  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.
