Birefringence is one of key performance for optical functional crystals owing to their indispensable role in angle phase-matching or adjusting the polarization of light. In the past few decades, commercial birefringent crystals MgF₂, α-BaB₂O₄, CaCO₃, YVO₄, TiO₂, and LiNbO₃ with the moderate birefringence ranging from 0.01 to 0.2 have been widely used as polarization devices. Aimed at the demands in large birefringence or new applied spectral region, a series of crystals with large birefringence have been developed. It is important to analyze the relationship between the structure and birefringence of potential optical functional crystals and propose new strategies to design optical functional materials.

Crystal Materials Research Center in the Xinjiang Technical Institute of Physics and Chemistry is devoted to the exploration of new nonlinear optical crystals and birefringent crystals. The team analyzed 60 optical functional crystal materials with large birefringence (≥ 0.1) in visible, ultraviolet and deep-ultraviolet wavelength ranges, including (fluorooxo-) borates, carbonates, cyanurates, iodates, molybdates, titanates, and tungstates, etc. In addition, the development of birefringent crystals is prospected, and the strategy of how to discover new optical materials achieving large birefringent and large band gap is proposed. In addition, the authors also suggest the combination of high-throughput screening, machine learning and reverse design in the future to accelerate the discovery of new materials.

This work was published in "Coordination Chemistry Reviews" (2022, 459, 214380).

Article links: https://doi.org/10.1016/j.ccr.2021.214380

(a) Potential optical functional crystals with experimentally measured birefringence（≥ 0.1）and the UV cutoff edges within 110–420 nm.
 (b) Compounds proportion of uniaxial/biaxial crystals, negative/positive uniaxial crystals and negative/positive biaxial crystals.