Giant ab‐Plane Birefringence in Quasi‐1D Fibrous Red Phosphorus

Abstract Quasi‐one‐dimensional (quasi‐1D) van der Waals crystal fibrous red phosphorus (RP) exhibits pronounced in‐plane optical anisotropy, positioning it as a potential candidate for polarization‐related micro‐nano devices. Unfortunately, a comprehensive investigation into the complex refractive index of fibrous RP and the structure–activity relationship connecting the distinctive quasi‐1D structure with optical anisotropy is currently deficient. Herein, we have collectively determined the complex refractive index of the fibrous RP flakes within the ab ‐plane through Kramers–Kronig (KK) analysis and theoretical calculation. Notably, the maximum birefringence of fibrous RP reaches 0.642@475 nm with an absolute extinction coefficient of only 0.08, superior to the reported traditional optical crystals and the emerging low‐dimensional materials as well. The remarkable birefringence can be attributed to the synergistic influence of the large electronic dipole polarizability, anisotropic electron density distribution and the distortion of stereochemically active lone pair (SCALP). This work demonstrates the potential of fibrous RP for polarization‐sensitive devices, illuminating possibilities to exploit novel giant birefringent crystals based on the structure–activity relationship.