I. Drevenšek Olenik

Diffraction gratings for neutrons from polymers and holographic polymer-dispersed liquid crystals

We discuss the applicability of holographically recorded gratings in photopolymers and holographic polymer-dispersed liquid crystals as neutron optical elements. An experimental investigation of their properties for light and neutrons with different grating spacings and grating thicknesses is performed. The angular dependences of the diffraction efficiencies for those gratings are interpreted in terms of a rigorous coupled wave analysis. Starting from the obtained results we work out the lines for the production of an optimized neutron optical diffraction grating, i.e. high diffraction efficiency in the Bragg diffraction regime with moderate angular selectivity.

Alignment of nematic liquid crystals on an electrically poled photopolymer film

We have observed liquid crystal alignment on an electrically poled polyvinylcinnamate film in which a polar symmetry was introduced by an in-plane electric field and stabilized by unpolarized ultraviolet light. The azimuthal anchoring energy and pretilt angle was measured in a twisted nematic or planar cell and is explained via the molecular structure of the unreacted cinnamates and of the photoproducts β-truxinates. Cinnamates mainly contribute via a strong flow alignment, while β-truxinate produces the anisotropic properties of the film such as its polar anisotropy, azimuthal anchoring, and pretilt angle.

Chirality in discotic lyomesophases

Abstract Properties of isotropic and columnar cholesteric lyomesophases of triphenylene derivatives dissolved in dodecane were investigated by selective reflection (SR) and dynamic light scattering (DLS) spectroscopy. The selective reflection measurements reveal a strong dependence of the helical pitch on concentration and temperature, which can be explained by competing effects of the molecular twisting power and the intercolumnar steric forces. The DLS measurements reveal a presence of three relaxational type dynamic modes in the pre-transitional region of the isotropic-cholesteric phase transition. Two of these modes exhibit an arrhenius like temperature dependence and does not seem to be directly related with the transition. The third mode shows a critical slowing down near the transition temperature and is presumably related to the translational diffusion of the columnar stacks, which rapidly increase in their length with decreasing temperature.

Optical second-harmonic generation at selective reflection band of the SmC* phase

Abstract The process of optical second harmonic generation (SHG) in a slab of the helically modulated SmC* material is analysed for a case when the SHG frequency is in the vicinity or inside the region of the selective reflection band. The analysis bases on the numerical solutions of the nonlinear wave equation. Various possible phase matched combinations of the optical eigen-modes are discussed and the dependence of the SHG intensity on the optical frequency and sample thickness is given.