B. Stebler

Submicrosecond electro‐optic switching in the liquid‐crystal smectic A phase: The soft‐mode ferroelectric effect

A new liquid‐crystal electro‐optic modulating device similar to the surface‐stabilized ferroelectric liquid‐crystal device is described. It uses the same kind of ferroelectric chiral smectics and the same geometry as that device (thin sample in the ‘‘bookshelf ’’ layer arrangement) but instead of using a tilted smectic phase like the C* phase, it utilizes the above‐lying, nonferroelectric A phase, taking advantage of the electroclinic effect. The achievable optical intensity modulation that can be detected through the full range of the A phase is considerably lower than for the surface‐stabilized device, but the response is much faster. Furthermore, the response is strictly linear with respect to the applied electric field. The device concept is thus appropriate for modulator rather than for display applications. We describe the underlying physics and present measurements of induced tilt angle, of light modulation depth, and of rise time.

Measurements of Hydrodynamic Parameters for Nematic 5CB

Abstract A nematic liquid crystal, pentyl-cyano-biphenyl (5CB) is studied in two different flow situations: Poiseuille flow and torsional shear flow, in both cases with and without the simultaneous application of an electric field. The Poiseuille flow set-up gives an accurate determination of the Miesowicz viscosity ηc and a less precise estimation of ηb. With ηc known it is possible to separately determine the elastic constants K 11 and K 33 and the viscous coefficients α2, α3, and thereby γ1, γ2 from the torsional shear flow experiment. Using the data we have also been able to calculate the actual director profile in the liquid crystal as a function of applied shear and electric field. Curves are given of K 11, K 33, K 11/K 33, α2, α3, γ1, γ2, γ1/γ2, ηb and ηc for 5CB as a function of temperature through the whole nematic range from 22.0 °C to 35.1 °C. After the completion of our manuscript we have noticed the recent article by J. Wahl, Z. Natur-forsch. 34a, 818 (1979). Wahl discusses shear flow in the ...

The soft-mode ferroelectric effect

Abstract In this paper a presentation is given of some of the basic physics of the soft-mode ferroelectric effect, along with its experimental background. Further, possible applications in electrooptic devices are elucidated. A fast (sub-microsecond) electrooptic switching can be achieved in the A* phase, and in similar orthogonal smectic phases made up of chiral molecules. Instead of using the phase variable Φ, it uses the tilt angle θ, which in principle is a “hard” variable, but is expected to soften on approaching the transition to a lower-lying adjacent tilted smectic phase. However, the switching is efficient in the whole range of the orthogonal phase (in fact, less efficient near the tilting transition) and is observed whether an adjacent tilted phase is present or not. As compared to the surface-stabilized (SSFLC) electrooptic mode, this soft mode (SMFLC) is based on the electro clinic, effect of essentially ferroelectric nature very closely related to the presently more investigated ferroelectric...

The molecular aspect of the double absorption peak in the dielectric spectrum of the antiferroelectric liquid crystal phase

Abstract The dielectric spectrum of the antiferroelectric smectic C∗ phase exhibits a low and a high frequency absorption peak (Pl, Ph) which have been studied as a function of temperature and bias electric field. Measurements from 10 Hz to 10 MHz were carried out with smectic layers parallel and quasi-perpendicular to the cell plates for the multicomponent mixture CS-4000 (Chisso). In addition to the orthogonal smectic A∗ phase, this material has four tilted phases, three narrow phases with a dielectric behaviour permitting us to classify two of them as C∗α (82·80° to 81·91°C) and C∗γ (80·10° to 79.17°C), and one broad antiferroelectric phase (79.17° to −10°C). On applying an increasing bias field, Δ∊ for both processes first increases by about a factor of two, then exhibits a maximum at a threshold field E c corresponding to the antiferroelectric-ferroelectric transition at which it decreases by almost one order of magnitude. In fact, at E c the PH peak vanishes and the PL peak shows up at a frequency s...

Device physics of the soft‐mode electro‐optic effect

The soft‐mode ferroelectric liquid‐crystal (SMFLC) effects utilize the electroclinic response of chiral orthogonal smectic phases (A*,B*,E*) in bookshelf geometry. The SMFLC cell may be considered a retardation plate with a field controllable optic axis, with a submicrosecond response time, and a wide continuous dynamic range. A great variety of electro‐optic components and devices can be constructed based on the SMFLC effect, many of them adding novel possibilities and challenges for optical design. We discuss in the present paper different combinations of SMFLC cells with retarders and polarizers, giving optical components capable of light and color modulation. Particularly, multiple electroclinic cells in a special constellation have the potentiality to give nearly achromatic full‐modulation light valves, as well as electrically controlled high‐speed color filters, which can be used for simple and compact TV cameras. Reflective single‐cell electroclinic devices are suitable for integration with semicon...

Polarization and Viscosity Measurements in a Ferroelectric Liquid Crystal by the Field Reversal Method

Abstract With the aim to develop a standard method for measurements of the polarization of ferroelectric chiral smectic C liquid crystalline materials, we have studied thin, well-aligned samples of MBRA-8 using the field-reversal method. The method also provides an estimate for the rotational viscosity and for the response times.

INVESTIGATIONS OF THE STRUCTURE OF A CHOLESTERIC PHASE WITH A TEMPERATURE INDUCED HELIX INVERSION AND OF THE SUCCEEDING SC-ASTERISK PHASE IN THIN LIQUID-CRYSTAL CELLS

Abstract Investigations of 4-[(S, S)-2, 3 epoxyhexyloxy]-phenyl-4-(decyloxy)-benzoate by polarizing microscopy, the Cano-Grandjean method, optical rotation dispersion and UV-VIS spectroscopy reveal a cholesteric phase with temperature induced reversal of the helical twist. Switching time experiments in the Sc* phase show that the intrinsic helix can be unwound reversibly and irreveribly by application of electric fields of different strengths.

A Family of Ferroelectric Liquid Crystals with Very High Spontaneous Polarization

Abstract A new family of ferroelectric liquid crystals is presented. In particular, some mixtures of compounds from this family show an interesting combination of high spontaneous polarization, good alignment properties and room temperature smectic C* phase. Measurements are presented for transition enthalpies, refractive indices, X-ray tilt angle, electro-optic tilt angle and spontaneous polarization. The electro-optic response is described as measured in 2 micron thick shear-aligned cells.

Sign Reversal of the Linear Electro-Optic Effect in the Chiral Nematic Phase

The chiral nematic phase aligned with its helical axis along one preferred direction, being parallel to the confining substrates, exhibits a linear electro‐optic effect when an electric field is applied normally to the substrates. The effect is considered to be of flexoelectric origin. A sign reversal of the effect is found when the helix of the chiral nematic phase changes its handedness, thus revealing the relationship between the sign of the linear electro‐optic effect and the helix handedness. Here, we present and discuss a simple model of this relationship.

Flow Properties of Nematic 8CB: An Example of Diverging and Vanishing α3

Abstract The temperature dependence of the Leslie viscosities α2 and α3 has been measured in the nematic liquid crystal 8CB (octyl-cyano-biphenyl), using a trosional shear flow apparatus. In most nematics studied so far α3 is found to be everywhere negative, but if the nematic has an adjacent semectic. A phase at lower temperatures, it can be expected that α3 changes sign and becomes positive in the vicinity of TNA as a precursory phenomenon of the transition. In fact this influence is dominating the major part of the nematic phase. We have found that for 8CB α > 0 in the whole nematic range (33.5°C-40.1°C), except for a small temperature interval of one degree just below the clearing point. This leads to intrinsic flow instabilities, but the flow can be stabilized by applied electric fields. The results further contain the first measurement of a diverging α3 in a flow experiment.