Photonics of liquid-crystal structures: A review

The original results of studies of the electro-optical and laser effects which have been performed at the Laboratory of Liquid Crystals of the Institute of Crystallography, Russian Academy of Sciences, over the last few years are reviewed. Cholesteric liquid crystals as vivid representatives of photonic structures and their behavior in an electric field are considered in detail. The formation of higher harmonics in the periodic distribution of the director field in a helical liquid crystal structure and, correspondingly, the new (anharmonic) mode of electro-optical effects are discussed. Another group of studies is devoted to bistable light switching by an electric field in chiral nematics. Polarization diffraction gratings controlled by an electric field are also considered. The results of studies devoted to microlasers on various photonic structures with cholesteric and nematic liquid crystals are considered in detail. Particular attention is given to the new regime: leaky-mode lasing. Designs of liquid crystal light amplifiers and their polarization, field, and spectral characteristics are considered in the last section.     

Dynamic photorefractivity guided by single-pulse voltage

The dynamic photorefractivity in a cell with photoconducting orienting layers, filled with a nematic liquid crystal (LC) 6CHBT and a mixture of anthraquinone dyes AD1 and AD2, has been investigated. The single-pulse mode, in which the polarity and amplitude of a dc electric field applied to an LC cell are switched for a fixed time interval, has been used. The scheme of dynamic self-diffraction of low-power laser beams was used in the experiment. The dependences of the width and intensity of diffraction pulses on the bias and switching voltages have been investigated. It is established that the width and intensity of the diffraction pulse arising after initial voltage recovery depends also on the switching pulse width. At the optimal width of the control pulse, the diffraction efficiency increases by two orders of magnitude.     

Simulation of electrooptical effects and dynamics of ferroelectric liquid crystals

The behavior of ferroelectric liquid crystals in an external electric field is simulated numerically. The equations that describe the dynamics of the director of a liquid crystal are derived within the continuum theory of elasticity with due regard for compressibility of smectic layers, finite anchoring energy, and dielectric properties of orienting coatings and external elements of a real electric circuit. These equations make the basis for simulation of the electrooptics of ferroelectric liquid crystals. The specific features and mechanisms of the surface-stabilized bistability and hysteresis-free electrooptical switching (the V-shape effect) are discussed.     

Electro-Optical Properties of Electric Field-Induced-Polymer Stabilization at Blue Phase of Chiral Nematic Liquid Crystal

Polymer stabilization of the blue phase induced a wide temperature range of more than 60 K, but the high driving voltage and the large switching hysteresis compared to the low molecular weight blue phase of the chiral nematic liquid crystal. In this study, we newly proposed electric field-induced-polymer stabilization of blue phase (EFIPSBP) which the electric field was applied to the blue phase during polymer stabilization and investigated the driving voltage and the switching hysteresis using various voltage-applied EFIPSBP cells. We confirmed that the driving voltage and the switching hysteresis of the EFIPSBP cell were decreased compared to those of conventional polymer stabilized BP at 8OCB chiral nematic liquid crystal mixture. This could be suggested as a potential method for improving the driving voltage and the switching hysteresis of conventional polymer stabilized BP.     

The Behaviour of Thin Samples of Ferroelectric Liquid Crystals

On a ferroelectric liquid crystal DOBAMBC transitions between helical and twisted structures are observed if changing the temperature. The twisted structure appears when the helical pitch becomes comparable to the sample thickness, From the switching process of the twisted DOBAMBC samples in an electric field the polarity of surface layers is determined. Low signal di- electric permittivity shows an increase below the transition to the ferroelectric phase, which is a consequence of the twisted structure deformation in the electric field.     

Scattering displays from 'banana-smectics'

Smectic liquid crystals of bent-shape molecules (banana-smectics) opened a whole new field in liquid crystals research, pioneered by groups in Tokyo [1], Boulder [2], Berlin [3], Halle [4], Budapest [5] and several other places. [6] The first scattering-type displays of these materials were presented recently. [7] It was observed that ‘racemic’ state of single-tilted polar smectic phase is scattering in the antiferroelectric OFF state and is optically clear in the ferroelectric ON state induced by electric fields. The chiral structure works in opposite way, i.e. it is transparent at zero fields and is scattering in the field ON state. In some cases these two structures may be reversibly interchanged implying their use in devices that consume energy only during switching from one stable state to the other. This article is a summary of a cooperative work with Prof. L-C. Chien, Prof. G. Heppke, Dr D. Krüerke, Dr G. Nair, Dr H. Sawade and S. Rauch.     

OPTICAL TRANSMISSION AND POLARIZATION SWITCHING PROPERTIES OF A FERROELECTRIC LIQUID CRYSTAL CELL WITH A STABLE TWISTED STATE

Abstract

Ferroelectric liquid crystal cells with a stable twisted state were prepared. Dynamic processes of molecular reorientation in a stable twisted state under an applied electric field have been investigated by observing the optical transmittance and polarization switching current with simultaneous observation of the texture change.     

Studying the Electrically Driven Switching of the Planar Light Guide

Liquid crystalline materials can be used as an active media for the new generation of planar light guides. The main characteristics which governs light-guiding and switching abilities of such devices are the spatial distributions of the refraction indices (defined via the distribution of nematic director) for the liquid crystal confined within a light-guiding pore. We aim to obtain these distributions from the molecular dynamics simulation of the liquid crystalline cell with the homeotropic boundary conditions being applied. We discuss the reorientation kinetics of the homeotropic-planar transition and obtain the equilibrium director profile upon application of the planar reorienting field.     

Experimenteller Nachweis der Temperaturfelder beim Schaltvorgang in Cu2O und Zeitlupenstudie des thermischen Durchschlages

The temperatures before and during the breakdown (switching) in planar Ag Cu₂O Ag structures were determined experimentally. They were measured by means of liquid crystals and thermo-chromatic cryons as temperature indicators as well as measuring the intensity distribution of the emitted IR radiation. Structural changes during electrical switching were observed using a slow-motion camera. The formation of the switching channel and the movement of the liquid phase in it were observed. The results show that the electrical switching is due to a thermal mechanism.     

Modulated Microdomain Switching of Nematic Liquid Crystals

Abstract

Modulated microdomain switching of nematic liquid crystals have been investigated. This display mode used for light switching of micron-sized domains regions using a complicated electric field with special mesh electrode structures. Under He-Ne laser irradiation, contrast ratio was 22:1 for homeotropic alignment cell and transmittance change was no dependent of incident polarization condition. Under white light illumination, the contrast ratio was 4.3:1.     

Field induced Increase of Pitch in Planar Cholesteric Liquid Crystals

In cholesteric liquid crystals the application of an electric field parallel to the helix axis can change the wavelength of selective reflection. For decreasing pitch of the studied mixtures, the colour shift becomes more stable against the field-induced transformation of the initial Grandjean structure into a confocal texture. In compounds with a low crossover frequency, a field induced increase of the pitch was observed at the switching between low and high frequency fields. The investigations allow new conclusions on the origin of the colour shift.     

Electrical switching in chalcogenide liquid semiconductors

Reversible electrical switching from a higher resistance off-state to a lower resistance on-state has been observed in several chalcogenide semiconductor alloys both in the solid and liquid phases. Experiments conducted between 100 and 500°C showed that the threshold switching voltage decreases with increasing ambient temperature, and that the on-state resistance remains relatively constant up to the melting region but generally increases thereafter. The alloy Se_0.77Te_0.13S_0.10 shows a preserved on-state which can be maintained using 60 Hz ac in both the solid and liquid phases. The on-state can be reversed to the off-state by decreasing the applied field, and hence is not a conventional memory state. Below the melting region (250°C) the on-state I–V curve has a distinct knee or barrier voltage above which the current increases steeply. Above the melting range the knee becomes less clearly defined and occurs at a lower voltage. Below 345°C the above alloy has an activation energy of 0.37 eV. At this temperature an abrupt slope change or a discontuinity exists in the temperature dependence of off-state resistance, threshold field, threshold power and on-state resistance.     

Field-Induced Orientation Transition in Ferroelectric Liquid Crystals

Static and dynamic effects of a fast bistable device containing a ferroelectric liquid crystal are investigated theoretically. The switching mechanisms are different at low and high electric fields. There is a critical voltage which separates a bulk switching regime from the regime of domain wall motion. This threshold is determined as a function of physical device parameters.     

Electrooptical investigations on a nematic two frequency addressing mixture

The change of the sign of the dielectric anisotropy of a nematic liquid crystal allows an active switching process in electrooptical cells in on and off direction. In the two frequency addressing mode the TN cell shows a qualitative and quantitative different dynamic behaviour in comparison to the conventional driving mode. During the switching on process hydrodynamical domains occur. After switching off the director configuration skips into the −3π/2 twist and thereby it realizes a loss in transmission. The director configuration relaxes into the quarter twist after some minutes. A superposition of low frequent and high frequent voltage gives a strong increase of the number of multiplexable lines.     

British Liquid Crystal Society Annual Conference, 1991 Reading,UK

This report describes the physical principles underlying the switching of nematic liquid crystals (LCs) by an electric field in the plane of the substrate. The results have contributed to development of an IPS-mode-TFT-LCD, which is now commercially available and one of the most popular TFT-LCDs with ultra-wide viewing angle characteristics. The report summarizes work in the Glenn Brown prize winning PhD thesis, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo, 152-8551 Japan.     

EFFECT OF AZIMUTHAL SURFACE LAYER ANCHORING ON FIELD-INDUCED LAYER REORIENTATION IN A SHORT PITCH FERROELECTRIC LIQUID CRYSTAL

Abstract

The concept of the azimuthal surface layer anchoring (ASLA) is introduced to describe the field-induced layer reorientation in a short pitch ferroelectric liquid crystal (FLC). It was found that for the larger ASLA, the smaller layer reorientation occurs and saturates in the high field regime, especially near both surfaces. The ASLA tends to renormalize the surface interactions which produce the asymmetric director switching. It is suggested that the ASLA acts as a hindered restoring force for the layer reorientation.     

A fast-switching vertically-aligned liquid crystal device based on a multi-functional mesogenic photocrosslinker

Abstract

A vertically-aligned liquid crystal (LC) device with fast-switching LC molecules and excellent electro-optical properties is proposed. The proposed method employs a mixture of a polyimide to achieve the desired vertical alignment, and a mesogenic photocrosslinker to obtain the fast LC response. More specifically, the photocrosslinkers were photopolymerized by UV irradiation under an electric field to pre-tilt the LC molecules. In addition, upon increasing the number of functional groups in the photocrosslinker, faster switching properties were exhibited due to enhanced pre-tilting. Following UV irradiation, the proposed system also exhibited stable homeotropic alignment properties similar to those of conventional polyimides.     

Conducting and switching characteristics of SiO films with Al and Ag electrodes

The field and temperature dependence of the conductivity in SiO films (550–2800 Å) was investigated using evaporated Al electrodes. Reversible switching of resistance occurred when one or both electrodes were made of Ag. Depending on the magnitude of the first voltage pulse applied to the sample (“forming”) we observed either threshold switching or one of two different memory effects. The resistance of the completely formed sample could be switched between 300 ohm and 10⁹ ohm simply by changing the duration of the voltage pulses. Even after a large number (> 500 000) of switching operations, the sample retained its memory element character. A model to explain the memory effect by metal diffusion from the electrodes is presented.     

Modulated Structures in Dielectric Liquid Crystals

Review of theoretical and experimental results concerned to formation of space-modulated orientational structures in liquid crystals is given. Properties of ideal liquid crystal dielectrics are considered. Origion conditions and properties of flexoelectric and flexoelectromagnetic structures in nematics, dielectric instabilities in cholesterics and smectics, ferroelectric structures in chiral smectics are discussed.

It is well known that an electric field induces a modulation of the orientational structure of liquid crystals. There are two types of such modulated structures (MS) having different physical natures. The dissipative one is connected with the transport of electric charges and mass of the substance. The thermodynamic MS are not accompanied by the irreversible transport processes. Such MS can exist in ideal liquid crystal dielectrics. In fact there are always some ion impurities and as a result two types of MS are possible in liquid crystals. However it is possible to change the thresholds of these orientational instabilities in dependence on the concentration of impurities, anisotropy of transport parameters, boundary conditions and frequency of an external field. Thus one can exclude or induce the effects of first and second types. Below we shall consider the thermodynamic orientational transformations and conditions of their observation.     

Nonlinear Optics of Nematic and Blue Phase Liquid Crystals

We present a critical reexamination of individual electronic and collective optical nonlinearities of nematic liquid crystals (NLC) in the context of all-optical transmission switching of femtoseconds – CW lasers, and report recent observations of similar optical nonlinearities in Blue-phase liquid crystals (BPLC) and their utilization in feasibility demonstrations of polarization-free nonlinear grating diffraction and transmission switching of CW as well as pulsed lasers.