A preview of some of the most interesting papers appearing in the
September 2008 issue of the Journal of the SID. To obtain access to these articles on-line, please go to www.sid.org
Edited by Aris Silzars
Vladimir G. Chigrinov (SID Fellow)
Hoi-Sing Kwok (SID Fellow)
Hiroshi Hasebe (SID Member)
Hirokazu Takada (SID Member)
Hong Kong University of Science and Technology
Abstract — Liquid-crystal (LC) photoalignment using azo dyes is described. It will be shown that this photoaligning method can provide a highly uniform alignment with a controllable pretilt angle and strong anchoring energy of the LC cell, as well as a high thermal and UV stability. The application of LC photoalignment to the fabrication of various types of liquid-crystal displays, such as VAN-LCDs, FLCDs, TN-LCDs, and microdisplays, on glass and plastic substrates is also discussed. Azo-dye photoaligned super-thin polarizers and phase retarders are considered as new optical elements in LCD production, in particular for transflective displays.
A novel photoaligned TN-LCD cell was fabricated by one-step illumination with oblique non-polarized UV light of an empty cell with azo-dye layers coated on indium tin oxide (ITO) substrates. If the incident angle was sufficiently large (>75°), the p-polarization of the light was more pronounced for the bottom azo-dye layer in comparison with that for the upper one. Thus, the photoalignment of the azo-dye layer on the bottom substrate became perpendicular to that of the upper one, and LC directors on the top and bottom substrates also oriented perpendicularly.
FIGURE 10 — A plastic full-color TN-LCD was produced using photo-aligned polymerized azo-dye film SDA2.
Institute of Physics, National Academy of Sciences, Ukraine
Abstract — A plasma-beam process, developed for the alignment of liquid crystal (LC) in electro-optic applications, has been successfully applied to align "non-standard" LC, such as crystalline materials with LC phases at elevated temperatures and reactive mesogenes. In addition to the high alignment quality of the materials, there is no need for an intermediate layer between the substrate and the LC layer. Furthermore, the construction of our source simplifies the alignment procedure of large-area rigid substrates and the roll-to-roll processing of flexible films. This method opens new horizons for optical retarders and polarizers, as well as anisotropic semiconducting films for organic electronics.
The exposure geometry preferably used is shown in Fig. 2. By rotation of the source, the incident angle of the plasma beam can be varied. The incident angle typically used was about 70°. The substrates were treated in a cycling (there and back) translation regime or in a roll-to-roll translation manner by mounting a corresponding moving system in the vacuum chamber. The translation speed was about 2 mm/sec. The substrates were treated entirely or partially. In the latter case, masks of different configurations were used.
FIGURE 2 — Plasma-beam exposure geometry. 1 – Cycling translation regime (mainly used for rigid substrates); 2 – roll-to-roll translation regime (used for flexible plastic strips).
Hoi-Sing Kwok (SID Fellow)
Fion Sze Yan Yeung (SID Student Member)
The pretilt angles are measured using a 5-μm-cell-gap homogeneously aligned (anti-parallel) test cell in a conventional crystal rotation setup. Figure 6 shows a set of 5-μm electrically controlled birefringence (ECB) cells. The top row cells are anti-parallel rubbed and are ECB cells. The bottom row cells are parallel rubbed. For the parallel-rubbed cells, an interesting transition from splay alignment to bend alignment occurs at around 45°. This is to be expected and is the basis of the no-bias bend cell for fast LCD applications.
FIGURE 6 — Test cells at various pretilt angles. The top row cells are anti-parallel rubbed and the bottom row cells are parallel rubbed.
Lachezar Komitov (SID Member)
Abstract — The solid-surface/liquid-crystal interactions, defining the field-free alignment of the liquid crystal in conventional liquid-crystal displays, are playing a vital role in their optical appearance and performance. Nano-scale changes in the solid-surface structure induced by light have been recently shown to affect the anchoring strength and the easy-axis direction. Fine tuning of the anchoring strength is also demonstrated by nano-structuring of the Langmuir–Blodgett monolayer employed as liquid-crystal alignment layers promoting homeotropic orientation. On the basis of nano-engineering of the surface alignment properties, two novel alignment concepts have been introduced: electrically commanded surfaces (ECS) and high-performance alignment layers (HiPAL). Nano-structured polymers related to these concepts have been designed, synthesized, and used as materials for alignment layers in LCDs. ECS materials belong to the category of active alignment materials designed to mediate switching of the liquid crystal, whereas the HiPAL materials make possible the control of the molecular tilt angle in a broad range, from 0° to 90°, and they seem to enable the control of the anchoring strength as well. The nano-structured alignment materials are strong candidates for implementation in a new generation of advanced liquid-crystal displays and devices.
The photo-induced changes that take place at the LC/solid-surface interface on a molecular (nano) scale level result in changes of the anchoring strength, which, in turn, affect the switching characteristics of the photosensitive nematic represented by UF, τrise, and τfall. When the surface density of cis-isomers exceeds a certain critical value, a spontaneous anchoring transition from the initial planar to homeotropic alignment of the bulk liquid-crystal molecules takes place due to the bent form of the cis-isomeric molecules [Fig. 1(b)]. The light-induced alignment transition demonstrates clearly that the alignment of the liquid crystal is very sensitive to the nano-scale changes of the structure of the alignment surface. Hence, the nano-engineering of the alignment surface seems to be a powerful approach for an efficient tailoring of the LCDs characteristics.
FIGURE 1 — Schematic presentation of light-induced transition from planar to homeotropic alignment of a photosensitive nematic upon exposure to UV light. (a) Before illumination with UV light all liquid-crystal molecules in the cell are in trans-form and the liquid-crystal material possess a planar alignment (for simplicity, only one of the liquid-crystal cell substrates is shown). Illumination with UV light results in generation of cis-isomers which are selectively adsorbed onto the surface of the confining substrates due to their higher polarity compared to one of the trans-isomers. This, in turn, results in a homeotropic alignment of the liquid-crystal bulk.
Alexander Muravsky (SID Student Member)
Vladimir Chigrinov (SID Fellow)
Hoi-Sing Kwok (SID Fellow)
Abstract — The rewritable azo-dye photoalignment (ORW) of liquid crystals (LCs) for application in optical rewritable electronic paper has been investigated. It was observed that a periodic change in the azimuthal aligning direction with polarized UV light (365 nm) brings about homeotropic alignment, while utilization of visible light (450 nm) does not affect the LC tilt angle. The wavelength dependence of the ORW photoalignment result and the behavior of the photoinduced anisotropy was explored. The dark amplification of film anisotropy after exposure was observed, which is believed to be the relaxation process related to hydrogen bonding in azo-dye film. New material, CD1, for azo-dye rotation photoalignment that possesses a high azimuthal anchoring energy (about 2 x 10–4J/m2) was found.
FIGURE 1 — Optical rewritable electronic paper: (a) structure design; (b) operation principle; (c) prototype on plastic.
Oleg Yaroshchuk (SID Member)
Hoi-Sing Kwok (SID Fellow)
Vladimir Chigrinov (SID Fellow)
Hiroshi Hasebe (SID Member)
Abstract — A method of preparation of positive O films with the tilt angle of the optic axis continuously controlled in the range 0–90° is proposed. It is based on the use of reactive mesogens and alignment materials that provide a wide range of pretilt angles. The method developed allows for further improvement in the viewing-angle characteristics of LCDs with O compensation films.
Optimization of LCDs with integrated O compensators, from the viewpoint of contrast angular dependence, gray-level stability, and color shift, requires the technology of O films, providing a continuous variation of O film parameters, such as thickness, birefringence, and optic-axis profile. The present paper offers such technology. An approach developed for conventional liquid crystals, which provides a con-tinuous change in the LC pretilt angle from 0° to 90°, is used. This variation is attained by using a mixture of two polyimides designed for planar and homeotropic alignment (polyimides p-PI and h-PI, respectively). The p-PI/h-PI layers are backed and unidirectionally rubbed to impart alignment function.
FIGURE 1 — The cells filled with RM UCL-011 viewed between a pair of crossed polarizers: (a) the in-plane projection of the optical axis is parallel to the polarization direction of incident light; (b) the in-plane projection of the optical axis is rotated with regard to the polarization direction of incidence light. The tilt angle of the optic axis is 3, 33, 56, and 89° in the cells 1, 2, 3, and 4, respectively.
Yi Huang (SID Student Member)
Philip J. Bos (SID Fellow)
K. H. Kim
J. K. Jang
H. S. Kim
Liquid Crystal Institute, Kent State University
Abstract — The basic factors related to the dynamics of a π-cell device are reviewed. Specifically, the director dynamics are studied for the case of a periodic drive voltage that is sometimes referred to as "impulse drive." It is found for this type of drive waveform the desired bend state is more stable against the twisting effect of transverse electric fields found in AMLCD devices. This effect causes the reduction in light transmission due to "impulse drive" to be smaller in π-cell devices than is expected to be found in other AMLCD modes.
To consider the effect of impulse drive on the twisting of the director field, two factors need to be considered. One is the transverse field that can result from "fringing fields" generated by adjacent pixels of different voltages or from gate electrodes near a pixel. It could be expected that these transverse fields will cause the director to twist and thereby affect VC, Fig. 2(b). Another is the effect of flow in the device, as the in-plane flow induced by the impulse drive could possibly stabilize the director in the bend state against the destabilizing affect of the fringing fields.
FIGURE 2 — The director configuration of π-cells. Ez is the applied field, Ey is the transverse field induced by the neighboring pixel: (a) desired state, since the cell should be operated in the bend state; (b) undesired state, due to the transverse field. The twist state is triggered at the lower applied voltage.
Dong-Woo Kim (SID Student Member)
Sin-Doo Lee (SID Member)
Seoul National University
Abstract — A novel deformed-helix ferroelectric liquid-crystal (DHFLC) mode in a vertically aligned (VA) configuration is described. In this configuration, several unique features of display performance such as uniform alignment, fast response, and analog gray-scale capability are obtained. Particularly, this VA-DHFLC mode allows for the defect-free uniform alignment of both the FLC molecules and the smectic layers over a large area without employing additional processes such as rubbing or electric-field treatment that are generally required for planar FLC modes. Based on the VA-DHFLC mode, a transflective display having a single-gap geometry with in-plane electrodes on two substrates in the transmissive regions and on one substrate in the reflective regions is described.
Figure 4 shows the operational principle of our transflective VA-DHFLC cell in a single-gap geometry. It is composed of two crossed polarizers, the upper and the lower quarter-wave plates (QWPs) and the DHFLC layer. The small (black) and curved (gray) arrows in the FLC layer denote the dipole moments of the FLC molecules and the electric-field directions, respectively. The R region has in-plane electrodes only on the top substrate while the T region has in-plane electrodes on both the top and bottom substrates. The thickness of our transflective VA-DHFLC cell was maintained using a glass spacer 6.5 μm thick.
FIGURE 4 — The operational principle of our transflective VA-DHFLC cell in a single-gap geometry with in-plane electrodes on two substrates in the T regions and on one substrate in the R regions: (a) under no applied electric field (a dark state) and (b) under an applied electric field (a bright state).
Hin Yu Mak (SID Student Member)
Tao Du (SID Student Member)
Vladimir G. Chigrinov (SID Fellow)
Abstract — A ferroelectric liquid-crystal (FLC) display was optimized as a transflective liquid-crystal display (LCD). In this configuration, the single-cell-gap approach was considered. The optimized configuration exhibits a high contrast ratio, wide viewing angles, and achromatic (black/white) switching in both the transmissive and reflective modes. Because no double-cell-gap structure, no subpixel separation, and no patterning polarizers and retarders are included in the configuration, the configuration is easy to fabricate and also possess a perfect dark state. This configuration is also suitable for bistable applications.
The structure of a transflective FLCD is shown in Fig 1. It is composed of two polarizers, a retardation film, a transflective film, and an FLC cell. The transflective film is used as a reflector in the sunlight or a bright place and as transmitter at night or in a dark place. An anti-reflection layer is inserted at the top of the configuration in order to reduce the surface reflectance. The FLC cell is prepared by using the photoaligned method.
FIGURE 1 — The structure of a transflective FLCD.
T. Takahashi (SID Member)
S. Kobayashi (SID Fellow)
Tokyo University of Science, Yamaguichi
Abstract — STN-LCDs embedded with special metal nanoparticles of Ag/Pd are shown to be useful for a direct-multiplexed dot-matrix STN-LCD with 320 x 240 pixels and show a fast response time by 3–5 times compared to those without nanoparticles. This phenomenon is shown to be attributed to the reduction of rotational viscosity by 70% at room temperature and by 30% at a low temperature (–20°C). The alteration of elastic constants by doping nano-particles could be also essential.
The fabrication and characteristics of direct-multiplexed dot-matrix STN-LCDs showing fast electro-optical response, particularly at low temperatures, e.g., –30°C, that is needed in current LCDs, particularly in automotive applications, mobile phones, and personal LCTVs, is reported. Our solution to this requirement is to dope metal nano-particles into the LCD host media using specially synthesized metal nano-particles that are particularly useful for dot-matrix LCDs.
FIGURE 1 — Conceptual scheme of STN-LCD doped with metal nano-particles.
Victor V. Belyaev (SID Member)
Vadim M. Novikovich
Peter L. Denisenko
Abstract — Light diffraction on optically anisotropic substrates using sine surface micro-relief has been calculated by using the OAGSM method. The influence of the microrelief depth and material birefringence on the diffraction intensity on the order of 0–3 is reviewed and discussed. The results are compared with the results of the calculation for a rectangular microrelief. The microrelief depth and material birefringence allows the realization of different polarization states of the light beam transmitted or reflected by the substrate. The approach can be used to control the light-beam propagation for different applications including LCD backlights.
Figure 1 illustrates the calculation parameters. A substrate has a solid part with thickness H and a microrelief part. The periodical sinusoidal or rectangular microrelief is characterized by the period Λ and depth h (both Λ and h are normalized to the light-beam wavelength λ). The material of the substrate is a uniaxial medium (a polymer or a liquid crystal) with the largest refraction index (ne= n2) in the direction of the grooves. For comparison, the case of the isotropic material (n1 = n2 = n3 = 1.50) was also considered too.
FIGURE 1 — Geometrical dimensions of a microreliefed substrate (H – thickness, Λ – microrelief period, h – microrelief height); its optical axis and TM wave are parallel to the grooves; TE wave is perpendicular to the grooves.
Multiple comparisons of Mura size showed that there were significant differences among each level of Mura size. However, there was no significant difference between 2.84° and 5.13°. In other words, when the visual angle fell to the range from 2.84° to 5.13°, the difference of visual contrast threshold was not significant. As illustrated in Fig. 6, there were some interactions among these three variables, but the results of analysis of variances showed that these interactions did not significantly affect visual contrast threshold.
FIGURE 6 — Average visual contrast thresholds of novices and experts under different color backgrounds and Mura sizes.