Xabier Quintana

Modifying electrooptics of orthoconic antiferroelectric liquid crystal cells by manufacturing procedures

Orthoconic antiferroelectric liquid crystals (AFLCs) having 45° tilt angle have been proposed to overcome the problem of reduced contrast usually exhibited by regular antiferroelectric displays. However, the pitch of the helix induced by chirality is customarily short in existing orthoconic materials, making it difficult to achieve surface stabilized configurations when the material’s pitch is smaller than the cell thickness.In this work, the influence of different manufacturing procedures on the electrooptical behaviour of orthoconic AFLCs is studied. Using the same orthoconic AFLC mixture, aligning surfaces and manufacturing protocol, we have observed two dissimilar responses, true orthoconic behaviour, and orthoconic V-shape response. The electrooptical response depends ultimately on the rubbing strategy adopted in either case.

Electrical response of liquid crystal cells doped with multi-walled carbon nanotubes

Summary The inclusion of nanoparticles modifies a number of fundamental properties of many materials. Doping of nanoparticles in self-organized materials such as liquid crystals may be of interest for the reciprocal interaction between the matrix and the nanoparticles. Elongated nanoparticles and nanotubes can be aligned and reoriented by the liquid crystal, inducing noticeable changes in their optical and electrical properties. In this work, cells of liquid crystal doped with high aspect ratio multi-walled carbon nanotubes have been prepared, and their characteristic impedance has been studied at different frequencies and excitation voltages. The results demonstrate alterations in the anisotropic conductivity of the samples with the applied electric field, which can be followed by monitoring the impedance evolution with the excitation voltage. Results are consistent with a possible electric contact between the coated substrates of the LC cell caused by the reorientation of the nanotubes. The reversibility of the doped system upon removal of the electric field is quite low.

Non-conventional Alignment Surfaces for Antiferroelectric Liquid Crystals

Antiferroelectric liquid crystals(AFLC)feature interesting properties for a number of photonic applications. However, alignment of planar structures of these materials using conventional alignment layers is not satisfactory.One of the main problems dealing with these materials is the limited alignment obtained with standard manufacturing procedures, and the contrast decrease in multiplexed devices derived from the presence of pretransitional effect.The influence of alignment layers on AFLC contrast and switching has been studied. Using different alignment materials or manufacturing processes, it is possible to obtain different threshold voltage, dynamic range, and time response for the same material.We have studied the influence of alignment in two kinds of AFLC materials, namely regular tristate AFLCs and orthoconic AFLCs. Regular tristate materials feature long pitch and relative small cone angle. Contrast is not very good, but dynamic switching properties are excellent. Orthoconic materials are short pitch, wide cone angle(90°)materials. Contrast is excellent(200–300), but dynamic behavior is below regular materials.

Novel addressing scheme for passive antiferroelectric liquid crystal displays

In this work, the use of antiferroelectric liquid crystals for high-end passive displays has been explored. Antiferroelectric gray levels arise from a double symmetric hysteresis loop that can be stabilized by a constant holding voltage. This driving scheme is passive multiplexing compatible, but limitations appear when the multiplexing rate increases. We have developed new waveforms and driving schemes for high multiplexing level at video rate. The problem of accumulated voltage on bias level arising from data voltages is tackled as well.

Design and simulation of single-electrode liquid crystal phased arrays

Liquid crystal (LC) phased arrays and gratings have been employed in optical switching and routing [1]. These diffractive optic elements are of great interest because they can be scaled up to a large number of elements and their optical properties can be electrically addressed with a low driving voltage. LC phase gratings have been achieved either by periodic addressing of pixels or by using periodically-modified structures. The latter approach leads to less reconfigurable devices but the addressing is simpler.In this paper we focus on optical phased arrays where the phase is varied either continuously or discretely and where the periodicity is induced by electrode configuration. We first describe a possible structure based on a conductive silicon wafer. We argue that this structure can induce either continuously or discretely varying arrays while applying single voltage to the array. In the second part we simulate the behaviour of such arrays. We base the simulation on a LC synthesized at the Military University of Technology, this high-birefringence nematic LC shows in a 4-μm thick cell a linear phase shift range of more than 360° between 1.2 V and 1.8 V. We calculate the distribution of the LC molecule director and assess the performance of the array with respect to the applied voltage. Finally, the relevance of such technology for switchable phased arrays is discussed.

Low threshold voltage asymmetric antiferroelectric liquid crystal cells

Asymmetric antiferroelectric liquid crystal displays (AAFLCD) are attractive since they show a very well defined off state and fast switching time. Moreover, they can be driven by a simple biasless DC compensated waveform. The electrooptical response of an AAFLCD allows for new addressing modes, including quasi-static intermediate greyscales maintained without applying a field and passively addressed multiplexed high-frequency displays and spatial light modulators. A new kind of asymmetric cells have been obtained by using fluorinated block copolymer (FBC) alignment, which enhances surface segregation and provides a low energy surface. In this work we combine FBC alignment with antiferroelectric liquid crystal mixtures containing strongly electronegative fluorinated components. Threshold voltages for the antiferroelectric-ferroelectric phase transition as low as 3 volts are observed. We report the time evolution of the shift of the electro-optical response.

Broad temperature range antiferroelectric regular mixtures

Tristate regular mixtures with different electro-optical properties such as threshold voltage, saturation voltage, holding ratio and response time are presented. The relation of properties with the structure of compounds is discussed. All of mixtures show only moderate dynamic and static contrast but big gray level scale without hysteresis for positive and negative field driving.

Evaluation of Orthoconic Antiferroelectric Materials for Photonic Applications

The use of antiferroelectric liquid crystals(AFLCs)on practical applications is often precluded by their low contrast. Orthoconic AFLCs, in principle, may overcome this limitation, for they behave as isotropic plates at normal incidence. A number of orthoconic AFLC materials have been evaluated. Multiplexed seven-level driving schemes have been employed for dynamic analysis. The most relevant results include average static contrast ratio over 200:1 and dynamic contrast ratio over 130:1. On the other hand, hysteresis often shows slight voltage shifts when the cells are driven with low frequency AC signals. This feature impairs symmetry of the transmission maxima on the positive and negative lobes of the hysteresis curve. The effect is attributed to the extremely low pitch of actual orthoconic materials–usually below 1 μm. The distinctive properties of orthoconic materials make them suitable for photonic applications requiring moderate response time such as routing or beam steering applications.

Security devices based on liquid crystals doped with a colour dye

Liquid crystal properties make them useful for the development of security devices in applications of authentication and detection of fakes. Induced orientation of liquid crystal molecules and birefringence are the two main properties used in security devices.Employing liquid crystal and dichroic colorants, we have developed devices that show, with the aid of a polarizer, multiple images on each side of the device. Rubbed polyimide is used as alignment layer on each substrate of the LC cell. By rubbing the polyimide in different directions in each substrate it is possible to create any kind of symbols, drawings or motifs with a greyscale; the more complex the created device is, the more difficult is to fake it.To identify the motifs it is necessary to use polarized light. Depending on whether the polarizer is located in front of the LC cell or behind it, different motifs from one or the other substrate are shown. The effect arises from the dopant colour dye added to the liquid crystal, the induced orientation and the twist structure. In practice, a grazing reflection on a dielectric surface is polarized enough to see the effect. Any LC flat panel display can obviously be used as backlight as well.

Electro-optics of antiferroelectric orthoconic reflective displays

The contrast of antiferroelectric liquid crystal (AFLC) displays working on transmissive mode is degraded by the presence of pretransitional effect. Contrast is significantly enhanced by the use of orthoconic materials, i.e., AFLC mixtures with 45° smectic cone half angle. However, current orthoconic materials usually show short helical pitch (<1 μm), what hinders the surface stabilization of the material with standard alignments. Indeed, cell thickness should be small compared to helical pitch, in order to surface-stabilize the material. Cell thickness nonetheless cannot be arbitrarily chosen, since AFLCs behave as linear retardation plates whose performance is a function of optical path. In the case of transmissive cells, thickness is fixed about 1.5 - 2 μm, i.e., wider than helical pitch. As an alternative, the use of reflective cells has been proposed. In these cells, the optical path is doubled; therefore, the same optical performance can be obtained, in principle, with 0.8 - 1 μm reflective cells than with transmissive cells twice as thick, whereas surface-stabilization is improved. In this work, the electro-optical behavior of orthoconic reflective cells is studied. Multiplexed seven-level driving schemes have been employed for dynamic analysis of orthoconic displays, allowing video-rate multiplexed analogue grayscale.