A preview of the papers appearing in the June 2006 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

Novel liquid-crystal materials with negative dielectric anisotropy for TV application

Matthias Bremer
Melanie Klasen-Memmer
Detlef Pauluth
Kazuaki Tarumi

Merck KGaA, Germany

Abstract — The development of materials for vertically aligned (VA) active-matrix display technology is briefly reviewed and the newest generation of liquid crystals for this application is disclosed. These new materials possess a unique combination of high dielectric anisotropy Δε and low rotational viscosity γ1 with good clearing temperatures TNI for fast-switching LC mixtures targeted for TV application.

Through systematic consideration of how to maintain adequately high Δε and low γ1 values, novel LC materials have been successfully identified. These new LC materials possess a unique combination of high Δε and low γ1 with good TNI that enables the design of improved LC mixtures for TV with significantly improved γ1 values. In addition, the LC mixtures for automotive as well as mobile applications could successfully be developed with improved γ values. These new LC mixtures can improve the switching time by ca. 15%. This improvement will contribute to the reduction of the blurred images that are caused by the holding-type driving of LCDs together with the techniques of black-stripe insertion or higher-frequency driving.

TABLE 1 — Polar "workhorses" used in state-of-the-art VA mixtures.

Ferroelectric nanoparticle/liquid-crystal colloids for display applications

Fenghua Li
John West
Anatoliy Glushchenko
Chae Il Cheon
Yuri Reznikov

Liquid Crystal Institute,Kent State University, U.S.A.

Abstract — A non-synthetic approach to modify liquid crystals (LCs) by dispersing low concentrations of ferroelectric nanoparticles is reported. These dilute colloids are stable and appear similar to a pure LC. However, by changing the concentration and the type of ferroelectric particles, the physical properties of the LC materials can be tailored, including the nematic isotropic transition temperature (TNI), the dielectric constants, the birefringence (Δn), elastic constants, and the threshold voltage. Specifically, doping low concentrations of BaTiO3 nanoparticles (~1%) into a LC MLC-6609 increases TNI up to ~40°C. This giant shift of TNI has never been previously reported and indicates strong interactions between the LC and ferroelectric nanoparticles on a molecular level. The doped LC also demonstrates significant enhancement in birefringence (by 10–30%), dielectric anisotropy (by ~50%), and the elastic constant K33 (by ~20%). Ferroparticles act as molecular additives and modify the intrinsic properties of LC materials without time consuming and expensive chemical synthesis. The new LC materials are very attractive and suitable for use in displays, switchable lenses, beam steering, as well as other light-controlling devices.

The enhancement of the physical properties of the colloids is a result of the ferroelectric particle increasing the order parameter of the LC. This may be accomplished by the local electric field of the particle aligning the molecules adjacent to the particle. This assumes that the particles are small enough not to disturb the LC phase through anchoring of the molecules at the curved particle surface. As important are the systems where, addition of ferroelectric nanoparticles has little or no effect. This is seen most clearly by comparing the effects of dispersing BaTiO3 particles in 5CB (as shown in Table 1).

TABLE 1 —Physical properties of pure LCs and ferroparticle-doped LCs.

An introduction to PSS-LCDs: A fast-optical-response smectic LCD

Akihiro Mochizuki

Nano Loa, Inc., Japan

Abstract — A new type of fast-optical-response liquid-crystal display is introduced. This display uses a certain type of smectic liquid-crystal material that has a fast optical response as well as a native wide viewing angle. Unlike well-known smectic-based LCD technologies, this new type of LCD technology is highly compatibile with most nematic-based LCDs. This compatibility provides advantages for practical uses. Here, the initial molecular alignment and drive concept as well as the general performance of this new display technology are discussed.

Unlike most of the smectic liquid crystals whose n-directors show some tilt from their layer normal, the initial molecular alignment of the PSS-LC is along the set initial direction, such as the direction of mechanical buffing. The most intrinsic requirement needed to make the PSS-LCD functional is to make its n-director's initial molecular alignment parallel to the set initial molecular direction. Because in most smectic liquid crystals whose n-directors show some tilt from their layer normal, such as smectic-C, smectic-H, chiral smectic-C, and chiral smectic-H phases, the PSS-LCD requires the use these types of smectic liquid crystals. The significant difference from the most common initial molecular alignment of these types of smectic liquid crystals is to make its n-director align along with the set initial molecular alignment direction by using strong azimuthal anchoring.

FIGURE 5 — Time-domain birefringence averaging in terms of the observed direction outside the LC panel. Unlike most of nematic-based LCDs, PSS-LCDs use time-domain averaging of birefringence, resulting in a very wide viewing angle with very low color shifting. This time-domain averaging provides a high aperture ratio in high-resolution LCDs.

High-speed nonchiral-smectic-C liquid-crystal modes for display applications

Chang-Jae Yu
Cherlhyun Jeong
Sin-Doo Lee

Seoul National University, Korea

Abstract — Three nonchiral-smectic-C (NSC) liquid-crystal (LC) modes having fast-response and wide-viewing characteristics applicable for next-generation LCDs are described. In the NSC LC modes, fast analog optical modulation is achieved by means of a coupling with an external electric field in a dielectrically driving scheme. The fast response results intrinsically from no interlayer interferences during molecular rotation in the layered structure of the NSC LC. Moreover, the self-formation of two domains and the in-plane variations of the optic axis produce the wide-viewing properties. A step-wise temperature annealing process is employed for a stable and well-aligned smectic layer structure in a time-efficient manner.

As shown in Fig. 1, in the absence of an electric field (dark state), the molecular director in the NSC LC is placed on the plane parallel to two glass substrates and is optically similar to that in a planar nematic LC. This NSC structure in the transverse electrode configuration corresponds exactly to a homogeneously aligned nematic structure except for the presence of the layer tilt. Therefore, a linearly polarized light incident along the rubbing direction is blocked completely by a crossed analyzer.

FIGURE 1 — The basic structure and the operational principle of the IPNSC LC mode with a positive dielectric anisotropy (De > 0) in the absence of an external electric field (off-state), the IPNSC LC structure corresponds exactly to a homogeneously aligned nematic structure except for the presence of smectic layers.

Method for fabrication of liquid-crystal cells with narrow gap, fast switching, and flexible plastic substrates

Qingbing Wang
Rui Guo
Satyendra Kumar

Liquid Crystal Institute,Kent State University, U.S.A.

Abstract — A phase-separation method for the construction of devices with a specific internal architecture of a LC/polymer composite system is presented. The method results in adjacent uniform polymer and LC films parallel to the substrates. Scanning electron microscopy was employed to investigate the internal structures. The results show that the thicknesses of the LC and the polymer films in cells constructed with fixed-size spacers directly depends on the LC/polymer in the initial mixture. This can be effectively used for precision cell-gap control and to fine tune the LC optical path length. Cells with a submicrometer gap, prepared with this method and with 35wt.% of LC in the mixture, exhibited a total response time approaching 1 msec. This method has also been used to fabricate devices with plastic substrates.

The mechanism responsible for the formation of phase-separated composite-film (PSCOF) structures is the spatially anisotropic phase separation of LC and polymer due to non-uniform polymerization of monomers. The LC molecules strongly absorb UV light at wavelengths near and below 350 nm. As a result, an intensity gradient is produced along the cell normal. The rate of polymerization of NOA-65 monomers near the UV source proceeds faster, depleting the monomer concentration,which in turn drives more monomers to diffuse to this side to equalize their concentration. The LC molecules are expelled from the polymerized volume and move toward the second substrate [Fig. 2(a)]. The final result is a complete phase separation of the two components of the mixture [Fig. 2(b)].

FIGURE 2 — Schematic representation of the formation of the PSCOF structure. (a) Directional diffusion of LC and prepolymer is caused by a non-uniform polymerization rate. (b) The bilayer structure of a PSCOF cell upon completion of phase separation.

Electro-optic effects of the optically isotropic state induced by the incorporative effects of a polymer network and the chirality of liquid crystal

Yasuhiro Haseba
Hirotsugu Kikuchi

Kyushu University, Japan

Abstract — The electro-optical properties of optically isotropic liquid-crystalline composites prepared by in-situ photo-polymerization of cross-linking monomers in the isotropic phase of chiral liquid crystals were investigated. The magnitude of the electric birefringence of the composites decreased as the chirality of the liquid crystal increased. The clear Kerr effect was observed for the composites with high chirality. A large Kerr constant, being relatively insensitive to temperature, was obtained for the composites even in a temperature range below the isotropic–chiral-nematic phase-transition temperature. The response time of the Kerr effect at room temperature was on the order of tens to hundreds of microseconds.=

It is a key challenge, from the viewpoint of applications in electro-optics, to expand the temperature range where the anomalously large Kerr effect occurs. In the case of usual nematics, ξ grows rapidly with decreasing temperature in the isotropic phase near TNIand diverges infinitely at T* as shown in Fig.1(a). If the growth of ξ is suppressed within a finite size less than the optical wavelength but larger than the molecular dimension below TNI as shown in Fig. 1(b), an optically isotropic state showing an anomalously large Kerr effect is expected to appear over a broad temperature range below TNI with low-temperature dependence.

FIGURE 1 — Schematic illustration of (a) temperature dependence of a coherence length (ξ) for conventional nematics and (b) possible temperature dependence of ξ expected in the optically isotropic nematics.

Material and device properties of highly birefringent nematic glasses and polymer networks for organic electroluminescence

K. L. Woon
A. E. A. Contoret
S. R. Farrar
A. Liedtke
M. O'Neill
P. Vlachos
M. P. Aldred
S. M. Kelly

University of Hull, U.K.

Abstract — Light-emitting nematic liquid crystals are promising materials for organic light-emitting devices because their orientational anisotropy allows polarized electroluminescence and improved carrier transport. Two classes of nematics, i.e., room-temperature glasses and crosslinked polymer networks, are discussed. The latter class has an additional advantage in that photolithography can be used to pixelate a full-color display. The order parameter and birefringence of a new light-emitting nematic liquid crystal with an extended aromatic core are both shown to have values greater than 0.9. The performance of green-light-emitting devices incorporating liquid crystals of different conjugation lengths is discussed. Efficacies up to 11.1 cd/A at 1160 cd/m2 at an operating voltage of 7 V were obtained. A spatially graded, color organic light-emitting device obtained by overlapping pixels of blue-, green-, and red-emitting liquid crystals were demonstrated. Some regions of the red pixel were only partially photopolymerized in order to obtain different hues in the overlapping region with green. It is also shown that the photolithographic process has micron-scale resolution.

The photolithographic process can produce a new type of color-graded image using partial photopolymerization. Figure 5 shows a photograph of an operating single-pixel multicolor OLED, prepared by the sequential deposition of the red, green, and blue materials. The red material was deposited first, irradiated through a mask and washed to remove the unirradiated material. The green and blue materials were then processed similarly with the crosslinked green region partially overlapping the insoluble red film and the crosslinked blue partially overlapping the green. This results in blue-green and green-red emission. A closer look at Fig. 5 shows that the overlapping green-red region has two distinct zones giving yellow and orange EL.

FIGURE 5 — Photograph of graded-color single-pixel OLED operating at 5 V. The logo represents the Humber bridge located near Hull, England. This is one of the longest single-span bridges in the world.

The emission characteristics of liquid-crystal lasers

S. M. Morris
A. D. Ford
C. Gillespie
M. N. Pivnenko
O. Hadeler
H. J. Coles

Cambridge University, U.K.

Abstract — Liquid-crystal lasers exhibit narrow linewidth, large coherence area, and low threshold laser emission. Moreover, the wavelength of the laser line can be readily tuned using a variety of different external stimuli, including electric fields. These combined features make them particularly attractive as compact tunable laser light sources. Recent experimental results with regards to the emission characteristics of chiral nematic photonic band-edge lasers are discussed. This type of liquid-crystal laser consists of a self-organizing one-dimensional photonic band structure and a gain medium in the form of a laser dye. Some of the generic features that are observed for these lasers are discussed, including the typical emission linewidth of the laser line, the change in emission energy of the laser for high excitation energies and high pump repetition rates, and the dependence of the excitation threshold and slope efficiency on the cell thickness. In addition, how the performance changes when either the molecular structure of the chiral nematic host or the gain medium is varied is considered. Results are presented on the laser emission for a wide-temperature-range blue phase I band-edge laser which consists of a self-organizing three-dimensional photonic band structure.

In Fig. 8, band-edge lasing from a dye-doped BPI laser is demonstrated. The figure shows the laser-emission spectrum for an excitation fluence of 10 mJ/cm2 at an operating temperature of T = 30°C along with the optical texture that is observed when viewed between the crossed polarizers of an optical polarizing microscope. The platelet from which laser emission occurs is highlighted in the figure and corresponds to the (200) orientation. It can be seen that lasing occurs at λ = 571 nm, corresponding to the band edge of the photonic band gap for the (200) platelet.

FIGURE 8 — A micrograph of the optical texture from optical polarizing microscopy between crossed polarizers. The circle in white indicates the platelet from which laser emission occurred.

Visual quality of watermarking for mobile devices

Konstantin Krasavin
Jussi Parkkinen
Timo Jaaskelainen

University of Joensuu, Finland

Abstract — In this study, the visual quality of watermarked images, displayed on mobile devices, are evaluated. A watermarking technique in the spatial domain where a watermark is embedded into an image is considered. Three different types of images are considered and for each image five values of watermarking magnitude were tested. The difference in human evaluation of visual quality of a watermarked image, depending upon image properties and the properties of a display, was evaluated. In visual assessment tests, images from a mobile phone, PDA, and CRT were used. The visual quality of watermarked image was evaluated by 20 human observers. The results were quantified using statistical testing. The results show that for high-end displays, the magnitude used for watermarking is a critical factor in image quality. In mobile devices, the magnitude used for watermarking has little visual effect, especially for technical drawings. Recommendations for proper parameters are given.

FIGURE 4 — Test images (left to right): Lena, map, sail boats. The upper row consists of the original images and the bottom row are the watermarked images with a watermark magnitude equal to 15.