A New Contribution to the Nonlinear Optical Susceptibilityi Arising from Unequal Atomic Radii

Abstract

Compounds containing a first-row atom are shown to have an important additional contribution to the nonlinear optical susceptibility $d_{\mathrm{ijk}}$. Inclusion of this effect explains the anomalous sign of $d_{33}$ for ZnO and leads to excellent agreement with experiment.