Abstract

Through the efforts in the past five years, we have preliminarily established a expert system for searching of inorganic NLO crystals, which include a structure database, computer graphics and structure evaluation systems, and computation programs for SHG coefficients, birefringence and absorption edge of NLO crystals. Performing the programs of the expert system, it is convenient to judge whether an anionic group is favorable to produce a larger microscopic second-order susceptibilities, moderate anisotropy linear susceptibilities and wider band gap or not. Under this pattern, compared with the routine currently used, it is much quicker to determine the linear and nonlinear optical properties of crystals, only if the space structure of the crystal is known. For example, with help of the above system the linear and nonlinear optical properties of KBBF (KBe₂BO₃F₂) crystal have been easily explained as below:¹ (1) (BO₃)³⁻ anionic group is the active group responsible to the NLO effects of KBBF; (2) the larger SHG coefficients of KBBF come from the co-planar arrangement of the (BO₃)³⁻ group in the crystal lattice, in which all the (BO₃)³⁻ planes keep perpendicular to the axis of the crystal; (3) the larger birefringence of KBBF (compared with LBO) results also from the co-planar arrangement of the (BO₃)^3_ grups; (4) the wide transparent range of KBBF on the UV side is mainly determined by the energy gap of (BO₃)³⁻, because that of the (BeO₃F)5⁻ group is larger than (BO₃)³⁻ (see Fig. 1). When the above theoretical considerations of molecular engineering approach are followed, it is reasonable that the suited candidates which may be easier to grow, while possesses linear and nonlinear optical properties similar to KBBF, may be ones that remain the same layer structure characteristics as KBBF, but with an appropriate bridged atom to be brought to produce interlinking of strong covalent bonds among the layers, in order to strengthen the chemical interaction between them. On the basis of above theoretical suggestions, we have successfully synthesized a new boron-beryllium compound (Sr₂BE₂B₂O₇) (SBBO).²

© 1995 IEEE