Ferroelectric Liquid Crystal Devices Research Articles

Alignment layers for improved surface-stabilized ferroelectric liquid-crystal devices

Ultrathin silane monolayer and silicon oxide alignment layers in surface-stabilized ferroelectric liquidcrystal devices are shown to enhance bistability and reduce degradation from ions.

Novel Switching with Gray Scale in Surface-Stabilized Ferroelectric Liquid Crystal Devices

The apparent tilt angle in surface-stabilized ferroelectric liquid crystals (SSFLCs) is dependent on the applied electric field for monostable and bistable devices. This behavior is attributed to the fact that the director profile of molecules in SSFLC cells is changed by an interaction between spontaneous polarization and the electric field. Based on this behavior, the transmission in SSFLCs can be controlled by the applied voltage, resulting in a smooth voltage-transmission curve.

All-optical implementation of a self-organizing map: a preliminary approach

Optically addressed ferroelectric liquid-crystal devices present certain characteristics that make them attractive for all-optical implementations of neural architectures. This is particularly true for the implementation of analog self-organizing systems such as Kohonen's self-organizing map. The optical architecture proposed here is based on the principle of a resonator memory. The main novelty lies in the two-dimensional, continuous, binary information encoding and organization. Basic operations such as neural decision, weight updating, and information spatial ordering are investigated. The principle of a learning procedure close to the Hebbian rule is discussed.

Analysis of ions in ferroelectric liquid crystals from hysteresis curves

Abstract We have developed an experimental method in which ion behavior in ferroelectric liquid crystal (FLC) devices is explored through periodic measurements of two types of FLC hysteresis as a test cell is subjected to a dc bias. We find that the integrated-current vs. applied voltage hysteresis curve contains separate components due to the FLC capacitance, resistance, and polarization switching. Over time the hysteresis curves shift in voltage and shape, indicating both trapping of ions at the alignment layers and changes in ion concentration. These shifts occur over hours, necessitating measurements to be made over hours. Through modeling of the FLC test cell, it is found that other mechanisms can also contribute to a voltage shift in the hysteresis curve.

Ferroelectric liquid crystal device using the τ-VMINmode

Abstract Ferroelectric liquid crystal (FLC) devices using the -Vmin mode were developed, combining FLC materials with negative dielectric anisotropy, the C2-uniform (C2U) orientational state and the “Malvern” drive schemes. The developed FLC materials in these devices showed the Vmin value under 35V, which is adequate to the drive voltage of ±40V. The C2U orientational state was found to be the preferred state for the -Vmin mode, and was obtained in FLC cells with medium pretilt aligning film. The FLC cells were driven by the Malvern drive schemes, achieving fast line address time (faster than 50μ sec/line), wide temperature range of driving (0∼40°C), high contrast (higher than 50:1) and drive voltage of ±40V.

The relationship between liquid crystal alignment in the nematic and smectic C∗ phases

Abstract The pretilt angle in the nematic phase is related to two different surface dependent factors, while the quality of surface stabilized ferroelectric liquid crystal (SSFLC) devices is dependent on only one of them. We have synthesized two series of polyalkyleneimides and found that the pretilt angles for thick nematic cells are not related to the cone angles in SSFLC cells.

Particle image velocimetry fringe processing using an optically addressed spatial light modulator

The use of an optically addressed spatial light modulator for processing images from a particle image velocimetry system is demonstrated. The results obtained by using a binary ferroelectric liquid-crystal device are compared with those obtained by conventional means (electronic Fourier transformation of the fringe patterns). It is shown that both methods of processing are capable of determining particle velocities to within 5% error when the autocorrelation peak separations are plotted against known particle separations. However, the optical processor is capable of operation speeds in excess of 3 kHz, whereas electronic-based systems perform the task at 1–10 Hz.

Observation of Dielectric Biaxiality in a Ferroelectric Liquid Crystal by the Excitation of Optic Modes

Abstract The method of AC stabilization to enhance the contrast of a ferroelectric liquid crystal (FLC) device is well established, however the actual process taking place is not well understood. Here a careful analysis of propagating optic mode data for an AC stabilized cell leads to the conclusion that the AC stabilization process involves a torque around the smectic cone, very similar to the DC/dipole interaction. This shows that the dielectric tensor of a FLC is BIAXIAL. The degree of this biaxiality is measured by comparison between AC and DC interaction energies.

Stripe electrodes for an electrically controlled gray scale in ferroelectric liquid-crystal devices

A stripe-electrode structure is proposed for making the electro-optic turn-on curves of ferroelectric liquid-crystal devices gradual and thus giving gray-scale capability. The feature of this structure is that the area that responds insensitively to an applied voltage is introduced into the pixel area. The applied voltage–transmission characteristics measured for a ferroelectric liquid-crystal cell with this stripe-electrode structure were directly related to the spacing and width of the stripe. With the stripe electrodes a cell’s voltage range between the transmission of the off-state level and the on-state saturation level was 3.6 times greater than with the conventional electrodes.

Measurement of the biaxial permittivities for several smectic C host materials used in ferroelectric liquid crystal devices

Abstract Dielectric biaxiality plays an important role in the electric field behaviour of ferroelectric liquid crystal devices, providing the necessary driving force for AC field stabilization and playing an important role in device dynamics. The static electric permittivities have been measured as a function of temperature for several mixtures with an N[sbnd]SA[sbnd]SC phase sequence. Three permittivities are defined for the biaxial SC phase, and were determined from measurements of homeotropic and planar cells and from the average permittivity extrapolated from the uniaxial nematic and smectic A phases. The SC cone angle is assumed to be equivalent at dielectric and optical frequencies. Results are reported for five common ferroelectric liquid crystal hosts and for two racemic mixtures, including the racemic analogue of the commercially available SCE 13. The dielectric biaxiality is sufficiently large below the SA to SC transition in each of the studied mixtures to have a strong influence on the electro-optic behaviour. This is consistent with the occurrence of AC stabilization in surface stabilized ferroelectric liquid crystal devices with the chevron layer configuration.

Bipolar optical neural networks using ferroelectric liquid crystal devices

Ferroelectric Liquid Crystal (FLC) electrooptic devices present some characteristics that make them attractive for the optical implementation of neural computers: high response speed and bistability, or intrinsic memory capability. Herein we will demonstrate how the electrooptic bistability of FLC materials can be used to implement bipolar optical neural networks, based on simple FLC cells that act as the neurons and as the interconnections among them.

Large area ferroelectric liquid crystal displays

Abstract Ferroelectric liquid crystals (FLCs) combine the established advantages of conventional liquid crystals with the additional advantages of fast switching speeds and bistable operation. This combination of properties offers the potential for producing highly multiplexed flat panel displays which have a wide angle of view, a high contrast, and are capable of showing a high information content at video rate. In addition, the fabrication of ferroelectric liquid crystal devices (FLCDs) is relatively straightforward, especially when compared with that employed for active matrix LCD technology, thus making FLCDs particularly well suited for use in large area displays operating at video frame rates.

The importance of dielectric biaxiality for ferroelectric liquid crystal devices

Abstract The dielectric biaxiality of SCE13, a commercially available ferroelectric liquid crystal (FLC) mixture, has been measured. The results have been used to compare theoretically predicted AC field stabilization effects and the voltages for response time minima with experiment.

Electro-Optic Characteristics of Charge-Transfer-Complex Doped Ferroelectric Liquid Crystal Device: Realization of Very High Contrast Ratio and Perfect Bistability

Abstract A surface stabilized ferroelectric liquid crystal electro-optic device using FLC material doped with a charge-transfer-complex (CTC), TMTFF-ODTCNQ, was shown to reveal a perfect bistability yielding a 100% memory capability with an inverted polarity accompanying a high contrast ratio reaching 55:1. The relationship between the EO characteristics and the dynamics of the ions of CTC has been investigated to understand this peculiar EO performance. It was shown that the inverted bistability is caused by the formation of strong internal depolarization field originated from the ions of the CTC molecules accumulating in the interfacial region. Both the values of the depolarization field and the mobility of ions were evaluated. The value of the former was shown to reach several volts and that of the latter was 5 × 10−7 cm2/V s. These values are thought to be enough to cause the inverted bistability phenomena.

Influence of thickness and conductivity of alignment layers on the bistability of ferroelectric liquid-crystal devices

The bistability conditions on both the thickness and conductivity of the alignment layers as a function of spontaneous polarization and other cell parameters for ferroelectric liquid-crystal devices have been obtained analytically based on a uniform switching model. It is found that there exists a practical threshold conductivity of the alignment layers, above which good bistability can be obtained. Below this threshold conductivity, there exists a critical total alignment-layer thickness, below which bistability can be achieved. For the latter case, the two quantities depend linearly on each other, with a coefficient increasing monotonically with cell thickness, the difference between the ratio of the square of the spontaneous polarization to the rotational viscosity, and the conductivity of the liquid crystal.

Speed and resolution in photoaddressed ferroelectric liquid-crystal devices

Abstract The speed and resolution issues in photoaddressed liquid-crystal spatial light modulator (SLM) devices are analyzed. It is shown that low-voltage, low-charge conditions favor high resolution. Higher speed switching in ferroelectric liquid crystals (FLCs) is associated with increased voltage and charge requirements. The compromises in device design and some experimental results are presented.

Behaviour of Ionic Effects on Response Times of Surface Stabilized Ferroelectric Liquid Crystal Devices

The response time measurements of a new room temperature ferroelectric liquid crystal have been done by using the field reversal method. The response time has been found to be frequency dependent and decreases considerably at frequencies above 10 kHz. The temperature dependence of response time showed the presence of ionic effects as is evident from the appearance of the second peak in the current response to an applied bipolar pulse.

The mechanism of ac stabilization in ferroelectric liquid-crystal-filled cells

The use of ac stabilization in ferroelectric liquid-crystal (FLC) devices is commonplace: It enhances the bistability and increases the extinction angles seen in a cell. The mechanism involved in this process is investigated here. Examining the optic tensor profile across an ac-stabilized FLC layer has revealed that it is very similar to that present under a forward bias dc field. Comparing the dc/dipole and ac/dielectric interaction energies shows conclusively the presence of biaxiality in the dielectric tensor. This biaxiality is measured through the comparison of the ac and dc electric energy densities in a cell.

Effect of alignment layer conductivity on the bistability of surface-stabilized ferroelectric liquid-crystal devices

Recent experimental results indicate that bistability is difficult to achieve in surface-stabilized ferroelectric liquid-crystal devices with high spontaneous polarization (Ps) materials. We formulated the basic equations to analyze the bistability as a function of the conductivity and thickness of the alignment layers using Ps as a parameter. It is found that a threshold value of the conductivity, which approximately scales with both the thickness and Ps, exists to achieve good bistability. Numerical results are given to provide important guidances for choosing the alignment layer materials to obtain excellent bistability using high Ps materials.

Polarization-independent, broadband, bistable, 2 × 2 optical exchange switch

A polarization-independent, broadband, bistable, 2 × 2 optical exchange switch utilizing a surface-stabilized ferroelectric liquid-crystal device is presented. The switch has <−15-dB cross talk over a 50-nm optical bandwidth, with a minimum of −18 dB at 615 nm. By modeling the individual components the theoretical output of the device is obtained, which agrees well with experimental results. Four independent 2 × 2 switches incorporated into one single compact device are also demonstrated. The typical switching time for these switches is ∼150 μsec.