Jürgen Nehring

A new, highly multiplexable liquid crystal display

A new, highly multiplexable liquid crystal display is described, which has a superior image quality than a twisted nematic display multiplexed at the same high level. The display cell consists of a chiral‐doped nematic layer with tilted boundaries and a twist angle of ∼270°. It operates in a birefringent optical mode between two ‘‘nonconventionally’’ oriented polarizers. Performance characteristics presented for a 120×240 dot matrix panel multiplexed at a 1/120 duty cycle include driving voltages compatible with complementary‐metal‐oxide‐semiconductor technology, 300‐ms response times, a contrast ratio of 10:1 at normal incidence, and ≥4:1 inside a viewing cone of 45° from the vertical.

Calculation of the Elastic Constants of Nematic Liquid Crystals

The ratio of the elastic constants of nematic liquid crystals is calculated using a simple inner field model. The result obtained is k11′: k22 : k33′: k24 : k13(2)=5:11:5:‐9:‐6. The first three constants, which are the constants for splay, twist, and bend, respectively, are found to be positive, in contrast with the result of an earlier calculation which was based on the same model but neglected second order splay‐bend contributions to the deformation energy.

On the schlieren texture in nematic and smectic liquid crystals

Observations on schlieren textures of nematic and smectic-C liquid crystals in thin layers are reported. The theory of schlieren textures is discussed and forces between singularities are considered. A singularity can be characterized by a positive or negative number which is proportional to the number of brushes it shows between crossed polarizers. In smectic-C, only singularities with four brushes are observed, nematics may in addition show singularities with two brushes. The latter singularities lead to conclusions on the molecular order and alignment: without the formation of inversion walls they can only occur in the absence of polarity and when the molecules align parallel to the surface. Singularities of opposite sign attract each other, those of equal sign repel each other. The sum of the characteristic numbers of the singularities in an extended layer tends to be zero. Under special assumptions about the elastic constants, it can be shown that the forces between singularities are additive and inversely proportional to the distances and that, for an isolated pair of singularities with the same absolute characteristic number but with opposite sign, the curves of equal molecular alignment are circles through the singularities.

Turn‐On Time Reduction and Contrast Enhancement in Matrix‐Addressed Liquid‐Crystal Light Valves

A new method of response time reduction and contrast enhancement in matrix‐addressed liquid‐crystal light‐valve arrays is described based on the effect that the threshold for hydrodynamic instability in certain nematic liquid crystals is increased by a superimposed electric field of sufficiently high frequency. The effect is discussed in terms of the Helfrich model.

High‐pretilt polyphenylene layers for liquid‐crystal displays

High‐pretilt angles (of about 20°), as required for highly multiplexable liquid‐crystal displays based on the ‘‘supertwisted birefringence effect,’’ can be obtained by rubbing polyphenylene layers. The aligning properties of such layers depend somewhat on the type of the precursor polymer from which the polyphenylene is made and on the substrate on which it is formed. A 120×240 matrix display prototype with 270° twist and rubbed polyphenylene orientation layers showed a uniform and stable surface alignment over the whole display area.

Anisotropies of the 19F Chemical Shifts in Fluorobenzene Compounds from NMR in Liquid Crystals

Fluorine‐19 shift anisotropies, which have been determined from NMR spectra of nematic solutions, are reported for 1,2‐, 1,3‐, 1,4‐difluorobenzene, 1,3,5‐trifluorobenzene, 1,2,4,5‐tetrafluorobenzene, and hexafluorobenzene. Combination of the anisotropies of different molecules leads to the following results for the principal values of the anisotropic part σa = σ − 13Trσ of the screening tensor σ for 19F nuclei without fluorine neighbors in ortho or para position: σ11a = −94 ± 12, σ22a = 27 ± 9, σ33a = 67 ± 3 in parts per million (2 axis in direction of C–F bond, 3 axis perpendicular to molecular plane). Fluorine substitution in ortho position changes the shielding tensor by Δσ11 = (+6 ± 3) × 10−5, Δσ22 = (−3 ± 3) × 10−5, Δσ33 = (+4 ± 1) × 10−5. There are difficulties in explaining the experimental results by the theory of Karplus and Das since a very high ionicity in the C–F bond has to be assumed that increases by ortho fluorine substitution.

Light propagation and reflection by absorbing cholesteric layers

The generalized Mauguin–Oseen–de Vries theory which includes absorption for light propagating along the helical axis of a cholesteric liquid crystal is investigated in detail. Expressions and examples are given that show the effect of absorption on the eigenmodes and selective reflection band. As for a nonabsorbing cholesteric, the eigenmodes in general are elliptically polarized waves when expressed in a coordinate system that rotates in step with the cholesteric structure, with the ellipses having fixed orientations relative to the local optic axis. Absorption is shown to change the ellipticity and, in some cases, the sense of the modes and to reorient the major axes of the ellipses. A Mauguin range, where nearly linearly polarized modes rotate in step with the cholesteric helix, and a selective reflection band are shown to be present even if the birefringence of the cholesteric liquid crystal vanishes. Absorption also introduces a coupling between the eigenmodes which is noticed when the energy propaga...

The formation of threads in the dynamic scattering mode of nematic liquid crystals

Abstract The formation of nematic threads in the dynamic scattering mode, which increase the scattered light intensity and the relaxation time, is reported. When the threaded dynamic scattering mode relaxes, well defined threaded textures containing different types of nematic threads are observed.

On the threshold condition for nematic layers in magnetic fields

Abstract General equations are given for the magnetic threshold field of nematic layers with originally one-dimensional director fields. These equations hold for all possible geometries and for materials having either positive or negative diamagnetic anisotropies.