THEORETICAL-STUDY OF THE NONLINEAR OPTICAL-PROPERTIES OF KTIOPO4 - AN EXTENDED HUCKEL SUM-OVER-STATES APPROACH

We investigate, within the framework of extended Huckel theory, the origins and structural dependences of the electronic hyperpolarizability tensors of asymmetric TiO2 and TiO68-. The TiO68- species, which is a basic structural unit of KTiOPO4 (KTP), is taken as a distorted octahedron with the trans oxygens placed asymmetrically above and below the TiO4 equatorial plane. A term-by-term analysis of the perturbative series for both TiO2 and TiO68- shows that second harmonic generation derives predominantly from pi excitations (relative to the asymmetric 0-Ti-O axis) and that the net hyperpolarizability results from the near cancellation of large contributions with opposite signs. Specific contributions to the hyperpolarizability, which we classify into three major pi families and three major sigma families, typically pass through minima and maxima as the difference between long and short Ti-O bonds is increased.