Valley-selective chiral phonon replicas of dark excitons and trions in monolayer WSe2

We observe a set of three replica luminescent peaks at similar to 21.4 meV below the dark exciton, and negative and positive dark trions (or exciton polarons) in monolayer WSe2. The replica redshift energy matches the energy of the zone-center E-mode optical phonons. The phonon replicas exhibit a parallel gate dependence and the same g-factors as the dark excitonic states, but follow the valley selection rules of the bright excitonic states. While the dark states exhibit an out-of-plane transition dipole and valley-independent linearly polarized emission in the in-plane directions, their phonon replicas exhibit an in-plane transition dipole and valley-dependent circularly polarized emission in the out-of-plane directions. Our results and symmetry analysis show that the K-valley dark exciton decays into a left-handed chiral phonon and a right-handed photon, whereas the K'-valley dark exciton decays into a right-handed chiral phonon and a left-handed photon. Such valley selection rules of chiral phonon replicas can be utilized to identify the valleys of the dark excitonic states and explore their chiral interactions with phonons.