First-principles study of static polarizability, first and second hyperpolarizabilities of small-sized ZnO clusters.

The static linear and nonlinear optical properties of (ZnO)N clusters of N= 2-12 are studied using finite field approach within the framework of density functional theory. Most of these clusters feature ring or cage structures in the specific size range, differing much from the wurtzite structure of bulk ZnO. The low-lying isomers with various structures possess remarkably different properties. The effect of ring structure, cluster stability, HOMO-LUMO gap, covalent/ionic bonding, etc., on the dipole moment, polarizability, first and second hyperpolarizabilites is discussed. Both the polarizability and second hyperpolarizability exhibit distinct size dependences, reflecting clearly the structural transition when the cluster grows. Our study reveals that the linear and nonlinear optical properties of ZnO clusters vary a lot in the size range studied, as a result of their structural diversity at small sizes.