Hideo Takano

Zig-Zag Defects and Disclinations in the Surface-Stabilized Ferroelectric Liquid Crystals

Characteristic features of zig-zag defects and disclinations in surface-stabilized ferroelectric liquid crystal cells are presented. It is pointed out that there exist two kinds of zig-zag defects, i.e., a lightning type and a hairpin type, which appear alternatively along the smectic layer normal. This fact indicates that two types of mismatching of a layer bend structure cause two kinds of zig-zag defects. Switching between two twisted states occurs with the formation of internal boat-shaped disclination loops in relation to the zig-zag defects. The nucleation process of the boat-shaped domains is quite characteristic; the bows appear on each side of the bright hairpin-type lines, while the sterns appear on both sides of the dark lightning-type lines. A model structure is proposed in order to interpret the experimental observation around the zig-zag defect lines on the basis of disclination dynamics.

Two Kinds of Switching Processes in Surface Stabilized Ferroelectric Liquid Crystals

Two kinds of molecular reorientation processes in SSFLC cells were observed. One (a) is accompanied by the nucleation and growth of domains and the other (b) proceeds by uniform brightness change without any domain formation throughout a cell. In transmittance change, (a) and (b) are also explained by two-step and single-step switchings, respectively. The switching speed of the case (b) is one hundred times faster than that of the case (a) at a low electric field. The models for these switching processes are presented: an internal disclination model for case (a), and a cooperative director reorientation model for case (b).

UV modification of surface pretilt of alignment layers for multidomain liquid crystal displays

Two types of ultraviolet (UV) light induced modification of the surface pretilt of alignment layers are reported. The UV modification allows the liquid crystal (LC) surface pretilt angle of a polyimide film to be selectively altered in a small area. Two device structures for fabricating two‐domain liquid crystal displays based on this selective alteration of the LC surface pretilt angle of polyimides are proposed. Results from 3D simulation show that each pixel splits into two domains for the proposed structures. Experimentally, two‐domain twisted nematic (TN) test panels and thin‐film‐transistor (TFT) addressed two‐domain TN panels were fabricated using this method. The UV modification described in this letter does not require conventional photoresist technology as do other methods of fabricating two domain liquid crystal displays (LCDs). This process is simple as it requires only one polyimide coating and one rubbing step for each substrate.

Cell gap measurement of filled twisted nematic liquid crystal displays by a phase compensation method

Two configurations of the phase compensation method are presented to measure the cell gap of a filled twisted nematic liquid crystal display (TN‐LCD). The twist angle of the TN‐LCD can be any value between 0° and 360° and the pretilt angle can be any practical value. The cell gap measured by this method is the true TN cell gap, namely the thickness of the LC layer which is primarily responsible for the TN optics. Due to this reason, the phase compensation method can also be applied to measure the cell gap of a color TN cell. The measurement uncertainty of this method is within ±1%.

Invited Lecture. Complexities in the structure of ferroelectric liquid crystal cells The chevron structure and twisted states

Abstract To demonstrate the generality of the chevron structure in ferroelectric liquid crystal (FLC) cells, an optical micrograph is presented, which shows a series of defect lines clearly indicating this structure. The generality is understood as a direct manifestation of the smectic layer undulation playing an important role in the SASC (S*C) phase transition. By establishing the relation between chevrons and asymmetric focal conics, both of which have two senses, ±z, the boundaries between them with opposite senses, i.e. so-called zig-zag defects, have been characterized; the type boundary does not exist as a line but shrinks into a point. The interplay between the chevron structure and the director alignment has been described in detail; in particular, the appearance of unwinding lines in the middle of thick cells, the characterization of internal disclination loops in SSFLC cells, and the law regulating the emission and absorption of internal disclinations by zig-zag defects. With a view to removing...

The Symmetry Property of a 90° Twisted Nematic Liquid Crystal Cell

Abstract A set of symmetry properties of a 90° twisted nematic (TN) cell under rotation of polarizers has been observed both cxpcrimentally and theoretically for the first time. These symmetry properties can he explained by an argument based on rotation and time reversal transformations. The use of these symmetry properties allows one to answer the long time puzzle as to whether the e-mode TN cell, in which the transmission axis of the entrance polarizer is parallel to the entrance rubbing direction, is superior to the o-mode TN cell, in which the transmission axis of the entrance polarizer is perpendicular to the entrance rubbing direction, for both normally white and normally black cases. In addition, these symmetry propertics provide us with a useful test for any existing TN LCD computer simulation program.

Measurements of twisted nematic cell gap by spectral and split‐beam interferometric methods

The transmissive Fabry–Perot interference peaks of a filled twisted nematic (TN) cell as a function of wavelength, from 400 to 600 nm and from 1100 to 2000 nm, have been used for the determination of the cell gap. Theoretical formulations have been developed to calculate the effective indices of refraction including the dispersive effect, as well as the twist geometry of the liquid‐crystal medium for both the ordinary and extraordinary waves. When the light passes through the TN medium an even number of times caused by the interfacial reflections, the equations governing the effective indices of refraction are well behaved. For an odd number of passes, the validity of these equations is restricted to a region adjacent to the Mauguin limit. The cell gaps were derived from the measured effective optical paths and the calculated indices of refraction, which include the dispersive effect with or without the twisted deformation of the liquid‐crystal medium. A general equation has also been obtained to derive t...

Charge Trapping Properties of uv-Exposed Polyimide Films for the Alignment of Liquid Crystals

A model has been developed to account for the occurrence of flickers and image-stickings in TN or two-domain TN cells aligned by polyimide (PI) films with one of the films subjected to uv radiations prior to rubbings. The probability of flickers and image-stickings is proportional to the imbalance in the charge densities trapped on the two interfaces between the LC medium and two alignment PI films. Rate equations are established to calculate this charge imbalance for the cases of low-frequency ac voltages applied on the TN cell. The modeled results are in good agreement with the experimental results using various PI material as alignment layers for the TN cells. The mechanism of uv-induced charge-trapping centers on the PI films is discussed.

Stability of UV-Type Two-Domain Wide-Viewing-Angle Liquid Crystal Display

We calculated the Gibbs free energy for the splayed-twist state, the reversed-twist state, and the reversed-tilt state of UV-type two-domain wide-viewing-angle thin film transistor liquid crystal display (TFT-LCD) panels to evaluate the stability of the three configurations. We show that the smaller tilt angle is the dominant fact on that influences the stability of the splayed-twisted state. Further more, at high applied voltage, the reversed-tilt state is the most unstable, and is easily deformed into the reversed-twist state. Methods are therefore proposed for reducing the reversed-tilt state area. General trends of the critical voltage as a function of the chiral pitch, twist angle, cell gap, dielectric constant, and elastic constant are also discussed.

Threshold Voltage Biased E-Mode TN LCD—Optimum Optical Design for Grayscale Application

Abstract In a normally white (NW) twisted nematic (TN) liquid crystal displays (LCDs), there are two optical eigenmodes, an extraordinary-ray mode (e-mode) and an ordinary-ray mode (o-mode). With zero or a low-bias voltage as the brightest state of the display, both eigenmodes have insufficient display performance for the analog grayscale application. The former has a large color change as a function of viewing zone. The latter has quite a narrow horizontal viewing zone. By applying a threshold voltage as a bias to represent the brightest level of the e-mode NW, the optical performance in the vertical viewing zone was much improved. Further optimization of the d*delta-n for this mode of operation resulted in a small color change and a wide viewing zone by choosing d*delta-n = 0.47 micrometers. We conclude that the threshold-voltage biased e-mode NW of the first-minimum thin-film-transistor (TFT) driven TN can achieve a wide viewing angle for analog-grayscale applications.