Nonlinear optical probes and processes in polymers and liquid crystals.

摘要

Nonlinear optical (NLO) techniques are important for materials research due to their high sensitivity to the molecular properties of the system. In this thesis, I consider both theoretical and experimental background for three NLO effects—the photorefractive (PR) effect, second harmonic generation (SHG) and electric field induced second harmonic generation (EFISHG) in polymers and liquid crystals.; I introduce the theoretical molecular model that describes the PR effect in polymers. The dynamics of both photoconductivity and photorefractive grating development is numerically simulated and analyzed. Factors limiting the PR speed in polymers are discussed. A procedure to determine various photoconductivity rates from experiments and, from them, to predict the PR speed is detailed. A complete study of the photoconductive and PR properties of various PVK-based composites is presented. An optimal polymer composite for the PR dynamical performance is suggested.; Dynamical EFISHG studies of chromophore orientation in PVK-based PR polymer composites are performed. The relationship between the speed of chromophore orientation as observed in EFISHG and in four-wave mixing (FWM) holographic experiments is established.; Surface SHG studies of polymeric Langmuir-Blodgett (LB) films and adsorbed liquid crystal (LC) monolayers are described. The molecular distribution in the monolayers is calculated using the polarization and angle dependence of SHG. The temperature stability of the LB films is studied by measuring the SHG temperature dependence.; The azimuthal angle and polarization dependence of the SHG signal for various point symmetry groups are numerically simulated. Then, using the azimuthal angle dependence of the SHG signal in various polarizations of the fundamental and second harmonic light, the in-plane anisotropy of the UV-illuminated adsorbed LC monolayer is explored.