Design Principles and Identification of Birefringent Materials

Birefringence (Δn) is the dependence of the refractive index of a material on the polarization of light traveling through it. Birefringent materials are used as polarizers, waveplates, and for novel light-matter coupling. While several birefringent materials exist, only a handful of them show large Δn > 0.3 and are primarily limited to the infrared region. The variation of Δn across diverse materials classes and strategies to achieve highly birefringent materials with transparency covering different regions of the electromagnetic spectrum is missing. We have calculated the Δn of 967 noncubic, formable crystals having vastly different structures, polyhedral connectivities, and chemical compositions. From this set of compounds, we have screened highly birefringent crystals (Δn > 0.3) having transparency in different regions of the electromagnetic spectrum. The screened compounds belong to several families such as A3′MN3, AMO2, AN3, and A′N6 (A = Li, Na, and K; A′ = Ca, Sr, and Ba; and M = V, Nb, and Ta). By analyzing the electronic structures of these compounds, we have distilled rules to enable the design of crystals with large Δn.