Chiral phonons in the indirect optical transition of a MoS 2 / WS 2 heterostructure

Since experimental verification of the predicted chiral phonon (CP), CPs have become a rising topic. The previous observation was a two-step process, which identified the CP by intervalley transfer of valley holes created from direct optical transition. It is highly desirable to simplify the creation and detection of CPs. Moreover, while indirect optical transition in transition metal dichalcogenide heterostructures has been observed, it is important to promote the understanding of the interplay between the phonon, electron, and photon in the indirect transition. Here, we take the ${\mathrm{MoS}}_{2}$/${\mathrm{WS}}_{2}$ heterostructure as a prototype and study CPs in the indirect optical transition. By first-principles calculations, the heterostructure with an indirect electronic gap exhibits CPs at the high-symmetry $\ifmmode\pm\else\textpm\fi{}\mathbit{K}$ points of the phonon Brillouin zone, which have sizable circular polarization and discrete pseudoangular momentum. CPs with distinct chiralities can be selectively and directly generated in indirect optical transitions by tuning the optical chirality and frequency. These findings provide microscopic understanding of the indirect optical transition by interacting chiral fermions and chiral bosons and enable more convenient detection and control of CPs.