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.

拉曼散射的极化选择规则对于半导体和相关电子系统中基本激发的对称性分析至关重要。在这里，我们报告了使用共振拉曼光谱在几层WS ₂中破坏拉曼选择规则的现象。当激发能量接近暗A激子态时，我们观察到一些红外激活模式和反向散射禁止模式。重要的是，我们发现所有观察到的声子模式遵循相同的平行极化行为。根据WS _2中带边缘附近的电子声子耦合，我们提出了基于带内Fröhlich相互作用的理论模型。在这种情况下，散射信号的极化响应不再由散射声子的原始拉曼张量确定。相反，它是由一个新的各向同性拉曼张量确定的，该张量是由暗A激子和声子之间的带内Fröhlich相互作用产生的。我们发现该理论模型与观察到的结果非常吻合。拉曼选择规则的破坏会违反光学响应的​​常规限制，并提供一种控制二维材料中拉曼散射信号偏振的有效方法。