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 ${\mathrm{WSe}}_2$ monolayers by optical spectroscopy in an out-of-plane magnetic field ${\mathrm{B}}_{\mathrm{z}}$. In agreement with the theoretical predictions deduced from group theory analysis, magnetophotoluminescence experiments reveal a zero-field splitting $\delta =0.6\pm 0.1\mathrm{meV}$ between two dark exciton states. The low-energy state is strictly dipole forbidden (perfectly dark) at ${\mathrm{B}}_z=0$, while the upper state is partially coupled to light with $z$ polarization (“gray” exciton). The first determination of the dark neutral exciton lifetime ${\tau }^{\mathrm{D}}$ in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure ${\tau }^{\mathrm{D}}\sim 110\pm 10\mathrm{ps}$ for the gray exciton state, i.e., two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at $T=12\mathrm{K}$.

• Received 7 August 2017
• Revised 25 September 2017

DOI:https://doi.org/10.1103/PhysRevB.96.155423