Design Principles and Identification of Birefringent Materials

Birefringence (Δ n ) 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 Δ n > 0.3 and are primarily limited to the infrared region. The variation of Δ n 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 Δ n 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 (Δ n > 0.3) having transparency in different regions of the electromagnetic spectrum. The screened compounds belong to several families such as A 3 ′ M N 3, AM O 2, A N 3, and A ′ N 6 ( 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 Δ n .