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

TL;DR

This study reveals valley-selective chiral phonon replicas of dark excitons and trions in monolayer WSe2, demonstrating their unique polarization and valley-dependent properties, which can aid in valley identification and chiral phonon interactions.

Contribution

It uncovers valley-dependent chiral phonon replicas of dark excitons and trions, providing new insights into their polarization and valley selection rules in monolayer WSe2.

Findings

Phonon replicas match zone-center E''-mode energies.

Replicas exhibit valley-dependent circular polarization.

Dark excitons decay into chiral phonons with valley-specific handedness.

Abstract

We observe a set of three replica luminescent peaks at ~21.4 meV below the dark exciton, 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 parallel gate dependence and same g-factors as the dark excitonic states, but follow the valley selection rules of the bright excitonic states. While the dark states exhibit out-of-plane transition dipole and valley-independent linearly polarized emission in the in-plane directions, their phonon replicas exhibit 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.