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.

中文翻译：

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准一维纤维红磷中的巨大离面双折射

准一维（准一维）范德华晶体纤维红磷（RP）表现出明显的面内光学各向异性，使其成为偏振相关微纳米器件的潜在候选者。不幸的是，目前还缺乏对纤维状RP的复折射率以及独特的准一维结构与光学各向异性之间的构效关系的全面研究。在此，我们通过 Kramers-Kronig (KK) 分析和理论计算共同确定了 ab 平面内纤维 RP 薄片的复折射率。值得注意的是，纤维状RP的最大双折射率达到0.642@475 nm，绝对消光系数仅为0.08，优于报道的传统光学晶体和新兴的低维材料。显着的双折射可归因于大电子偶极子极化率、各向异性电子密度分布和立体化学活性孤对电子（SCALP）畸变的协同影响。这项工作展示了纤维状 RP 在偏振敏感器件中的潜力，阐明了开发基于构效关系的新型巨型双折射晶体的可能性。