Evaluating refractive index and birefringence of nonlinear optical crystals: Classical methods and new developments

Nonlinear optical (NLO) materials play an increasingly important role in laser technology. Birefringence is one of the most important parameters for NLO materials to realize angle phase-matching conditions. In comparison with other desirable optical properties, the availability of birefringence and refractive index dispersion is especially problematic owing to the strict requirements for single crystals. In this review, we described how to obtain the refractive index and birefringence of NLO materials from crystals sub-millimeters to centimeters in size. Especially, recently developed methods including the minimum deflection angle method, auto-collimation method, prism coupling method, oil immersion technique, interference color method, and theoretical calculation (DFT) for rapid assessment of birefringence are summarized, the contents of which are mainly focused on the principles and typical applications, together with the advantages and drawbacks. In addition, representative examples of birefringent measurements were presented. The purpose of this work is to provide a useful perspective on the characterization of birefringence for NLO materials. It is hoped that this review can give a clear description of the birefringence measurements and accelerate the discovery of new NLO crystals.