V. Reshetnyak

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.

Preparation of ferroelectric nanoparticles for their use in liquid crystalline colloids

In this paper we summarize our many years of experience in the preparation and optimization of stable colloids of ferroelectric nanoparticles dispersed in an isotropic carrier and in a liquid crystal host. The colloids are of interest for use in electro-optic devices, photorefractive hybrids and nonlinear optical elements. We also outline some of the most interesting features the nanoparticles bring to liquid crystals, along with the potential of these relatively new colloids.

Light scattering by optically anisotropic scatterers: T-matrix theory for radial and uniform anisotropies

We extend the T-matrix approach to light scattering by spherical particles to some simple cases in which the scatterers are optically anisotropic. Specifically, we consider cases in which the spherical particles include radially and uniformly anisotropic layers. We find that in both cases the T-matrix theory can be formulated using a modified T-matrix ansatz with suitably defined modes. In a uniformly anisotropic medium we derive these modes by relating the wave packet representation and expansions of electromagnetic field over spherical harmonics. The resulting wave functions are deformed spherical harmonics that represent solutions of the Maxwell equations. We present preliminary results of numerical calculations of the scattering by spherical droplets. We concentrate on cases in which the scattering is due only to the local optical anisotropy within the scatterer. For radial anisotropy we find that nonmonotonic dependence of the scattering cross section on the degree of anisotropy can occur in a regime to which both the Rayleigh and semiclassical theories are inapplicable. For uniform anisotropy the cross section is strongly dependent on the angle between the incident light and the optical axis, and for larger droplets this dependence is nonmonotonic.

Nano-colloids of Sn2P2S6 in Nematic Liquid Crystal Pentyl-cianobiphenile

It has been shown experimentally that the ferroelectric nano-particles, in common with other molecular additives, shift the clearing temperature, Tc, extending or reducing the two-phase coexistent region and changing the average order parameter of the single-component nematics. An increase of up to 11 C or a smaller decrease of as much as 3 C of Tc, and corresponding changes of the order parameter were observed. In all cases the order parameter of the colloid followed the universal temperature behaviour, which is a characteristic of LCs with molecular additives.

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.

An Oblique Orientation of Nematic Liquid Crystals on a Photosensitive Aligning Polymer

Abstract The problem of an oblique orientation of a liquid crystal in cells with light induced easy orientation axis is considered. Theoretical and experimental studies of the director distribution in a cell with a photosensitive orientant, fluorinated polyvinylcinnamate polymer film, are performed. The two exposure technique for liquid crystal oblique orientation and the condition for a new anchoring transition in a cell with a photosensitive orientant are also presented.

Electrically Controlled Light Scattering of the Aerosil-Liquid Crystal System

Abstract Optical and electro-optical characteristics of a silica suspension in a liquid crystal were studied. Their possible applications in different electro-optical devices are analyzed.

Light-induced changes of the refractive indices in a colloid of gold nanoparticles in a nematic liquid crystal

It was shown that irradiation of a nematic liquid crystal doped with metal nanoparticles in the visible near the plasmon resonance band led to strong thermal changes of the refractive indices. The effect was studied by recording of dynamic optical gratings in the colloid. Nanoparticles “worked” as effective nano-heaters in a matrix causing the order parameter decrease around the particles. A large nonlinearity parameter (n2 ≈ 10-2 cm2/kW and fast response (≈ 0.7 ms), with no detectable particles’ aggregation and excellent photo- thermo-stability make these colloids potentially attractive nonlinear optical media. Application of a dynamic holography technique allowed measuring the coefficients of thermal conductivity of the liquid crystal along the director k|| = (0.4 ± 0.02) W m−1K−1 and perpendicular to the director k⊥ = (0.2 ± 0.01) W m−1K−1.

Formation and dynamics of easy orientation axis in magnetic field on PVCN-F surface

We describe the experiments on a magnetically-induced drift of the easy axis on a soft surface of photoaligning material fluoro-polyvinyl-cinnamate. We found unexpected partial relaxation of the drift of the easy axis after switching the magnetic field off. This relaxation cannot be explained in a framework of the existing models and requires additional assumptions about the drift process. We propose a model that explains the experimental data suggesting elastic-like behaviour of the polymer fragments during the drift of the easy axis.

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.