Resonant Raman Scattering in Anisotropic Black Phosphorus Crystals

Black phosphorus (BP), a remarkable elemental layered semiconductor with puckered structure, has attracted significant attentions in the past three years. Due to the in-plane low symmetry, few-layer BP exhibits distinct anisotropic electron-photon and electron-phonon interactions in the visible region, giving rise to significant linear dichroism feature and anisotropic Raman scattering. Previous literature reported that the totally symmetric (A_g) Raman modes of BP show abnormal angle-dependent polarized Raman intensities, which are dependent on the excitation energy, thickness and Raman modes too. Several models have been proposed to explain this abnormal phenomenon from the perspective view of complex Raman tensor, birefringence effect, and interference enhancement. However, the resonant Raman effect, which significantly enhances the Raman intensity has not been taken into consideration. Here we investigated the resonant Raman scattering of BP along armchair and zigzag directions with different excitation wavelengths in the visible range. Our results show that the resonant Raman effect plays a crucial role in observing the abnormal polarized Raman scattering in BP.