Theory of the Optical Second-Order Nonlinear Polarizability of Wurztite-Type Crystals

抄録

In this paper we describe the theory of nonlinear polarizability of wurtzite-type crystals using sp³ hybridized wave function model. Negative sign of χ₃₃₃⁽²⁾ of BeO, ZnO and SiC follows naturally from the model with Slater-type wave function and Pauling’s ionicity without any further empirical assumptions. It is pointed out that the overlap of the wave functions of anion and cation leads to a contribution to χ_ijk⁽²⁾ as large as other terms. The abnormal ratio χ₃₃₃⁽²⁾⁄χ₃₁₁⁽²⁾ in distorted wurztite-type crystals (BeO, ZnO, SiC and CdS) is also discussed. Its main cause is the bond-ionicity anisotropy Δf_i=f_ix−f_iz in crystals. Δf_i is proportional to the spontaneous lattice distortion Δ(c⁄a), where c and a are the lattice constants. Δf_i⁄Δ(c⁄a) is found to be about 3.