J. S. Patel

A reliable method of alignment for smectic liquid crystals

An alignment method for smectic liquid crystals is described. Alignment is obtained by the deposition of a certain class of polymer on a substrate followed by unidirectional rubbing. Only linear polymers capable of being elongated in bulk samples are found to produce alignment. The quality of alignment is found to be exceptionally good and this method can be used to produce alignment over large areas.

Ferroelectric liquid crystal devices.

Current developments in ferroelectric liquid-crystal device physics are reviewed and experimental work in shutter and photoaddressed spatial light modulators is presented.

Electrically tunable and polarization insensitive Fabry-Perot étalon with a liquid-crystal film

A polarization insensitive, electrically tunable Fabry–Perot structure is demonstrated. The polarization insensitivity is realized by using a twisted structure of a nematic liquid crystal such that the molecules at one surface are orthogonal to those at the other surface. In the high‐field regime, this configuration can be thought of as a structure composed of two orthogonal, birefringent slabs with the electrically controllable thickness. At low voltages the device is polarization sensitive but becomes polarization insensitive at relatively higher voltages. In the polarization insensitive region, the linewidth of the transmission peak is of the order of 0.5 nm and the tuning range is about 15 nm.

Fast linear electro‐optic effect based on cholesteric liquid crystals

Flexoelectric effect in cholesteric liquid crystals can be used to produce optical switching on a microsecond time scale. In a simple geometry, this effect produces a light modulation the amplitude of which is linear in field. The switching time from one optical state to the other is predicted to be independent of the magnitude of the electric field. We confirm this prediction experimentally in a room‐temperature cholesteric liquid crystal.

Contributions of the molecular species to the magnitude of the spontaneous polarization in ferroelectric liquid crystals

Abstract In certain ferroelectric liquid crystals that contain the biphenylyl moiety, the spontaneous polarization undergoes a change in direction relative to the optical axis of the system as a function of temperature. Consequently, at a characteristic temperature for a particular derivative, the polarization vanishes. A model based on the presence of at least two species, interconvertible via an activation barrier is proposed to explain this extraordinary behaviour of the polarization. It is suggested that these species are most likely identified with the rotational conformers of these materials while they are in their liquid-crystalline states.

Nonlinear behavior of the field‐induced molecular tilt near the smectic A–C* transition

A deviation from the linear relationship between induced molecular tilt and applied electric field has been observed in the vicinity of the transition from the smectic A to the chiral smectic C* phase. Near the transition temperature Tc, the induced tilt angle θ initially grows linearly with the strength of an applied electric field E, and clearly exhibits a crossover to Eα (1/3 <α<1) above a certain field strength. As temperature increases, however, the exponent α approaches 1 in a wide range of the electric field we studied. A simple mean field model is presented to explain the observed nonlinear behavior.

Effect of flexoelectric coupling on helix distortions in cholesteric liquid crystals

The static and dynamic properties of the flexoelectric effect in cholesteric liquid crystals are presented. The application of an external electric field, below the critical strength for helix unwinding, perpendicular to the cholesteric helix, results in a linear electro‐optic effect. The experimental results are in good agreement with theoretical predictions made under the assumption of the uniform distortion of the helix due to the flexoelectric coupling with the field. The temperature dependence of the dynamic response to the field is found to be the Arrhenius type. It is suggested that the surface boundary conditions play a role in the properties of the flexoelectric effect.

The ferroelectric and liquid-crystalline properties of some chiral alkyl 4-n-alkanoyloxybiphenyl-4′-carboxylates

A variety of chiral esters of a range 4-n-alkanoyloxybiphenyl-4′-carboxylic acids have been prepared in order to produce three closely related homologous series of liquid-crystalline materials. The majority of the materials synthesized were found to exhibit smectic liquid-crystalline phases close to room temperature. Many of these phases were also found to be ferroelectric. Moreover, the ferroelectric behaviour observed was found to be very unusual, and in the case of some materials a peculiar change in sign of the spontaneous polarization direction with temperature was detected.

Temperature and field dependence of the switching behavior of induced molecular tilt near the smectic A–C* transition

A fast electro‐optic switching has been achieved in the smectic A phase of a electroclinic (EC) mixture, based on large EC coupling to the applied electric field. Near the transition from the smectic A to the chiral smectic C* phase, we have observed for the first time strong field dependence of the switching time, resulting from nonlinear behavior of the induced molecular tilt. As temperature increases, however, the switching time remains essentially constant. In addition, there is no temperature dependence at sufficiently high fields. A simple‐mean field description is presented to account for the experimental results.

Epitaxial growth of aligned polydiacetylene films on anisotropic orienting polymers

We report an epitaxial method for orienting thin films of polydiacetylenes on ordinary alignment polymers that have been stretched by rubbing. By selectively removing the alignment polymer using standard photolithographic techniques, we show that it is possible to obtain well‐oriented polydiacetylene films in desired regions. Typical values for the birefringence are 0.14±0.01 for epitaxially grown films, enough to allow the formation of gratings and channel waveguides on patterned surfaces.