| The relationships between microstructure and macroscopic nonlinear optical properties are investigated in thin film materials. The fundamentals of nonlinear optics are used to systematically develop the theoretical basis of second harmonic generation experimental techniques. These techniques are shown to be useful for determining the macroscopic orientation of textured polycrystalline thin films in cases where x-ray diffraction measurements are not applicable. Experimental studies of the nonlinear optical properties of wide bandgap semiconductors, GaN, AlN, and SiC, are presented and the relevance for technological applications is discussed. The response of polymeric systems containing hyperpolarizable polar chromophores is investigated and correlated to theoretical predictions. In experimental arrangements in which charge injection into the polymer film occurs, anomalous second harmonic generation is observed, in contrast with the classical model of noninteracting polar molecules commonly applied to organic systems. Measurements of the polarization dependence and the frequency dispersion of second harmonic response are used to elucidate the origins of anomalous nonlinear optical responses observed under these conditions. These results suggest that the observed anomalous response arises from charged chromophore aggregates. |
