| Optical second harmonic generation (SHG) are forbidden in media with inversion symmetry but are necessarily allowed at surfaces and interfaces, making it a powerful and versatile tool for surface studies. In this thesis, we applied SHG to study interfaces involving liquid crystals (LC) and polymers.; SHG has been used to investigate the mechanism for surface-induced bulk alignment of nematic and smectic LC. First, the orientational distribution of LC monolayers on rubbed-polymide surfaces was measured by SHG. A Landau-de Gennes formalism was then developed to predict the bulk LC alignment from the measured surface-monolayer orientational distribution. The excellent agreement between the predicted bulk alignment and the one obtained from ellipsometry measurement suggests that the mechanism responsible for aligning LC film by rubbed-polyimide substrates is the short-range molecular interactions between LC and substrates. Further SHG study of the orientation of a homologous series of alkyl-cyanobiphenyl molecules on various polar substrates provided us a microscopic understanding of intermolecular interactions between LC and substrates.; We have also demonstrated the applicability of SHG to study surface chemical reactions of polymers, using photo-isomerization of azo-dye-functionalized polymer Langmuir films as an example. The azo-dye chromophores were found to isomerize with two distinct reaction rates depending on their local environment.; Nonlinear optics of LC is a fascinating field. The very strong optical nonlinearity and numerous mesophases of LC can lead to many interesting nonlinear optical phenomena. In this thesis, we have investigated two of them, concerning guest-host interaction and rotational dynamics of LC, respectively.; We found that the large optical nonlinearity of LC in both nematic and isotropic phases can be further enhanced by one order of magnitude by dissolving 0.1% of anthraquinone dye in the LC. The same guest-host effect also exists in non-LC liquids. It can be explained by a model based on the change of guest-host interaction resulting from optical excitations of the dye.; A transient optical Kerr setup was built to study the optical nonlinearity of LC in the smectic A phase, which yielded information on the ultra-fast rotational dynamics of LC molecules. |
