Dark Exciton and Optical Selection Rules Revealed by In-plane Propagation of Light in Transition Metal Dichalcogenides Monolayers

The optical selection rules for interband transitions in WSe2, WS2, and MoSe2 transition metal dichalcogenide monolayers (ML) are investigated by polarization-resolved photoluminescence (PL) experiments with a signal collection from the sample edge. We observe clear signatures of the emitted light with the electric field oriented perpendicular to the ML plane, corresponding to the optical transition of “dark” neutral excitons forbidden at normal incidence used in standard optical spectroscopy. These studies, performed at zero external magnetic field B, yield a direct determination of the bright-dark exciton splitting : we measure 40 and 55 meV in WSe2 and WS2 ML [1].
Then magneto-PL experiments with B perpendicular to the ML plane evidence the fine structure splitting δ of these “dark” excitons. We measure δ = 0.6 meV in WSe2 ML. The low-energy state is strictly dipole forbidden (perfectly dark) at B=0, while the upper state is partially coupled to light with z polarization (“gray” exciton). Finally the lifetime of this « gray » exciton state is measured by time-resolved Pl ; we find τ ~110 ps, i.e. two orders of magnitude larger than the bright exciton [2].
[1] G. Wang, C. Robert et al, Phys. Rev. Lett. 119, 047401 (2017)
[2] C. Robert et al, Phys. Rev. B 96, 155423 (2017)