Vibrational Dichroism of Chiral Valley Phonons

Valley degrees of freedom in transition metal dichalcogenidesthoroughlyinfluence electron-phonon coupling and its nonequilibriumdynamics. We conducted a first-principles study of the quantum kineticsof chiral phonons following valley-selective carrier excitation withcircularly polarized light. Our numerical investigations treat theultrafast dynamics of electrons and phonons on equal footing withina parameter-free ab initio framework. We report the emergence of valley-polarizedphonon populations in monolayer MoS2 that can be selectivelyexcited at either the K or K & PRIME; valleys depending on the lighthelicity. The resulting vibrational state is characterized by a distinctivechirality, which lifts time-reversal symmetry of the lattice on transienttime scales. We show that chiral valley phonons can further lead tofingerprints of vibrational dichroism detectable by ultrafast diffusescattering and persist beyond 10 ps. The valley polarization of nonequilibriumphonon populations could be exploited as an information carrier, therebyextending the paradigm of valleytronics to the domain of vibrationalexcitations.