Design and synthesis of anisotropic crystals with π-conjugated rings toward giant birefringence

Abstract

Birefringent crystals play a significant role in linear optical devices due to their ability to modulate the polarization of light. Commercial birefringent crystals MgF₂, α-BaB₂O₄, CaCO₃, YVO₄, TiO₂, and LiNbO₃ have been widely used as polarization devices in the past decades. A variety of crystals have been developed to meet the requirements of large birefringence and have great potential as optical functional crystals. However, a key question is how to balance the conflict between the energy gap and birefringence. In this review, aiming to better judge the integrated properties of optical crystals, we came up with the birefringent quality factor. We also summarised our recent findings on the birefringent crystals, which contain structural units similar to [B₃O₆]³⁻ rings, from these perspectives, including crystal structure features, optical performances, and structure–property relationships. We summarised the strategy to achieve the balance between the energy gap and birefringence by adjusting the delocalized π-conjugation and the confined π-conjugation to improve the performance of birefringent crystals, which will open up a new window for the exploration of novel birefringent crystals.