S. T. Lagerwall

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...

Antiferroelectric liquid crystals with 45° tilt - a new class of promising electro-optic materials

Abstract Antiferroelectric liquid crystals with a tilt angle of 45 degrees have very interesting optical properties, which seem to have been overlooked so far - perhaps because such materials have hardly been available. We have prepared a four-component mixture of partially fluorinated compounds with a SmCa* phase in the interval between 27.4°C and 121.6°C, in which the tilt angle θ saturates at 45 degrees for T≤80°C, and we investigate the optical properties, theoretically and experimentally. One of the surprising features of 45 degree materials is that they permit a remarkably high contrast by virtue of an excellent dark-state, in spite of the fact that AFLC materials are notoriously difficult to align. This is because a 45° AFLC turns out to be (negatively) uniaxial instead of biaxial. We describe these properties and propose a number of potentially interesting new applications, including a polarizer-free display mode and a three-level “phase-only” modulator.

Elastic and flexoelectric properties of chiral smectic-C phase and symmetry considerations on ferroelectric liquid-crystal cells

Abstract The elastic energy expression of de Gennes for the non-chiral and chiral smectic-C phases is reformulated, and it is shown that the maximum size of a uniformly oriented sample is limited, not only by the chiral helix but also by a spontaneous bend of the smectic layers. A description of the flexoelectric effects is given: 9 different vector fields are involved, and they all remain in non-chiral smectic-C phase. These vector fields are connected to the divergence terms in the elastic free energy. The complexity of the boundary conditions for ferroelectric liquid crystals is discussed, and the concepts ‘bookshelf geometry’ and ‘φ-can’ are introduced. It is shown how the φ-can may be used to describe various monostable and bistable configurations of smectic-C cells. The symmetry of the cell can determine the configuration. Some general rules concerning the optical behaviour of smectic-C cells are given.

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...

Linear electro‐optic effect based on flexoelectricity in a cholesteric with sign change of dielectric anisotropy

The linear electro‐optic effect in short‐pitch cholesterics is based on the linear coupling of the medium with an applied electric field. It has a number of remarkable properties. The electric field causes the optic axis to tilt in a plane parallel to the surfaces of the cell glass plates, giving the same symmetry as the electro‐optic effects in the smectic C* phase (surface stabilized ferroelectric liquid crystals and deformed helix mode) and the smectic A* phase (soft mode/electroclinic effect). For shutters and displays this guarantees a very wide viewing angle. The induced tilt is a linear function of the applied field, at least for small fields, which gives a well‐controlled continuous grey scale. Furthermore, it is practically independent of temperature. Response times of the order of 100 μs are easily achievable. The most interesting development in this effect would be to extend the linear regime to much larger tilt angles, in particular to 22.5°, where light could be modulated from 100% to zero tr...

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.

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.