Planar Nematic Cell Research Articles

Threshold spatially periodic structure of the director in a nematic flexoelectric cell with finite anchoring energy

The influence of the finiteness of the anchoring energy of a director and the value of flexoelectric polarization on the threshold of a spatially periodic reorientation of the director and the period of the arising structure is considered for a planar nematic cell. The threshold and the period are calculated numerically and the corresponding analytical expressions are obtained for the case of a strong anchoring of the director. It is shown that for a finite azimuthal anchoring energy the range of admissible values of the flexoelectric parameter ν widens, while for a finite polar anchoring energy this range narrows as compared to the case of an absolutely rigid orientation of the director at the cell surface.

Molecular reorientation dynamics due to direct current voltage-induced ion redistribution in undoped nematic planar cell

We discuss the influence of mobile ions in liquid crystalline materials on the dc voltage-induced molecular reorientation dynamics. We investigate the conventional case of an undoped planar nematic sample aligned by thin polymeric films deposited on the electrodes. A simple model for the ion drift towards the electrodes has been developed in the limit of negligible diffusion current in order to obtain the time dependence of the internal electric field and, subsequently, the dynamics of the director distribution. This model describes the buildup of two electric charge double layers near each electrode reducing the effective voltage drop across the nematic sample. We report the measurement of the dynamics of the phase retardation in a conventional 5CB planar cell when a stepwise dc voltage above the Freedericksz threshold is applied to the electrodes. We demonstrate that our model reproduces the experimentally determined curve, thereby providing reliable values for the physical parameters used in the fitting procedure.

Photorefractive effect due to a photoinduced surface-charge modulation in undoped liquid crystals.

We demonstrate that some peculiarities of the surface-induced photorefractive effect (SIPRE) in undoped nematic planar cells can be simply explained considering the electric field induced by a modulated surface-charge distribution. Polarization-dependent forced light scattering and two-beam coupling experiments indicate that the observed anisotropy of the diffraction efficiency and the energy transfer between the pump beams strongly depend on the experimental geometry. The investigation suggests that the unusual dichotomy between local and nonlocal behavior can be ascribed to a modulated longitudinal electric field component, in phase with the interference pattern, which is not accountable by the conventional photorefractivity. In a simple and general approach we demonstrate that the conceived charge distribution model for the SIPRE produces a space-charge field having two orthogonal modulated components, in-phase and pi/2 out of phase, respectively. The electric field configuration within the nematic sample gives reason for the main experimental features.

Dynamic grating features for the surface-induced photorefractive effect in undoped nematics

We investigate the highly sensitive surface-induced photorefractive gratings, observed in undoped nematic planar cells for low dc voltage (few volts) and laser intensity (few milliwatts per square centimeter). Forced-light-scattering and two-beam coupling measurements on E7 nematic samples, aligned with rubbed polyvinyl alcohol layers, verify the orientational effect of a space-charge field and the crucial role of the polymer–liquid-crystal interface in the photoinduced processes, through a wavelength-dependent photoelectric activation. The experimental behavior supports the surface-induced photorefractive effect model, according to which light with the proper wavelength locally reduces the interfacial charge density, thus producing a space-charge electric field in the cell.

Phase-conjugate reflection and self-starting optical phase-conjugate oscillation in planar nematic liquid-crystal cells

A theoretical model for phase-conjugate reflection of planar nematic liquid-crystal cells is presented based on the continuum theory of liquid crystals and the optical theory of degenerate four-wave mixing. An analytical solution for the phase-conjugate reflectivity R is derived under the undepleted-pump approximation, and the necessary conditions for self-starting optical phase-conjugate oscillation are obtained. The dependence of these conditions on the related physical parameters is also discussed. The reconstruction phenomenon of phase-conjugate light and the self-starting optical phase-conjugate oscillation have been observed experimentally. The intensities of the phase-conjugate light versus the biased voltages for various incident optical intensities have been measured, and the results agree very well with the prediction of our theoretical model.

Dielectric, viscous and elastic properties of nematogenic 1-(4- trans -propylcyclohexyl)-2-(4-cyanophenyl)ethane

This paper presents the results of measurements of the static and dynamic electric permittivity, the shear viscosity of a freely flowing sample, and the splay and bend elastic constants for 1-(4-trans -propylcyclohexyl)-2-(4-cyanophenyl)ethane (C3H7-CyHx-CH2CH2-Ph-C=N) (3CCPE). The static permittivity of the isotropic phase shows pre-nematic critical behaviour which is discussed in the frame of the fluid-like model of Mukherjee. From the temperature dependence of the dielectric relaxation time (corresponding to the molecular rotation around the short axis) and the shear viscosity, the strength of the nematic potential and the effective length of the 3CCPE molecule (in the isotropic phase), were estimated. The splay and bend elastic constants were determined from the voltage dependence of the capacitance of a planar nematic cell.

The Elastic Constants of Nematic n-Hexylcyanobiphenyl Determined with the Capacitance Method

Abstract The splay (K 11) and bend (K 33) elastic constants of n-hexylcyanobiphenyl (C6H13-Ph-Ph-C=N) were determined from the voltage (U) dependence of the capacitance (C) of the planar nematic cell with a small molecular pretilt angle. The capacitance changes are due to distortion of the director n driven by the applied electric field. K 11 was obtained from the Freedericksz threshold voltage (Uth) and K 33 from the C(U) dependence above the threshold voltage by means of the method proposed by Gruler et al. and Uchida et al. The significance of the pretilt angle in the determination of K 33 is discussed.

Active Q-Switching of a Solid State Laser Using Nematic Liquid Crystal Modulators

Fast electro-optical switching with planar nematic liquid crystals in multipass and also Fabry-Pérot modulators has been realized. By using short driving pulses of 150V, we achieved microsecond switching times for nematic planar cells (NPC) and even sub micro-second switching in case of nematic Fabry-Pérot modulators (NFPM), which have been applied as active Q-switches in Nd:YAG solid state lasers. Four different resonator configurations have been investigated. Typically, giant pulses of 35–70 ns duration (FWHM) with energies between 10–25 mJ are emitted. A rather plain Q-switch resonator design consists of only two optical components: a cholesteric liquid crystal polarizing mirror and a reflecting nematic liquid crystal modulator.

Observation of self-starting phase-conjugate oscillation in a planar nematic liquid-crystal cell

We report the observation of self-starting optical phase-conjugate oscillation in a nematic liquid-crystal cell. A theoretical model based on the continuum theory of liquid crystals and optical theory of degenerate four-wave mixing is developed to explain the observed phenomena.

Freedericksz Transition in a Planar Nematic Cell and the Surfacelike Elastic Constant Problem

On the basis Of local stability analysis We have studied how the splay-bend term, that is known as the K13-term, influences Freedericksz transition in a nematic liquid crystal (NLC) sandwiched between two parallel plates. Two casea are considered: (a) the homeotropic anchoring is favored at both walls and magnetic field is assumed to be parallel to the confining surfaces; (b) the boundary conditions are planar and the magnetic field is normal to the walls. Under certain conditions it if found that the splay-bend term can change the order of the Freedericksz transition. In particular, we encounter the bistability induced by the term.

Optically aligned liquid crystals: physics and applications

We study the photo-induced alignment of a nematic liquid crystal in contact with an azo-dye-doped polyimide alignment layer. Using polarized light to illuminate a planar nematic cell with one active alignment layer results in the formation of twisted nematic domains. We measured the dynamics of domain growth resulting from light-induced changes in the surface anchoring. We interpret the results in terms of a simple model for the evalution of the orientational distribution function of the dye and the resulting anchoring potential. We discuss possible applications of optical alignment.

Propagation of Optical Fields in a Planar Liquid Crystal Waveguide

Abstract A model for the propagation of an optical field in a planar nematic cell is presented. The effects produced by the hydrodynamic couplings due to backflows and to an externally induced flow are explicitly taken into account. It is shown that when the resulting flow is a planar Couette flow, the liquid crystal distortion induced by the field gives rise to an index-gradient which generates a wave guiding effect that concentrates the ray trajectories and the electromagnetic energy density into the central part of the cell.

WAVEGUIDING EFFECT IN A CELL WITH A LIQUID CRYSTALLINE CORE

We present a model for the propagation of an optical field in a nematic planar cell where hydrodynamic flows are neglected. We show that the distortion induced by the field produces an index-gradient which produces a waveguide effect that concentrates the electromagnetic field and its energy density in the central part of the cell. This is shown by first performing a perturbative analysis of the reorientational dynamics and the coupled dynamics of the electromagnetic field in powers of the dielectric anisotropy of the nematic. Up to the first order, we calculate analytically the normal modes of the field, its dispersion relation and the electromagnetic energy density spatial distribution within the cell. The existence of this effect is also shown in a non-perturbative way by calculating the ray trajectories from the eikonal equation for a low intensity beam in the optical limit. We show that the trajectories indeed bend towards the middle part of the cell.

Anchoring Properties of Nematic Liquid Crystals Studied Using the Attenuated Total Reflection Method

Abstract By using the attenuated total reflection method associated with the excitation of surface plasmons, the tilt angle of the liquid crystal director and its gradient at the surface are measured in a planar nematic cell as a function of the applied voltage. The surface anchoring anisotropy δπ of the liquid crystal and the surface elastic constant ks , are found to be δπ = 0.288 erg/cm and ks , = 9·12 × 10-11 erg, respectively, when the boundary condition suggested by Barbero et al is used. The theoretical and experimental values obtained with this boundary condition and that of Mada are discussed. The results show that the boundary condition proposed by Barbero et al is in better agreement with the experiment.