Yuri Reznikov

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.

Dye-doped liquid crystals as high-resolution recording media.

Light-induced anchoring of the molecular director is reported to be an efficient method for writing permanent holographic gratings in dye-doped liquid crystals. We have achieved higher sensitivity and spatial resolution in these materials with other methods. An energy density as low as 10(-1) J/cm(2) was sufficient to write gratings with a resolution higher than 100 lines/mm.

Ferroelectric Particles in Liquid Crystals: Recent Frontiers

In this article we describe electro-optical properties of recently discovered ferroelectric particles/liquid crystal colloids. We show that the presence of ferroelectric particles in a liquid crystal changes its birefringence and dielectric anisotropy. In contrast to the traditional time consuming and expensive chemical synthetic methods, this method to create liquid crystals with enhanced properties is relatively simple and has a great potential. We also demonstrate the performance of these new materials in various devices, including displays, light modulators, and beam steering devices.

Ferroelectric nanoparticle/liquid-crystal colloids for display applications

— A non-synthetic approach to modify liquid crystals (LCs) by dispersing low concentrations of ferroelectric nanoparticles is reported. These dilute colloids are stable and appear similar to a pure LC. However, by changing the concentration and the type of ferroelectric particles, the physical properties of the LC materials can be tailored, including the nematic isotropic transition temperature (TNI), the dielectric constants, the birefringence (Δn), elastic constants, and the threshold voltage. Specifically, doping low concentrations of BaTiO3 nanoparticles (∼1%) into a LC MLC-6609 increases TNI up to ∼40°C. This giant shift of TNI has never been previously reported and indicates strong interactions between the LC and ferroelectric nanoparticles on a molecular level. The doped LC also demonstrates significant enhancement in birefringence (by 10–30%), dielectric anisotropy (by ∼50%), and the elastic constant K33 (by ∼20%). Ferroparticles act as molecular additives and modify the intrinsic properties of LC materials without time consuming and expensive chemical synthesis. The new LC materials are very attractive and suitable for use in displays, switchable lenses, beam steering, as well as other light-controlling devices.

Tilted Photoalignment of a Nematic Liquid Crystal Induced by a Magnetic Field

Nematic liquid crystal cells with polyvinyl cinnamate coated substrates were subjected to ultraviolet light. When this was done in the presence of an oblique magnetic field the photoalignment was found to be temporally and thermally robust, with a large pretilt angle and weak polar anchoring. Moreover, two easy axes with equal and opposite pretilt angle were obtained, such that a magnetic field could switch the director from one easy axis to the other.

Fast optical recording in dye-doped liquid crystals

Light-induced director reorientation in dye-doped nematic liquid crystals was recently reported to be an efficient method of writing permanent holographic gratings with high sensitivity [Phys. Rev. Lett. 82, 1855 (1999)]. We report the achievement of stable director reorientation in the same materials by means of a single 4-ns pulse of the second harmonic of a Nd:YAG laser. Fast recording of high-resolution holographic gratings (more than 500 lines/mm) can be obtained with an energy density as low as 7x10(-3)J /cm(2) .

Photo-Alignment using Adsorbed Dichroic Molecules

Abstract In this paper we report photo-alignment using low molecular weight dichroic materials. We produced photo-alignment by exposing thin films of dichroic materials such as methyl red, 4,4′-dihydroxyazobenzene and 4,4′-biphenol to linearly polarized light. The irradiation generates a preferred orientation of liquid crystals perpendicular to the polarization direction. The mechanism of the photo-alignment process was studied. Alignment appears to be produced by a single mono-layer of the dichroic material adsorbed on the substrate. Polarized exposure produces anisotropic desorption and distribution of the remaining dichroic material. Photo-alignment of these materials requires the presence of polar groups in the molecules. Strong adsorption and stable alignment can be achieved by properly matching the chemical characteristics of the dichroic and surface materials. We hope to adjust the energy and tilt angle of the alignment layer by simply adsorbing mixtures of different dichroic materials.

Colloidal particles at a nematic-isotropic interface: Effects of confinement

Abstract.When captured by a flat nematic-isotropic interface, colloidal particles can be dragged by it. As a result spatially periodic structures may appear, with the period depending on particle mass, size, and interface velocity (J.L. West, A. Glushchenko, G.X. Liao, Y. Reznikov, D. Andrienko, M.P. Allen, Phys. Rev. E 66, 012702 (2002)). If liquid crystal is sandwiched between two substrates, the interface takes a wedge-like shape, accommodating the interface-substrate contact angle and minimizing the director distortions on its nematic side. Correspondingly, particles move along complex trajectories: they are first captured by the interface and then “glide” towards its vertex point. Our experiments quantify this scenario, and numerical minimization of the Landau-de Gennes free energy allows for a qualitative description of the interfacial structure and the drag force.

38.1: Study of Ion Beam Alignment of Liquid Crystals on Polymer Substrates

Various polymer substrates, including polyimide and polystyrene were bombarded with Ar+ ion beam in high vacuum at an oblique angle. The alignment direction of liquid crystal 5CB on these polymer substrates is always parallel to the ion beam propagation direction regardless of the type of polymer used. The alignment mechanism is studied using surface sensitive techniques such as Polarized Infrared Attenuated Total Reflection Spectroscopy (ATR), and X-ray Photoelectron Spectroscopy. We propose that selective destruction of the weakest bonds (π bonds) in the polymer by the Ar+ ion beam results in a net excess of the remaining π bonds that align liquid crystals parallel to the ion beam propagation direction.

Study of Ion Beam Alignment of Liquid Crystals on Polymer Substrates

Various polymer substrates, including polyimide and polystyrene were bombarded with an Ar+ ion beam in high vacuum at an oblique angle. The alignment direction of the liquid crystal 5CB on these polymer substrates is always parallel to the ion beam propagation direction regardless of the type of polymer used. The alignment mechanism is studied using surface sensitive techniques such as Polarized Attenuated Total Reflection Infrared Spectroscopy (ATR-IR), and X-ray Photoelectron Spectroscopy. We propose that selective destruction of the weakest bonds (π bonds) in the polymer by the Ar+ ion beam results in a net excess of the remaining π bonds. The anisotropy in these π bonds align liquid crystals parallel to the ion beam propagation direction.