Victor Yu. Reshetnyak

Ferroelectric nematic suspension

We report on the development of a dilute suspension of ferroelectric particles in a nematic liquid-crystal (LC) host. We found that the submicron particles do not disturb the LC alignment and the suspension macroscopically appears similar to a pure LC with no readily apparent evidence of dissolved particles. The suspension possesses enhanced dielectric anisotropy, and is sensitive to the sign of an applied electric field.

Orientational Coupling Amplification in Ferroelectric Nematic Colloids

We investigated the physical properties of low concentration ferroelectric nematic colloids, using calorimetry, optical methods, infrared spectroscopy, and capacitance studies. The resulting homogeneous colloids possess a significantly amplified nematic orientational coupling. We find that the nematic orientation coupling increases by approximately 10% for particle concentrations of 0.2%. A manifestation of the increased orientational order is that the clearing temperature of a nematic colloid increases by up to 40 degrees C compared to the pure liquid crystal host. A theoretical model is proposed in which the ferroelectric particles induce local dipoles whose effective interaction is proportional to the square of the orientational order parameter.

Control of an Easy-Axis on Nematic-Polymer Interface by Light Action to Nematic Bulk

An appearance of an easy orientation axis of a nematic liquid crystal on the isotropic aligning surface under the light absorption by in-bulk azo dye molecules is observed. It is shown that orientation of LC molecules in the irradiated region is close to the direction of the exciting beam polarization. Accompanying to aligning, the surface driven reorientation effect has been studied. A threshold as well as a nonthreshold reorientation of the director toward the light-induced easy axis were observed. The value of the light-induced anchoring energy depends on the exposure time and achieves 2.5×10-3 erg/cm2. The experimental data are explained within the framework of adsorption of phototransformed molecules from the LC bulk onto the aligning surface.

Electrically controlled surface diffraction gratings in nematic liquid crystals

Photorefractive diffraction gratings were studied in cells of homeotropically aligned pentyl-cyanobiphenyl liquid crystal. These holographic gratings were induced by the simultaneous and nonsimultaneous application of dc and coherent optical electric fields. The observed behavior was consistent with a predominantly surface-mediated photorefractive effect. Beam coupling was observed in all cases and led to a model involving screened and unscreened interfacial trapped charges driving a modulation of the easy axis. Holographic gratings could be switched on and off by the application of a small voltage.

Enhanced two-beam coupling in colloids of ferroelectric nanoparticles in liquid crystals

We report on the first, to the best of our knowledge, studies of photorefraction in nematic liquid crystal (LC) doped with nanoferroelectric particles. We found the strong enhancement of two-beam coupling in the colloid of ferroelectric nanoparticles in LC. The effect originated from an increased birefringence of the colloid and a stronger LC reorientation torque. Our measurements allowed us to suggest that increased birefringence is caused by the contribution of polarizability anisotropy of the ferroelectric particles. Stronger reorientation torque is caused by the permanent dipole moment of the particles contributing to the dielectric anisotropy of the colloid eacol. The enhancement of two-beam coupling in LCs by doping with ferroelectric nanoparticles at extremely small concentration shows the strong potential of ferroelectric nanoparticles for improving the optical response of LCs, especially for those materials where a method of chemical synthesis has reached its limit.

Magnetic sensitivity of a dispersion of aggregated ferromagnetic carbon nanotubes in liquid crystals

Using carbon nanotubes filled with α-Fe, we have shown that aggregated ferronematic colloids demonstrate reliable and very effective response to a weak (<5 mT) magnetic field. The magnetic field realigns the aggregates of the particles which results in a non-threshold reorientation of the LC nearby, leading to the optically observed director distortions. The distortion regions expand with the increase of the magnetic field and achieve maximum size of several micrometres, comparable with the size of the agglomerates. In the non-distorted regions the reorientation of the director begins at the magnetic field reaching the Freedericksz transition value. Taking into account the extreme sensitivity of aggregated ferronematics to magnetic field, the following experimental and theoretical studies of the individual response of the aggregated nanoparticles to magnetic field may became the topical task of the physics and applications of ferronematics.

Surface-mediated light-controlled Friedericksz transition in a nematic liquid crystal cell

Surface-mediated director reorientation in a pure nematic liquid crystal (LC) cell in the presence of both a dc and low-power optical field has been observed. A surface-dependent lowering of the Friedericksz transition voltage in a planar cell upon light irradiation was found. It is believed that this effect is due to a light-induced change in the near-surface ion concentration in the presence of a dc field and a photosensitive surface. Enrichment in the ion concentration near the surface causes a redistribution of the electric field in the cell and its localization near the surface. As a consequence, the energy of interaction between the LC and the dc field near the surface overcomes the anchoring energy and results in the director reorientation.

Asymmetric Freedericksz Transitions from Symmetric Liquid Crystal Cells Doped with Harvested Ferroelectric Nanoparticles

The electrical Freedericksz transition characteristics of planar aligned liquid crystal cells doped with harvested single ferroelectric domain 9 nm nanoparticles of BaTiO(3) have been measured. We demonstrate for the first time that the electrical pre-history of the cells imparts significant polarity sensitivity to the Freedericksz characteristics. The presence of harvested single domain ferroelectric nanoparticles enables cells to be programmably semi-permanently polarized. This reduces or increases the Freedericksz transition threshold by 0.8 V, depending on the polarity of the applied voltage, giving a net 1.6 V Freedericksz threshold asymmetry for 8 mum thick cells filled with TL205 liquid crystal.

Peculiarity of an Oblique Liquid Crystal Alignment Induced by a Photosensitive Orientant

The distribution of the director of a nematic liquid crystal (NLC) in a cell with photosensitive aligning layers has been studied. It is shown that a doubly degenerate easy orientation axis is induced on the interface between the nematic and orienting surface. It was also found that the degeneracy of the easy axis orientation can be removed by NLC molecules flow during LC cell filling and an oblique liquid crystal orientation is realized.

EFFECTIVE DIELECTRIC FUNCTION OF FERROELECTRIC LC SUSPENSIONS

We study dielectric properties of a dilute suspension of ferroelectric particles in a nematic liquid crystal(LC)host. It is supposed that submicron particles do not disturb the LC alignment and the suspension macroscopically appears similar to a pure LC. We propose theoretical model for effective dielectric function of ferroelectric LC suspension. It is found that particles permanent polarisation may significantly increase the effective value of suspension dielectric function in comparison with pure LC. For more elongated particles depolarisation factor is smaller and respectively the contribution of induced polarisation of particles to the effective dielectric function is higher.