Chiral phonon replicas of dark excitonic states in monolayer WSe₂

We observe a set of replica luminescent peaks below the dark excitonic states (including excitons and trions) in monolayer WSe₂. The replica redshift energies match the energies of the zone-center and zone-corner optical phonons. The phonon replicas exhibit parallel gate dependence and same g-factors as the dark excitonic states, but follow the valley selection rules of the bright excitonic states. Our results and symmetry analysis show that a K-valley dark excitonic state can decay into a left-handed chiral phonon and a right-handed photon, whereas a K’-valley dark excitonic state can decay 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.