A simple molecular-orbital theory of the nonlinear optical properties of group III-V and II-VI compounds

The second-order nonlinear optical susceptibility in the low-frequency limit for Group III-V and II-VI semiconductors can be understood in terms of a very simple and yet surprisingly accurate molecular-orbital model of the tetrahedral bonds of the crystal. The physical origin of the nonlinearity is the field-dependence in the ionicity of the bond due to the transfer of valence charge from one atom to the other forming the bond in the presence of the applied field. The sign of the second-order nonlinear susceptibility is directly related to the polarity of the permanent electronic dipole moment of the valence charge distribution on the bond.