ON THE APPLICATION OF A GENERALIZED FORM OF MILLER DELTA-COEFFICIENT TO NONLINEAR REFRACTIVE-INDEXES IN PARTIALLY IONIC-CRYSTALS

The applicabilities of Miller's delta coefficient to the second- and t hird-order electronic susceptibilities are compared. To discuss the be haviour of Miller's delta generalized to the third-order susceptibilit y, the nonlinear refractive index is considered. The results obtained by employing the available set of 45 mainly centrosymmetric compounds and the set of about 130 non-centrosymmetric crystals show that the as sumption of weak changes in Miller's delta should not be limited only to the second-order susceptibility. This assumption is even found to w ork better for the third-order electronic susceptibility than it does for the second-order one. The coefficient delta(w; omega, omega, omega ) is employed to estimate the nonlinear refractive index. It is found that the estimation of its magnitude in partially ionic dielectrics ma y be improved by taking into account the ionicity of crystal bonds. An empirical formula to calculate delta(omega; omega, omega, omega) is p resented.