Effective index of refraction, optical rotation, and circular dichroism in isotropic chiral liquid crystals

This paper concerns optical properties of the isotropic phase above the isotropic-cholesteric transition and of the blue phase BP III. We introduce an effective index, which describes spatial dispersion effects such as optical rotation, circular dichroism, and the modification of the average index due to the fluctuations. We derive the wavelength dependance of these spatial dispersion effects quite generally without relying on an expansion in powers of the chirality and without assuming that the pitch of the cholesteric $P$ is much shorter than the wavelength of the light $\lambda$, an approximation which has been made in previous studies of this problem. The theoretical predictions are supported by comparing them with experimental spectra of the optical activity in the BP III phase.Comment: 15 pages and 7 figures. Submitted to PR

Behavior of Inclusions with Different Value and Orientation of Topological Dipoles in Ferroelectric Smectic Films

Abstract—Cholesteric droplets in ferroelectric free standing films with tunable anchoring on the droplet boundary are investigated. A droplet and satellite topological defect(s) form a topological dipole. We obtained droplets with different angles α between two radial lines from the droplet center to –1/2 topological defects. Droplets with parallel dipoles form linear chains in which the interparticle distances decrease with increasing the defect angle α. For the first time, the dependence of the interparticle distance on the angle between topological defects was measured. We can adjust the magnitude and orientation of topological dipoles formed by the droplets. For the first time, the droplets with antiparallel topological dipoles were prepared in a smectic film. Interaction of the droplets with parallel and antiparallel dipoles differs drastically. Formation of antiparallel dipoles leads to a decomposition of the droplet pairs and chains of droplets. Our observations may be used to change the magnitude, anisotropy of the interparticle interaction, and structures of inclusions in liquid crystal media

The Effect of Spontaneous Polarization on Two Dimensional Elasticity of Smectic Liquid Crystals

The influence of polarity on orientational elasticity and on structures formed in the director field is studied in free standing smectic films. Periodic stripe patterns and 2π walls in a magnetic field are investi gated. Measurements are performed on a nonpolar racemic mixture, on an optically pure ferroelectric com pound, and in mixtures with different concentrations of the chiral isomers of opposite signs. The structure of periodic stripes changes drastically with the polarity of the film. The ratio of the bend KB and splay KS elastic constants for smectic films is determined as a function of polarization from the structure of periodic stripe patterns and 2π walls. We find that the elastic anisotropy KB/KS increases essentially with increasing the polarity of the film. Changes of the elasticity and the structure of periodic stripes are explained by polarization charge effects

Instability of the lattice structure of crystalline liquids near the BPI-BPII phse transition

Essential change of the splitting between reflections (200) and (110) with temperature in the Blue Phase I (BPI) transmission spectrum in an electric field has been observed. The electrostriction coefficient $\gamma_{1111}$ in BPI increases anomalously, while approaching the BPI-BPII phase transition temperature. These results show that the phase transition is connected with lattice instability with increasing temperature

Rearrangement of topological defects and anchoring on the inclusion boundary in ferroelectric smectic membranes

We report experiments on a ferroelectric membrane and droplets with tunable surface properties. In smectic membranes the configuration of the c-director field near inclusions may be rearranged drastically with temperature. The transformation of the c-director field results from the competition between the elastic and polar properties of the membranes.We demonstrate that anchoring conditions on the inclusion boundary are not fixed but depend on the temperature. A dipolar c-director configuration near droplets can evolve to a mixed configuration and to a quadrupolar one. These modifications of the c-director field near the inclusions lead to a change of the interaction between the inclusions, their self-organization, and even to the destruction of structures already formed by the inclusions. Our observations open new possibilities for manipulating inclusions and controlling their self-organization

Shape of nematic droplets in smectic membranes

We found that the nematic droplets in smectic membranes and the configuration of the c-director field near droplets may rearrange drastically with droplet size and temperature. Investigations were made in the membranes with quadrupolar droplets. Large droplets are nearly circular, whereas small droplets become elongated. The elongation of the droplets increases with decreasing droplet size. At high temperature the droplets are found to be spindlelike with two cusps on their boundary. The observed transformations are explained by the competition between the elastic energy of the director field and the surface tension of the interface between the droplets and the smectic membrane

Electrooptical Effects and Experimental Probes of the Structure of Blue Phase III

Optical transmission spectrum measurements in an electric field are used to study the short-range structure of the blue phase III (BPIII). The data are compared with the theoretical models for BPIII. Our results can be explained by the models in which BPIII has the simple-cubic local structure or cubic bond orientational order.

VIBRATIONAL DENSITY OF STATES OF AMORPHOUS AND CRYSTALLINE LIQUID CRYSTAL SUBSTANCES

We have measured by the TOF neutron spectroscopy method the amplitude-weighted frequency distribution function of the glassy and crystalline phases of the EBBA, MBBA and PAA compounds. Evident changes in the vibrational density of states in the frequency range up to 300 cm-1 are observed when passing from the amorphous to ordered crystalline phases. The partly deuterated d6-PAA spectrum have shown that these vibrations are due to translational and librational motions of the whole molecule and due to low frequency oscillations of the end groups of the molecule