We present the observation of second-harmonic (SH) whispering-gallery modes (WGMs) in the hexagonal cross-sections of the tapered ZnO nanotetrapod legs. Because of the continuously changing diameter in the leg, several orders of the WGMs at different SH wavelengths can be on resonance simultaneously at different locations along the leg. The strongest SH WGMs occur when the polarization of the fundamental excitation beam is parallel to the c-axis of the crystal along the leg.

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Second harmonic generation (SHG) measurements were performed in the reflection geometry using the femtosecond Ti:sapphire laser at the wavelength of 810nm for Pb0.35Sr0.65TiO3 films, which were epitaxially deposited on (001) MgO substrates by pulsed laser ablation under different oxygen pressures. We formulated the procedures to measure the ratios of the compensated fractions of both c domains and in-plane domains and the ratios of the components of the nonlinear susceptibility tensor under only a non-normal incidence of the fundamental beam. We applied this technique to characterize the domain microstructures of the Pb0.35Sr0.65TiO3 films at three typical temperatures (78, 150, and 300K) and found these films to exhibit a larger compensated fraction of c domains. The ratios of the components of the nonlinear susceptibility tensor were calculated to be relatively constant regardless of the temperature and the oxygen pressure. On the other hand, their SHG intensities were found to increase as the oxygen pressure goes lower, which is attributed to the higher density of the oxygen vacancies in the films. These films also exhibit the diffuselike phase transition in a very wide temperature range, which is attributed to the structural inhomogeneity and the nonuniform distribution of Pb2+ and Sr2+ in the Pb0.35Sr0.65TiO3 films.

The second harmonic (SH) generation from the highly epitaxial Al-doped ZnO film on sapphire was measured, using the femtosecond Ti:Sapphire laser at the near-resonant SH wavelength, in reflection geometry to avoid the sapphire's contribution in the conventional Maker fringes technique. By investigating SH intensities as a function of the azimuthal angle along the film's normal, we found that the sapphire substrate had a negligible contribution to the reflective SH signal and the film had a pure and well-aligned c-domain. We also developed a new method to calculate the component's ratios of the nonlinear susceptibility tensor by analyzing the polarization diagrams of SH intensities under the incidence with two different angles. The ratios indicate that Kleinman's symmetry is broken due to the absorption at SH wavelength and the dominant component of the nonlinear susceptibility tensor is d(33). Calibration using the Z-cut quartz shows a possible overestimate of the nonlinear response by Maker fringes technique.

Optical nonlinearities of periodically poled LiNbO(3) crystals were investigated by the single beam Z-scan technique with a continuous wave (cw) laser beam at 532 nm. The nonlinear optical absorption coefficient and refractive index change are determined to be 8.1 x 10(-6) cm/W and 2.6 x 10(-4) at 0.5 MW/cm(2) light intensity, respectively. Both sign and magnitude of the measured refractive nonlinearity are considerably different from the Z-scan results in congruent LiNbO(3). The nonlinearities in the periodically poled LiNbO(3) induced by 532 nm continuous waves are believed to be mainly due to the photorefractive effect.