenide monolayers. We have performed a detailed investigation of the spin-forbidden dark excitons in WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field B-z. In agreementwith the theoretical predictions deduced from group theory analysis, magnetophotoluminescence experiments reveal a zero-field splitting delta = 0.6 +/- 0.1 meV between two dark exciton states. The low-energy state is strictly dipole forbidden (perfectly dark) at B-z = 0, while the upper state is partially coupled to light with z polarization (gray exciton). The first determination of the dark neutral exciton lifetime tau(D) in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure tau(D) similar to 110 +/- 10 ps for the gray exciton state, i.e., two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at T = 12K.