X. Quintana

Addressing waveforms for tristable aflcs in active matrix displays

Abstract In this work, addressing waveforms for grayscale generation in tristable AFLC cells are proposed. The behaviour of an active matrix device have been simulated. Electric limitations arising from TFTs (compatible voltages, storing capacities) have been taken into account. These limitations have been made compatible with the LC characteristics, i.e., threshold voltage, switching and holding times, and operating range. Optimised working conditions have been derived from previous considerations.

Asymmetric switching of antiferroelectric liquid-crystal cells

Antiferroelectric liquid-crystal cells usually show symmetric electro-optic switching response from the antiferroelectric state to two opposite ferroelectric states. Intermediate transmission levels (analog gray scale) can be stabilized, applying a constant dc bias voltage after switching. Modifying the manufacturing process and using narrow dynamic-range antiferroelectric materials, a fully asymmetric response has been achieved. This asymmetric switching allows the antiferroelectric cell to be driven as a ferroelectric cell, reducing or eliminating the bias voltage, and ultimately leading to analog optical multistability, i.e., devices whose optical transmission may be arbitrarily set and maintained without power supply.

Measurement of Pretilt Angle and Cell Gap in Reflective Vertically Aligned Nematics

An experimental setup to measure the pretilt angle and cell gap in reflective VAN (vertically aligned nematic liquid crystals) has been developed. This setup, based on the rotation method, has been modified in order to measure simultaneously high pretilt angles in reflective cells along with the cell gap. The determination of the pretilt angle is achieved by finding the rotation angle for maximum and minimum transmittances.

Addressing Waveforms for Asymmetric Antiferroelectric Liquid Crystal Displays

Antiferroelectric Liquid crystal cells having a SiOx on one side and a conventional alignment layer on the other side have been prepared. This manufacturing process allows the antiferroelectric cell to be driven as a ferroelectric cell. This behavior opens the possibility of driving the cell with simple bipolar pulses, allowing some interesting effects such as video-rate multiplexed analogue gray scale.

Flickering and stability in AFLC analog gray scale

Abstract Antiferroelectric liquid crystals have recently demonstrated the possibility of producing analog, multiplex-compatible, video-rate gray scales by addressing with fairly simple waveforms. These waveforms, however, usually rely on DC compensation by alternative use of the two opposite electrically-induced FLC phases as clear state, and the AFLC relaxed phase as dark state. This strategy may induce flickering derived from asymmetry in optical transmission of the FLC phases, FLC-AFLC relaxation, and reset or saturating prepulses. A theoretical and experimental study of flickering sources in AFLC multiplexed displays has been carried out. The relative importance of each source is analyzed, and performance of the display in actual working conditions is derived.

Reliability of Liquid Crystals in Space Photonics

Passive liquid crystal (LC) devices are becoming an interesting alternative for the manufacturing of photonic devices in spatial applications. These devices feature a number of advantages in this environment, the lack of movable parts, and of exposed electronics being among the most outstanding ones. Nevertheless, the LC material itself must demonstrate its endurance under the harsh conditions of space missions, including launch and, perhaps, landing. In this paper, we present the environmental testing of an LC device for space applications. A number of LC based beam steering devices were manufactured, characterized, and tested in a series of destructive and nondestructive tests defined by the European Space Agency (ESA). The purpose was to evaluate the behavior and possible degradation of the LC response in simulated space environments. Device fabrication and testing was done within an ESA-funded project, whose purpose was the design, manufacturing, and characterization of adaptive optical elements, as well as the execution of qualification tests on the devices in space-simulated conditions.

Asymmetric electro-optic response in antiferroelectric liquid crystals

A new manufacturing procedure producing asymmetric hysteresis curves in surface-stabilized antiferroelectric liquid crystal cells has been developed. It is based on the use of dissimilar surface conditionings on either side of the cell. The central relaxed AFLC state is not located at zero volts. The AF→F transitions on either side, consequently, are not produced at the same switching voltage. This behavior opens the possibility of driving the cell with several new modes, allowing some interesting effects such as video-rate multiplexed analogue grayscale with simple bipolar pulses, and longterm multistability.

Automated characterisation system for liquid crystal displays

A complete program suite for the automated characterisation of liquid crystal displays (LCDs) has been developed in the Lab View 7.1 environment. It includes routines for basic electro-optical characterisation, i.e. generation of transmission-voltage curves applying triangular waveforms while measuring the switching voltages. And it provides an easily accessible interface for design of arbitrary waveforms, both for active matrix addressing (often necessary in nematic LCDs) and for passive matrix addressing (applicable to ferro-and antiferro-electric LCDs as well as some nematic LCDs). The software includes automated methods for generation of dynamic grey scales, for measurement of grey-grey transition times and for analysis of image sticking.

Improving Electro-Optic Response in V-Shape Chiral Smectic Displays

Addressing waveforms for reflective LCOS devices filled with V-shape chiral smectic liquid crystals, have been tested and optimised. Analogue greyscale generation has been tested on passive devices mimicking the behaviour in active matrix based devices. A comparative study of a number of experimental chiral smectic mixtures showing V-shape response has been carried out. Specific driving waveforms have been designed and optimised in order to improve the materials electro-optic response. This procedure is lengthy since every voltage level of the waveform ought to be separately optimised, and interdependences between the different waveform fragments lead to an iterative optimisation process. The effect of a short blanking pulse named well pulse has been particularly remarkable, thus forcing a thorough tune-up as a function of working temperature and video rate.

Mechanism of Grayscale Generation in Antiferro-Electric Liquid Crystal Displays

Abstract The statistical origin of grayscales showed by surface-stabilized antiferroelectric liquid crystal cells has been tested. Long-term stable microdomains have been detected, thus supporting the statistical mechanism. However, the relaxation time required for these microdomains to stabilize at room temperature is fairly large for current materials under actual working conditions. Therefore, a combined time-modulation/relaxation mechanism is proposed for video-rate grayscales in AFLC displays.