M. I. Barnik

A Study of a Ferroelectric Liquid Crystal Using Second Optical Harmonic Generation

Abstract The phase-matched generation of the optical second harmonic was investigated in the chiral smectic C* phase of a classical ferroelectric liquid crystal (DOBAMBC). The phase-matching was achieved either by incidence angle or by temperature tuning. The temperature behaviour of the second order nonlinear susceptibility was studied.

Dielectric Regime of Electrohydrodynamic Instability in Nematic Liquid Crystals

Abstract The frequency dependencies of the threshold voltage of electrohydrodynamic instability in nematic liquid crystals were investigated as functions of the dielectric anisotropy (-1.5 O and the homeotropic orientation at all the frequencies (the circular and fingerprint-type domains); c) in the isotropic phase. The threshold voltage in the nemataic phase was shown to be independent of the anisotropy parameters and the orientation of a liquid crystal and to coincide with the threshold of the instability in the isotropic phase. Thus, we came to the conclusion that the instability has the isotropic character in all the cases investigated.

Photonics of liquid-crystal structures: A review

The original results of studies of the electro-optical and laser effects which have been performed at the Laboratory of Liquid Crystals of the Institute of Crystallography, Russian Academy of Sciences, over the last few years are reviewed. Cholesteric liquid crystals as vivid representatives of photonic structures and their behavior in an electric field are considered in detail. The formation of higher harmonics in the periodic distribution of the director field in a helical liquid crystal structure and, correspondingly, the new (anharmonic) mode of electro-optical effects are discussed. Another group of studies is devoted to bistable light switching by an electric field in chiral nematics. Polarization diffraction gratings controlled by an electric field are also considered. The results of studies devoted to microlasers on various photonic structures with cholesteric and nematic liquid crystals are considered in detail. Particular attention is given to the new regime: leaky-mode lasing. Designs of liquid crystal light amplifiers and their polarization, field, and spectral characteristics are considered in the last section.

Mesomorphism and Dielectric Properties of Phenyl 4 -Alkylbiphenyl-4′ -Carboxylates and Phenyl 4 (4-Alkylphenyl) cyclohexanecarboxylates

Abstract 4-Alkylbiphenyl-4′-carboxylic and trans-4(4-alkylphenyl)cyclohexanecarboxylic acids have been synthesized. Their p-cyanophenyl esters formed only nematic mesophase in a wide temperature range, and p-hexyloxyphenyl esters-both smectic and nematic mesophases. The replacement of the benzene ring in carboxylic part of molecule by the 1,4-disubstituted transcyclohexane ring leads to decrease of nematic thermal stability and all dielectric constants.

Interaction of light with a NLC–dendrimer system

The interaction of light with a nematic liquid crystal (NLC) doped with dendrimers containing terminal light-absorbing azobenzene fragments and a low-molar-mass dye identical to the fragments was studied in detail. Both the dendrimers and the dye induce orientational nonlinearity in a transparent nematic host. For the fifth-generation dendrimer, the nonlinearity is negative (director is rotated away from the light field direction). For the first-generation dendrimer and the dye, the nonlinearity sign depends on the light propagation direction with respect to the NLC director, light polarisation and external electric field. The results obtained show that complication of the molecular structure of the dopant, i.e. passing from the ‘free’ dye molecule to the first- and fifth-generation dendrimers, results in an increase of the efficiency of the orienting effect of the light.

Light-Induced Director Reorientation in Nematic Liquid Crystals Doped with Azobenzene-Containing Macromolecules of Different Architecture

The results of the studies of the orientational optical nonlinearity of a nematic host doped with high-molar-mass compounds of different molecular architecture such as a comb-like polymer and a dendrimer containing azobenzene chromophores are presented. The nonlinearity induced by these compounds was found to be higher than that induced by the low-molar-mass dyes similar in structure to the light-absorbing constituents of the polymer and dendrimer. Possible reasons for the nonlinearity enhancement for macromolecular dopants are discussed.

Orientational optical nonlinearity induced by comb-shaped polymers in a nematic liquid crystal

The effect of optical orientation in nematic liquid crystals containing small additions of high-molecular compounds, i.e., comb-shaped polymers with light-absorbing azobenzene side fragments, was studied. The effects of light-induced reorientation of the director of nematic liquid crystals caused by light absorption of polymers and a low-molecular compound with a structure similar to side fragments of the polymers were compared in detail. An explanation was proposed for large values of the orientational nonlinearity induced by polymers.

Fine structure of the repolarization current in the B 2 phase of a banana compound

A detailed study of the field induced polarization switching has been carried out over the whole temperature range of the antiferroelectric B2 phase of a banana-shaped compound, the tetradecyl homologue (n=14) of the series 4-chloro-1,3-phenylene bis[4-(4-n -alkylphenylimino)benzoate]s. The fine structure of the current response oscillograms was attributed to the field switching of two antiferroelectric subsystems, most probably the racemic and homogeneously chiral domains which, according to the Boulder group, appear as a result of the spontaneous breaking of mirror symmetry. The temperature and field dependences of polarization as well as switching times and the corresponding viscosity of the substance have been measured.

Multiwave out-of-normal band-edge lasing in cholesteric liquid crystals

We have investigated the lasing effect in dye-doped cholesteric liquid crystals, where the photonic stop-band for light propagating along the helical axis coinciding with the layer normal is strongly shifted to longer wavelengths with respect to the luminescence band of the dye. In the absence of overlapping of the photonic and luminescence bands, the well-known lasing effect along the normal at the stop-band edges is forbidden. However, the lasing is still allowed for out-of-normal directions, for which the photonic band is shifted to shorter wavelengths corresponding to the luminescence band. Despite of the out-of-normal light propagation, the positive feedback necessary for the lasing is still available in a case of the monodomain cholesteric layer, when its thickness is much less than the optical pumping aperture. We have observed a significantly increased energy threshold for the out-of-normal lasing modes despite they are characterized by lower threshold gain compared to the modes propagating along ...

Modern Classification of Electrohydrodynamic Instabilities in the Nematic Phase

Abstract Physical models for various electrohydrodynamic instabilities are reviewed and an up-to-date classification is presented.