Large second harmonic response in ZnO thin films

Abstract
The second-order susceptibilities in the ZnO films have been studied systemically. In very thin films, we observe an enhancement of second-order susceptibilities, which is larger than that of single-crystal ZnO. It was also observed that the second-order susceptibilities for ZnO films depend on the thickness but not on the film growth technique used (reactive sputtering or plasma-enhanced chemical vapor deposition). We suggest a mechanism to explain the reduction in the second-order susceptibilities of the thicker films.