The understanding of nonlinear optical effect for narrow band and wide band gap crystals

Determining theoretically the respective contributions of cations and anionic groups in inorganic crystals to their linear and NLO effects is meaningful when searching for new NLO materials. In the past we used a theoretical model called anionic group theory to evaluate the relationship between microstructure of the oxide NLO crystals and their macro-optical properties, and calculate their n/sub i/, /spl Delta/n and d/sub ij/ coefficients. The investigations of the band energy calculation method confirm that anionic group theory is successful for the oxide series NLO crystals. In the case of refractive index and birefringence in particular, the results are consistent with experiments. For calculating d/sub ij/ coefficients, the theory is basically successful. Its major shortcoming is that it is unable to consider the effect of cations on the d/sub ij/ coefficients.