Breakdown of Raman selection rules by Fröhlich interaction in few-layer WS2

The Raman selection rules arise from the crystal symmetry and then determine\nthe Raman activity and polarization of scattered phonon modes. However, these\nselection rules can be broken in resonant process due to the strong\nelectron-phonon coupling effect. Here we reported the observation of breakdown\nof Raman selection rules in few-layer WS$_2$ by using resonant Raman scattering\nwith dark A exciton. In this case, not only the infrared active modes and\nbackscattering forbidden modes are observed, but the intensities of all\nobserved phonon modes become strongest under paralleled-polarization and\nindependent on the Raman tensors of phonons. We attributed this phenomenon to\nthe interaction between dark A exciton and the scatted phonon, the so-called\nintraband Fr\\"{o}hlich interaction, where the Raman scattering possibility is\ntotally determined by the symmetry of exciton rather than the phonons due to\nstrong electron-phonon coupling. Our results not only can be used to easily\ndetect the optical forbidden excitonic and phononic states but also provide a\npossible way to manipulate optical transitions between electronic levels.\n