Review on birefringence in borates based on birefringence-active functional groups and arrangements

As a fundamental parameter of the optical crystals, birefringence plays a vital role in many optical applications, such as phase modulation, light splitting, and polarization, especially the phase matching process of the nonlinear optical crystals. The big birefringence not only benefits to the miniaturization of related devices, but also broadens the phase-matching wavelength range of nonlinear optical crystals. The design and synthesis of crystals with large birefringence becomes a hot research topic due to its more and more important applications in the optical modulation and laser technology fields. Herein, crystals with birefringence greater than 0.05 in the borate system are reviewed and classified according to different birefringent active groups, and the relationship between structure and properties is thoroughly explored. It is hoped that this review will provide a clear understanding of what kinds of building units and arrangements would have more opportunity to get adequate birefringence in borate systems and provide the statistical references to encourage the emergence of better crystal materials with large birefringence.