CALCULATION OF THE LINEAR AND NONLINEAR OPTICAL SUSCEPTIBILITIES IN KTP AND MXO₄ CRYSTALS (M=Ga, Al; X=P, As) Ch. Schetelich and P.A.Thomas Physics Department, University of Warwick, UK.

Bond polarisability calculations were carried out using Levine's model of the linear and nonlinear susceptibilities, which is based on the Phillips - Van Vechten dielectric theory. Bond properties, such as homopolar and heteropolar energy gaps, the ionicities and susceptibilities have been computed for the different bonds in KTP (KTiOPO₄) and MXO₄ crystals (M=Ga, Al; X=P, As). We found a satisfactory agreement between the theoretical and experimental values of the averaged macroscopic linear susceptibilities. The simple extension to the anisotropic linear case using a sum over the products of the direction cosines and the isotropic bond polarisability has been proved as inappropriate. A different approach using anisotropic bond polarisabilities suggested by Levine is under investigation.

A comparision of the values of the nonlinear optical susceptibility obtained by both calculations and experiments is given for KTP.