E. Ouskova

Dielectric relaxation spectroscopy of a nematic liquid crystal doped with ferroelectric Sn 2 P 2 S 6 nanoparticles

It was found that doping a nematic liquid crystal (LC) with a small amount of ferroelectric nanoparticles strongly affects the dielectric properties of the system. In particular, adding the ferroelectric particles results in a shift of the absorption bands corresponding to the rotation of liquid crystal molecules around their short axes to lower frequencies and in an increase of the amplitude and with of the absorption bands. This suggests that strong interactions occur between the LC molecules and the particles, caused by the large dipole moment and high polarizability of the ferro-particles. The ferroelectric particles affect not only dielectric losses, but also dielectric permittivity of the system. Specifically, the static dielectric permittivity and the dielectric anisotropy of the suspension are more than twice that of the pure LC.

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.

Enhanced Dielectric Response of Liquid Crystal Ferroelectric Suspension

It was found that doping of tiny ferroelectric particles to a nematic liquid crystal(LC)strongly affects dielectric properties of the system. The doping increases dielectric response of the LC due to interaction between LC molecules and particles, which possess large dipole moment and high polarisability. The suspension reveals high dielectric response on either side of the Curie temperature, TCurie, and peculiarity at the vicinity of TCurie. This result points at retention of dipole ordering above Curie temperature that may be a result of a memory effect.

Enhanced Magneto-Optical Properties of Suspensions of Spindle Type Mono-Dispersed Hematite Nano-Particles in Liquid Crystal

We explored how to increase liquid crystal sensitivity to external magnetic fields. Suspensions consisting of a mixture of liquid crystal and spindle type mono-dispersed hematite nano-particles at concentrations lower than 1 wt% were prepared. The Fréedericksz transition threshold for the suspensions appeared to be lower than for the pure liquid crystal. It was proved that adding canted antiferromagnetic nano-particles in liquid crystals increased their sensitivity to magnetic field while no change of the basic mesogenic properties of the matrix occurred.

Magnetically induced alignment of FNS

In this work we report the observation of magnetically controlled anchoring of ferro-nematic suspensions. We found that application of a weak magnetic field to a cell with the ferro-suspension induces an easy orientation axis with weak anchoring energy on a glass surface. Varying the direction of the magnetic field can change the easy axis orientation. We believe that the magnetically induced alignment of the ferro-liquid crystal (LC) suspension is caused by adsorption of the ferro-particles on the glass surface and this alignment can be manipulated with a magnetic field. The developed system is very promising for ultra-sensitive magnetically controlled LC devices for information processing and storage.

Anchoring and gliding of easy axis of 5CB on photoaligning PVCN-F surface

Abstract— The photoaligning properties of the popular photoaligning material polyvinyl-4(fluorocinnamate) (PVCN-F) are presented. The aligning quality and azimuthal and zenithal anchoring energy were measured and the drift of the easy orientation axis (gliding effect) on the PVCN-F surface, depending on UV exposure, was studied. Special attention is paid to unraveling the contribution of the adsorption liquid-crystal molecules onto the aligning surface to the anchoring properties of PVCNF and measuring the drift of the easy orientation axis over the PVCN-F surface. It is shown that a relatively weak azimuthal anchoring energy (Waz ∼ 10−7 − 10−5 J/m2) leads to strong drift of the easy axis in the azimuthal plane that was observed in a moderate (∼0.1–0.3 T) magnetic field. A much stronger polar anchoring (Wzen ∼ 10−4 J/m2) allowed us to observe the essential gliding of the easy axis in the zenithal plane in a rather strong electric field (∼5 V/μm).

Strong Cubic Optical Nonlinearity of Gold Nanoparticles Suspension in Nematic Liquid Crystal

We report on the observations of extremely strong and fast cubic optical nonlinearity in the suspension of gold nanoparticles in nematic liquid crystal (LC). The nonlinearity was observed by recording dynamic phase sinusoidal holograms with a continuous operating laser. It is caused by changes of the refractive index of a LC due to efficient light-induced heating of the nanoparticles at plasmon resonance excitation and following thermal transport of heat to LC matrix. Large nonlinearity parameter (n 2 ∼ 10−2 cm2/kW) and fast characteristic times, together with excellent photo- and thermo-stability of the system make its extremely promising for optical processing applications.

Light-Controlled Alignment of Cholesteric Liquid Crystals on Photosensitive Materials

We report on investigation of photoalignment of cholesteric liquid crystal on phenylone-based photosensitive material as aligning layer. We found that reflective and electro-optical characteristics of cholesteric cells strongly depended on the dose of irradiation of a photoaligning layer with polarized UV light. Irradiation resulted in orientation of initially chaotically oriented planar domains in a direction determined with incident UV-light polarization and in narrowing of the dependence of light scattering by a planar structure on the scattering angle. The long enough exposure resulted in scattering and electro-optical characteristics not worse than obtained with standard rubbing technology. The photosensitive orientant possesses evident advantages of the effective control of cholesteric textures.

Asymmetry Peculiarities of Surface-Mediated Liquid Crystals Gratings Recorded Due to Light-Induced Anchoring

The peculiarities of the director and polarization gratings recorded due to light-induced anchoring effect in liquid crystal media doped with azo-dye were found and studied. Depending on the ratio of the grating period to the cells thickness, either asymmetrical LC director distribution or quasi-sinusoidal director modulation were formed. The observed behavior can be explained in terms of surface and bulk interaction of twist domains presenting the grating stripes. We showed the possibility to control the grating diffraction efficiency by applying the ac electric field, increasing the diffraction efficiency up to 30%.

Properties of Bulk-Mediated Photoalignment of Doped Liquid Crystal

Abstract We have found that producing a light-induced easy axis in a liquid crystal cell doped with methyl red can result in surface director reorientation both toward and outward the polarization of the incident light. This pointed out two different mechanisms of light-induced anchoring in the system. We proposed that these mechanisms are light-induced anisotropy in the adsorbed MR-layer and light-induced adsorption of MR molecules on the substrate. The study of light-induced anchoring in the isotropic phase showed that the mechanism of light-induced anisotropy in adsorbed MR-layer prevails at small light intensities, whereas the mechanisms of light-induced adsorption dominates at high intensities.