  Single-layer transition metal dichalcogenides (TMDs) provide a promising
material system to explore the electron's valley degree of freedom as a quantum
information carrier. The valley degree of freedom in single-layer TMDs can be
directly accessed by means of optical excitation. The rapid valley relaxation
of optically excited electron-hole pairs (excitons) through the long-range
electron-hole exchange interaction, however, has been a major roadblock.
Theoretically such a valley relaxation does not occur for the recently
discovered dark excitons, suggesting a potential route for long valley
lifetimes. Here we investigate the valley dynamics of dark excitons in
single-layer WSe2 by time-resolved photoluminescence spectroscopy. We develop a
waveguide-based method to enable the detection of the dark exciton emission,
which involves spin-forbidden optical transitions with an out-of-plane dipole
moment. The valley degree of freedom of dark excitons is accessed through the
valley-dependent Zeeman effect under an out-of-plane magnetic field. We find a
short valley lifetime for the dark neutral exciton, likely due to the
short-range electron-hole exchange, but long valley lifetimes exceeding several
nanoseconds for dark charged excitons.
