INFLUENCE OF D-ORBITALS ON THE NONLINEAR OPTICAL-RESPONSE OF TRANSPARENT TRANSITION-METAL OXIDES

The bond-orbital theory of linear and nonlinear electronic response in optically transparent materials, developed earlier for pretransition-metal halides and chalcogenides, is expanded to embrace the transition-metal (TM) oxides. The extension requires an explicit recognition of the influence of cationic empty d orbitals on electronic polarizability. Two competing mechanisms, involving, respectively, virtual electronic excitations to the d orbitals and to the conduction-band "sp orbitals," are shown to be essentially additive for linear polarizability chi(1) and lowest-order nonlinear polarizability chi(2), but not for chi(3). The d-orbital contributions to linear and nonlinear response are found to be negligible for bond lengths d greater-than-or-similar-to 2.3 angstrom, but to increase rapidly as a function of decreasing bond length within each TM series to become dominant when d less-than-or-similar-to 2.0 angstrom. Numerical evaluations of non linear refractive index n2 are presented for each series of TM oxides.