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Observation of Chiral Phonons

(A) Atomic rotation in two chiral phonon modes with same linear momentum but different angular momenta; (B) Circular polarization selection in the infrared absorption spectrum proves chiral photon-phonon interaction and yields the energy of the generated phonons.

Scientific Achievement
First experimental discovery of the chirality of phonons that intrinsically consist of atomic rotation.

Significance and Impact
Route for studying chiral Boson theories in solid states, active control of valley and spin, and phononic circuits.

Research Details

  • Chiral phonons are predicted in inversion asymmetric lattice of monolayer WSe2, but their period is at atomic level and deep sub-wavelength, demanding novel excitation mechanism.
  • Optically injected electronic vacancies in K valley provide the required large momentum to phonons, and momentum transfer is verified by the K' valley emission from the final electronic states.
  • Circularly polarized mid-infrared light provides the angular momentum of the phonons. The phononic chirality is verified by the imbalance of absorption for the opposite polarizations.
  • The energy transferred from the electrons and photons to the phonons agrees with the modes calculated by first principle.

H. Zhu, J. Yi, M.-Y. Li, J. Xiao, L. Zhang, C.-W. Yang, R.A. Kaindl, L.-J. Li, Y. Wang, X. Zhang, Science 359, 579–582 (2018) DOI: 10.1126/science.aar2711