Emerging Photoluminescence from the Dark-Exciton Phonon Replica in Monolayer WSe2

TL;DR

This study reveals that coupling between dark excitons and chiral phonons in monolayer WSe2 enables intrinsic photoluminescence of dark-exciton replicas, opening new avenues for manipulating valley-spin properties in 2D materials.

Contribution

It demonstrates the intrinsic photoluminescence of dark-exciton replicas via exciton-phonon coupling, a novel mechanism in monolayer WSe2.

Findings

Circularly-polarized dark-exciton replica peak observed below dark exciton

Coupling selectively involves spin-forbidden dark and bright excitons

Phonon replica energy matches Se vibration phonon energy

Abstract

Tungsten-based monolayer transition metal dichalcogenides host a long-lived dark exciton, an electron-hole pair in a spin-triplet configuration. The long lifetime and unique spin properties of the dark exciton provide exciting opportunities to explore light-matter interactions beyond electric dipole transitions. Here we demonstrate that the coupling of the dark exciton and an optically silent chiral phonon enables the intrinsic photoluminescence of the dark-exciton replica in monolayer WSe2. Gate and magnetic-field dependent PL measurements unveil a circularly-polarized replica peak located below the dark exciton by 21.6 meV, equal to E" phonon energy from Se vibrations. First-principles calculations show that the exciton-phonon interaction selectively couples the spin-forbidden dark exciton to the intravalley spin-allowed bright exciton, permitting the simultaneous emission of a chiral phonon and a circularly-polarized photon. Our discovery and understanding of the phonon replica reveals a chirality dictated emission channel of the phonons and photons, unveiling a new route of manipulating valley-spin.