Potential optical functional crystals with large birefringence: Recent advances and future prospects

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 have appeared and possessed great potential as optical functional crystals with large birefringence. Herein, the authors review the developments of emergent candidates with the experimentally measured birefringence ≥ 0.1 in the field of optical materials. Recent efforts and active progress in exploring crystal with large birefringence according to different optical transmittance regions are discussed. The relationship between the crystal structures and birefringence was analyzed. It is hoped that this review will expound a clear perspective on structural chemistry of potential optical functional crystals with large birefringence and expedite the discovery of new birefringent crystals with expected properties to satisfy the urgent demands.