Birefringence refers to the phenomenon that when a beam of light waves is incident on an optical heterogeneous body, it is decomposed into two beams of linearly polarized light that are perpendicular to each other in the direction of polarization, and the maximum difference between the refractive indices of the two rays (Δn) is the birefringence index. Birefringent crystal is a key material for the preparation of photoelectric modulation devices such as polarizers, optical isolators, circulators and phase retarders, and is widely used in laser polarization technology, optical communication, polarization information processing, high-precision optical devices and other fields. The larger the birefringence index of the birefringence crystal, the easier it is to control the polarization state of light at a smaller scale, so it is very important to find optical materials with a large birefringence index for the miniaturization of devices. In recent years, the design and synthesis of traditional bulk birefringence crystals have developed rapidly, but their size has limited their application in the field of micro-nano optics. In contrast, low-dimensional materials with anisotropic optical properties and nanoscale thickness have attracted more and more attention in the field of birefringence optical materials.
Fiber red phosphorus is an allotrope of elemental phosphorus, as a newly discovered quasi-one-dimensional van der Waals crystal, its unique structure and excellent electrical and optical properties have aroused people's research interest. However, due to the limitation of sample preparation method and size, although it exhibits extraordinary optical anisotropy, there is still a lack of in-depth research on the complex refractive index of fiber red phosphorus, especially the experimental determination of in-plane birefringence index.
Based on this, Yan Qingfeng's team from the Department of Chemistry of Tsinghua University successfully achieved direct growth on substrates on the basis of (001) preferential fiber red phosphorus single crystal thin slices (Nat. Commun. 2023, 14, 4398), the anisotropy of fiber red phosphorus single crystal lamellae was determined by microscopic means, the complex refractive index in the ab plane was extracted based on the Kramers–Kronig equation, and the potential structure-activity relationship between the giant birefringence index (0.642@475 nm) and its quasi-one-dimensional structure was clarified by DFT calculation. The research results were published in Angewandte Chemie International Edition under the title "Giant ab-Plane Birefringence in Quasi-1D Fibrous Red Phosphorus". Wujia Chen, a 2021 Ph.D. student in the Department of Chemistry at Tsinghua University, is the first author of the paper. Yan Qingfeng, an associate professor in the Department of Chemistry at Tsinghua University, is the sole corresponding author of the article. This research was supported by the National Natural Science Foundation of China (No. 1). 21671115 and No. 52072198).
【Core Innovation】
1. The authors successfully extracted the complex refractive index of the ab plane from the micro-reflectance spectra of the fiber red phosphorus single crystal sheet by using the Kramers-Kronig equation, and combined with theoretical calculations, it was confirmed that there is a birefringence index of up to 0.642@475 nm in the visible band, which is far higher than most of the low-dimensional materials that have been reported so far.
2. It is found that the phosphorus atom has a higher electric dipole polarizability than other atoms, which helps the elemental phosphorus to exhibit a larger birefringence index. On this basis, the authors further calculated the birefringence index of purple phosphorus, which has a similar structure to fiber red phosphorus, and the results showed that the anisotropic arrangement of stereochemically active lone pairs (SCALP) caused by the quasi-one-dimensional structure of fiber red phosphorus was the fundamental reason for its giant birefringence index.
3. Through theoretical calculations, the authors further found that there is a layer-dependent birefringence change in fiber red phosphorus. Through the analysis of stereochemically active lone pairs, it is found that with the increase of the number of layers, the van der Waals force between the layers will cause the distortion of the electron cloud around the phosphorus atom in the direction of the fiber red phosphorus a-axis, resulting in more localized electrons, weakening the electric dipole moment in this direction, and then affecting the birefringence index.
【Data Overview】
Fig.1 Crystal structure and basic characterization of fiber red phosphorus
Fig.2. In-plane optical properties of fiber red phosphorus flakes
Fig.3 Optical constants of fiber red phosphorus extracted from KK relation
Figure 4: Potential mechanism of the giant birefringence index of red phosphorus in fibers
Figure 5: The refractive index and birefringence index dependent on the thickness of the fiber red phosphorus
Bibliography:
Wujia Chen,Bowen Zhang, Kezheng Tao, Qiang Li, Jia-Lin Sun, Qingfeng Yan* ，Giant ab-Plane Birefringence in Quasi-1D Fibrous Red Phosphorus ，Angew. Chem. Int. Ed., 2024, e202403531, DOI: 10.1002/anie.202403531
Literature Links:
https://onlinelibrary.wiley.com/doi/10.1002/anie.202403531
Source: Frontiers of Polymer Science