摘要:

With progressing resolution of dot matrix displays, effects of inhomogeneous electric fields at the edges of structured electrodes become more and more important in the optical performance. We present a new method of polarizing microscopy for the study of the director field in two-dimensionally inhomogeneous nematic LC layers. Planar cells with electrode strips are investigated. The novel effects observed in these systems are two types of inversion walls, depending upon the preferred director orientation at the glass plates with respect to the electrodes.

ISSN:0267-8292    

DOI:10.1080/02678299308027729

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The occurrence of bend-stripes also involving twist distortion is investigated in hybrid aligned nematic layers, with stronger tilt-anchoring at the homeotropic substrate than at the planar one. The modulated structure is found to exist above the thickness thresholddafor the aperiodic bend-splay deformation. The periodicities of the director azimuth and polar angle are incommensurate along the normal to the cell plates. In principle, two independent periodic modes are possible, the first one stable, with thresholddp=da, the second one metastable, with thresholddp>da, according to the values of the torsional extrapolation lengths. Both transitions are continuous with respect to the in-plane wavenumber. The second mode can appear only if the difference between the torsional and the tilt extrapolation lengths is positive at the planar substrate. Moreover, both thresholds are independent of the saddle-splay elastic constant.

ISSN:0267-8292    

DOI:10.1080/02678299308027730

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The imaging process in an ideal optical system with an essentially inhomogeneous source-object, based on a liquid crystal, is discussed.

ISSN:0267-8292    

DOI:10.1080/02678299308027731

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We have measured the dielectric constants of 6CHBT. The results from studies of various alignments and thicknesses measured under different electric and magnetic fields are presented. We discuss how the dielectric properties depend on boundary conditions, sample thickness and the magnitudes of electric and magnetic fields. Experimental results and discussion in the terms of continuum theory make it possible to compute the diamagnetic anisotropy (Δχ), as well as the splay and bend elastic constants (K11,K33) of 6CHBT.

ISSN:0267-8292    

DOI:10.1080/02678299308027732

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Soft mode and Goldstone mode properties have been studied for a fluorinated substance. The dielectric spectra have been measured on the planar oriented samples, in the frequency range from 10 Hz to 10 MHz. The thickness of the sample was 10 μm and two kinds of capacitors were used: (i) a low resistance EHC cell and (ii) gold coated electrodes. A bias field up to 10kV cm−1has been applied to show the existence of both the soft mode and domain mode in the S*cphase belowTc. In the S*cphase a strong Goldstone mode has been observed with a low critical frequency (vGC= 15 Hz). The high frequency side at the Goldstone mode spectrum is accompanied by a shoulder which consists of the soft mode and domain mode as well. In the vicinity of the S*A-S*Ctransition the dielectric parameters of the soft mode obey a Curie-Weiss law. The slope ratio is equal to −1·62 for the inverse of dielectric increments (Δ∈−1) and −1·73 for the critical frequencies (vc) obtained by using gold electrodes. The respective values received for the EHC cell are −4·14 and −2·1. The dielectric parameters of the domain mode have been obtained versus temperature and bias field. We can qualitatively show that a high frequency molecular relaxation is present in the S*Aand S*Cphases as the high frequency limit of dielectric permittivity is distinctly higher than the refractive index squared.

ISSN:0267-8292    

DOI:10.1080/02678299308027733

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

We consider a cylindrical nematic liquid crystal waveguide with an infinite homogeneous isotropic cladding, and study the propagation of TM and TE modes. The dielectric tensor of the liquid crystal core which governs wave propagation is determined by the configuration of the nematic director. For TM modes, propagating optical fields alter the director configuration, and thus change the dielectric tensor. For both TM and TE modes, we consider the effects of the propagating fields on the order parameter tensor. We use an iterative numerical scheme to determine the propagation constant as a function of optical power. For the TM modes, the propagation constant increases continuously with the power. For the TE modes, an abrupt increase is found.

ISSN:0267-8292    

DOI:10.1080/02678299308027734

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A pressure system specially designed for the study of liquid crystal materials at high pressures up to 4kbar is presented. The pressure system is based on a hydrostatic screw injector and uses either oil or gas as the pressure transmitting medium. The type of measurements which can be performed with the instrument include polarized microscopy, optical spectroscopy, electrooptic and electrical measurements. The different measurements performed place specific constraints on the design of the apparatus and the pressure cells, and details are given. A preliminary study of the smectic A modification of the twist grain boundary phase (TGBA) at high pressures is presented. The pressure versus temperature phase diagram shows (i) a negative gradient of the TGBA/isotropic phase boundary line and (ii) that the TGBAphase does not exist at pressures above about 250 bar. Following Lubensky's analogy between the TGBAphase and type II superconductors, the disappearance of the TGBAphase at high pressure may imply that the Landau-Ginsburg parameterKdecreases with pressure.

ISSN:0267-8292    

DOI:10.1080/02678299308027735

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The particular phase in which a liquid crystal system will exist is that which has the lowest free energy at a certain temperature. The free energy may depend on variables such as temperature, pressure, chirality, etc. One way in which the stability of a particular thermodynamic phase, relative to its neighbours, would be manifest is in its temperature range. The effect of chirality, in particular, on the temperature range or stability of blue phases has been well-studied both experimentally and theoretically. To date these studies assume that chirality is the only parameter which will influence the existence of blue phases. However, blue phases with relatively low chirality and broad range, which should in principle only show very narrow blue phases, have been reported. This suggests that factors other than chirality are involved in blue phase stability. In this paper we investigate the phase stability of various blue phase mixtures containing equal amounts of a chiral dopant, via their blue phase temperature range. Correlation between blue phase stability and the elastic constantsk2andk22, molecular length of the nematic host, and the order parameter at the blue phase to cholesteric transition is discussed. We have confirmed that for our mixtures the total blue phase temperature range may be related equally to the elastic constantk22and the chirality. We also present the first data displaying an odd-even effect in blue phases. Finally, we have found an apparent correlation between the stability of the blue phases and the magnitude of the orientational order parameter of the cholesteric phase at the cholesteric to blue phase transition.

ISSN:0267-8292    

DOI:10.1080/02678299308027736

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

This is a scanned image of the original Editorial Board page(s) for this issue.

ISSN:0267-8292    

DOI:10.1080/02678299308027698

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor