Angular dependent optical transmission of twisted nematic liquid-crystal layers

The angular-dependent light transmission of a twisted nematic layer between crossed polarizers in an electric or magnetic field depends mainly on the induced orientation of the director in the middle part of the layer. Measurements and computations compare favourably.     

Deformation of nematic layers in crossed electric and magnetic fields

A parallel nematic layer will show a Fréedericksz-transition in an electric field when the voltage exceeds a threshold value U_c. If a magnetic field is applied parallel to the layer but perpendicular to the molecular orientation the threshold voltage U_c increases with increasing magnetic field field H above a threshold field H_c2. From a plot of U_c vs H all three elastic constants can be obtained. Theoretical results are compared with data of HBAB.     

Rotational reorientation of the director in twisted nematic liquid-crystal cells

The orientational relaxation of the director to its equilibrium orientation under electric, elastic, and viscous torques arising in twisted nematic liquid-crystal cells is investigated. It is shown that the relaxation time of the director depends strongly on the external electric field strength and weakly on the energy of anchoring of liquid-crystal molecules to the surfaces of the cell. The relaxation time of the director anomalously increases in electric fields close to the Fréedericksz threshold. It is established that, at specific strengths of the external electric field, the relaxation can occur in the form of traveling waves propagating from one edge to the other edge of the twisted nematic liquid-crystal cell. The calculations of the relaxation processes in the vicinity of the nematic-smectic A phase transition temperature demonstrate that the distortion of the director field is uniform over the entire cross section of the liquid-crystal cell and does not depend on the strength of anchoring of the liquid-crystal molecules to the surfaces of the cell.     

Coexisting nematic and smectic-A phases in a twisted liquid-crystal cell

Twisted homogeneously planar-aligned nematic liquid-crystal cells are cooled into the smectic- A phase. The expected defective structure does not form. Instead the cells still show good optical-guiding characteristics. Exploration of the cells using a half-leaky guided-mode arrangement reveals that the liquid-crystal phase separates into three or more regions. Adjacent to both the upper and lower boundaries is a region of highly twisted nematic liquid crystal. In the center of the cell is one or more homogeneous smectic- A regions with smectic layers normal to the cell surfaces, separated by twisted nematic. As the cell is cooled so the smectic- A regions grow in thickness with the nematic regions progressively thinning but with increased twist gradient. A theoretical model of these novel results is presented.     

Entanglement-free determination of pretilt angles of twisted nematic liquid-crystal cells by phase measurement

The pretilt angle of a twisted nematic(TN) liquid crystal display has a strong effect on viewing angles and response times of the display as well as shifting the light-leaking disclination lines into the black-matrix area within pixels of a thin-film-transistor driven TN panel for a high contrast ratio. In this Letter, we develop an optical method to determine the TN pretilt angle based on heterodyne measurements of the phase versus the incident angle of a thin TN cell subjected to out-of-plane rotations. We believe that our method of measuring phases has advantages of simpler setup and calculations, better stability, and less sensitive to ambient electromagnetic interference than the commonly used method of measuring intensities.     

Photon echoes produced by switching electric fields

We demonstrate photon echoes in Eu3+:Y2SiO5 by controlling the inhomogeneous broadening of the Eu3+ 7F0<-->5D0 optical transition. This transition has a linear Stark shift, and we induce inhomogeneous broadening by applying an external electric field gradient. After optical excitation, reversing the polarity of the field rephases the ensemble, resulting in a photon echo. This is the first demonstration of such a photon echo, and its application as a quantum memory is discussed.     

Surface-anchoring properties related to the distribution of polyimide chains in a twisted nematic liquid-crystal cell

On the basis of a general Rapini and Papoular equation and the unified surface-anchoring energy, surface-anchoring strength is newly defined theoretically as a function of the azimuthal deviation angle of the surface liquid-crystal directors and the statistical distribution of polyimide chains in a typical twisted nematic liquid-crystal cell. Then these are determined experimentally by the surface second-harmonic generation method, which is nonlinear optics, and the optical phase retardation method. We assume that the anisotropy distribution of polyimide chains induced by the rubbing strength is dominated by a Gaussian distribution around the rubbing direction.     

Dual-mode operation of a twisted nematic liquid crystal cell by switching between dynamic and memory modes

We propose a twisted nematic liquid crystal device that can be operated in dynamic or memory mode, based on the information content to be displayed at that time. +90°-twisted and -90°-twisted states are used as two stable states for operation in the memory mode. A vertical electric field is applied to realize gray levels for operation in the dynamic mode. The proposed device has a memory retention time of over a month for the memory mode and a response time of 12 ms for the dynamic mode. Contrast ratios of over 500∶1 can be obtained in both the dynamic and memory modes.     

Dynamic response of twisted nematic liquid crystal cells to transiently pulsed fields

The transient optical response of a TN cell to single field pulses is experimentally investigated. The optical threshold for pulses is investigated as a function of pulse width and cell parameters d and θ. The subsequent relaxation of the cell below optical threshold is inferred by investigation of the sensitivity of the cell to a second delayed pulse. A method of calculating tilt angles from capacitance measurements of LC cells is also described.     

Numerical study of deformations induced by ac electric field in insulating flexoelectric nematic layers

The influence of the frequency f of applied ac electric field on the time dependence of electric field induced deformations of homeotropic nematic layers is studied numerically. Three kinds of nematic liquid crystals were considered:

Coherent information storage with photon echoes produced by switching electric fields

We demonstrate modified photon echoes in Eu³⁺:Y₂SiO₅ by controlling the inhomogeneous broadening of the optical transition in Eu³⁺. These modified photon echoes are shown to be capable of storing phase and amplitude modulation.     

Instabilities in nematic elastomers in external electric and magnetic fields

In this paper we study the macroscopic behavior of nematic side-chain liquid single crystal elastomers exposed to an external electric or magnetic field. For this purpose we use the framework of a continuum model. The geometries investigated comprise the bend and the twist geometry known from the classical Frederiks transition in low molecular weight liquid crystals. For the bend geometry we find a laterally homogeneous and a two-dimensional undulatory instability, which may compete at onset. In the case of the twist geometry three instabilities can occur at onset, two of which are two dimensional and clearly show undulations. As a major result we propose how the values of the twist coefficient K(2) and the values of the material parameters D(1) and D(2) connected to relative rotations between the director field and the polymer network can be determined from experimental observations. In addition, we explain why a twist experiment is probably the most suitable set-up in order to measure the material parameter D(1).     

The effect of applied electric fields on chemisorption

A simple model is used to study the effects of applied electric fields on the neutral chemisorption bond at a metal-vacuum interface. The adsorbate-substrate complex is represented by a model diatomic molecule in an electric field which varies spatially due to substrate screening. The field induced changes in binding energy, equilibrium position and vibrational frequency of the adsorbate are calculated. The latter changes are due to the dependence of the effective field coefficients on the adsorbate distance from the surface. Measurable effects, which reflect the bond characteristics, are predicted for experimentally attainable fields.     

Equilibrium structures of planar nematic and cholesteric films in electric fields

Liquid crystal layers subjected to an electric or a magnetic field can have several types of instabilities. This paper reviews recent theoretical studies concerning equilibrium structures of planar layers.

Using the Oseen-Frank elasticity theory, the Freedericksz transition and the transition to static periodic domains of planar nematic and cholesteric films are reconsidered. A perturbation treatment of nonlinear torque balance equations for the director is suitable to derive amplitude equations for the film distortions under the action of a field and to predict the topological features of phase diagrams for equilibrium states. The competition between the Freedericksz effect and the formation of periodic distortions is studied varying material and geometrical parameters. Some results are useful to optimize cholesteric mixtures for application in display devices in such a manner, that the occurrence of periodic domains is avoided.     

Semisoft nematic elastomers and nematics in crossed electric and magnetic fields

Nematic elastomers with a locked-in anisotropy direction exhibit semisoft elastic response characterized by a plateau in the stress-strain curve in which stress does not change with strain. We calculate the global phase diagram for a minimal model, which is equivalent to one describing a nematic in crossed electric and magnetic fields, and show that semisoft behavior is associated with a broken symmetry biaxial phase and that it persists well into the supercritical regime. We also consider generalizations beyond the minimal model and find similar results.     

Diffraction behavior of an azo-dye-doped nematic liquid crystal without applied electric field

We investigate the dependence of the two-beam coupling diffraction efficiency on the experimental parameters, and hence achieve the optimum experimental conditions for holographic recording in planar Methyl Red-doped nematic liquid crystal film without external electric field. Under the optimum experimental conditions, the diffraction efficiency of a holographic grating is improved to 21.4%, and holographic image with long-term stability is stored in the film. These results provide an experimental basis for holographic data storage in this liquid crystal film.     

扭曲向列相液晶电光效应的研究

分析了扭曲向列相液晶的电光效应.采用溴钨灯作为光源,运用光栅单色仪选择380~1 100 nm波段的光进行照射,测量在0~8 V电压条件下扭曲向列相液晶对于不同波长入射光的透射率.     

Dielectric properties of an ytterbium-based nematic liquid-crystal complex

The dielectric properties of an yttrium-based paramagnetic thermotropic nematic liquid-crystal complex have been studied. The conditions for the measurement of the magnitude and sign of the dielectric anisotropy of the mesophase have been determined. The relaxation of the components of the relative permittivity of a macroscopically homogeneously oriented sample has been observed. A high polarity of the metallomesogenic complex has been revealed.     

Giant momentary readout produced by switching electric fields during two-wave mixing in sillenites

We show theoretically and experimentally that switching an applied square-wave field produces strong and short pulses of the outgoing signal during two-wave mixing in sillenite crystals. These pulses originate from the strong effect of the field on the optical eigenmodes and can be used in new optical schemes based on time-separated recording and readout processes.