The polarization selection rule of Raman scattering is crucial in symmetry analysis of elementary excitations in semiconductors and correlated electron systems. Here we reported the observation of breakdown of Raman selection rules in few-layer WS₂ by using resonant Raman spectroscopy. When the excitation energy is close to the dark A exciton state, we observed some infrared active modes and backscattering forbidden modes. Importantly, we found that all observed phonon modes follow the same paralleled-polarization behavior. According to the electron-phonon coupling near the band edge in WS₂, we proposed a theoretical model based on the intraband Fröhlich interaction. In this case, the polarization response of the scattering signal is no longer determined by the original Raman tensor of scattered phonons. Instead, it is determined by a new isotropic Raman tensor that generated from this intraband Fröhlich interaction between dark A exciton and phonons. We found that this theoretical model is in excellent agreement with the observed results. The breakdown of Raman selection rules can violate the conventional limitations of the optical response and provide an effective method to control the polarization of Raman scattering signals in two-dimensional materials.

Electron-phonon coupling affects the properties of two-dimensional (2D) materials significantly, leading to a series of novel phenomena. Inelastic light scattering provides a powerful experimental tool to explore electron-phonon interaction in 2D materials. This review gives an overview of the basic theory and experimental advances of electron-phonon coupling in 2D materials detected by Raman and Brillouin scattering, respectively. In the Raman scattering part, we review Raman spectroscopy studies of electron-phonon coupling in graphene, transition metal disulfide compounds, van der Waals heterostructures, strongly correlated systems, and 2D magnetic materials. In the Brillouin scattering part, we extensively introduce Brillouin spectroscopy in non-van der Waals 2D structures, including temperature sensors for phonons and magnons, interfacial Dzyaloshinsky-Moriya interaction and spin torque in multilayer magnetic structures, as well as exciton-polariton in semiconductor quantum well.