K. Skarp

Dielectric studies of the soft mode and Goldstone mode in ferroelectric liquid crystals

Abstract Different contributions to the dielectric permittivity in ferroelectric liquid crystals are discussed, with emphasis on the soft mode and the Goldstone mode and their location in the dielectric spectrum. Experimentally, the complex dielectric permittivity has been studied as a function of temperature and frequency in the range 5 Hz - 13 MHz for three different ferroelectric liquid crystal materials. The main problems encountered in dielectric measurements at low and high frequency are discussed in some detail. The soft mode dielectric behaviour has been studied as a function of temperature, frequency and bias electric field. The applicability of the Curie-Weiss law for the soft mode dielectric contribution in the A* phase was analyzed. In the C* phase the temperature dependence of the dielectric contribution of the Goldstone mode has been measured. By applying a bias electric field, we have been able to study the soft mode dielectric behaviour also deep into the C* phase. In the A* and C* phases ...

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...

Ferroelectric Liquid Crystals. Material Properties and Applications

Abstract The last decade has seen a tremendous growth in the research and development activities in the area of ferroelectric liquid crystals. The field was opened up by the work of Meyer in 1974, and has now grown rapidly into one of the major research areas in liquid crystal science. The emphasis of the earlier work was on basic material characterization and phenomenology. Only very few liquid crystalline compounds exhibiting a ferroelectric phase were known at that time, and furthermore these Schiff-base organic compounds were chemically and thermally unstable and presented their ferroelectricity only at elevated temperatures. After the work by Clark and Lagerwall in 1980 on a fast electro-optic effect in a surface-stabilized ferroelectric liquid crystal (SSFLC) structure, both basic and applied research have accelerated, and today a great number of papers deal with development and characterization of ferroelectric compounds and mixtures and various aspects of device physics and manufacturing. It is of...

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...

Material Properties of Ferroelectric Liquid Crystals and Their Relevance for Applications and Devices

Abstract Recent developments in understanding structural and material properties of ferroelectric liquid crystals are reviewed with emphasis on their relevance for electro-optic effects and device applications. In only a few years the material situation has changed from one characterized by an almost complete lack of technically useful materials to one, not yet adapted to technical needs, but beginning to offer interesting, commercially available ferroelectric mixtures. The present device limitations can, however, be tracked down, not so much to the principal non-availability of materials as to the lack of understanding of which material properties are desirable and needed, in particular to permit simple and efficient matrix addressing. Thus we have centered our discussion to properties of prime importance for device dynamics in this review, which also contains a general discussion of material properties based on the molecular structure, as well as an assessment of the present industrial development.

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.

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.

Properties of some broad band chiral smectic C materials

Abstract A comparative study of three related families of ferroelectric liquid crystals and some distinctly different analogues has been made with the aim of clarifying systematic trends in smectic C phase stability and the appearance of macroscopic polarization. The substances are interesting not only because the C phases are broad and occur around ambient temperature, but also because the more highly ordered tilted phases (smectic I, J, …) are suppressed. Their collective behaviour also sheds light on the molecular origin of the spontaneous polarization.