Progress in Liquid Crystal Science and Technology

The following sections are included:

• Dedication

• PREFACE

• CONTENTS

A series of transitions from one technology to another were made by the author since 1968. This long journey included: from dislocations in inorganic crystals to disclination in liquid crystals in 1950's to 1970's, from microwave semiconductors to infrared holography and LCD optics in 1960's to 2010's, from DSM-mode LCD to TN-mode LCD and STN-LCD and the realization of defect free LCD from 1968 to 1987, from rubbing to non-rubbing technology in 1996's, from SS-FLCD to defect free PSV-FLCD in 1990's, from color filter to color filterless PSV-FLCD-FSC-LCD and NTN-FSC-LCD in 1985–2012, and then to nanoparticle doped LCDs for the reduction of operation voltage and response time, and finally to the early work on organic films for substrates and active matrix LCD driving.

The following sections are included:

• Introduction

• Nematic Liquid Crystals

• Smectic Liquid Crystals

• Discotic Liquid Crystals

• Nematic Elastomers

• Conclusion

• References

Thermal polarized light microscopy of liquid crystal systems primarily involves the identification of phase types. Recently, X-ray diffraction has assumed more importance as a method of structural characterization, which has resulted in the classification of numerous novel phases. However, microscopy allows for the rapid investigation of the bulk structures of phases under ambient and various applied conditions. In this chapter we show how certain defect textures of smectic phases are related to phase structure, and how subtleties of investigation can provide information that are not normally available through modern methods of study.

The electric birefringence effect of nematic liquid crystals is discussed both theoretically and experimentally. In the isotropic (I) phase below the critical temperature, a paranematic (pN) order is formed under an applied electric field, and there occurs the first-order transition to the nematic(N) phase at a threshold field strength. As a consequence, the system exhibits electric birefringence with hysteresis. Above the critical temperature, a hysteresis-free continuous pNN phase transition is induced, and the induced birefringence by electric field above the critical strength is as large as that of the intrinsic N phase. It is found experimentally that the substrate surface treatment affects the electric birefringence effect in I phase. It is also confirmed that the response and relaxation times are below the millisecond range because of short-range nematic order. The experimental results appear to support this effect to be applicable to novel liquid crystal displays featuring fast switching, high contrast ratio, wide viewing angle, etc.

Liquid Crystal Display (LCD) technology has a striking history that traces its beginnings in its application in the form of pocket calculators around the 1970s and is now utilized in broad product portfolios, from small cellular phones to large television sets. The successful developmental history of LCD technology and the associated liquid crystal materials are reviewed in this chapter.

We briefly review our recent simulation studies that elucidate the structures of confined chiral liquid crystals exhibiting cholesteric blue phases in the bulk. We show that, depending on the anchoring conditions, cell thickness, and temperature, the liquid crystal exhibit various exotic defect structures that do not resemble those of bulk cholesteric blue phases. These defect structures can be regarded as the result of frustrations arising from the competition between the bulk blue phase chirality-driven ordering and the confining surfaces whose anchoring disturb the ordering. Structures include a hexagonal array of Skyrmion excitations, which have been shown to play an important role in a variety of condensed matter systems. Our results clearly indicate that liquid crystals can exhibit much more exotic ordered structures than we already know or we can imagine.

Smectic liquid crystal–based display device technologies are reviewed from the viewpoint of chiral smectic liquid crystals and both their panel internal charges role and surface interaction. A technical migration from a surface-stabilized ferroelectric liquid crystal display to an analog-like electro-optic response characteristic as well as some certain unique physical properties are chronologically overviewed.

We review the development of nanostructured alignment surfaces for liquid crystals. These surfaces are not uniform and consist of nanostructured domains of different alignment materials. They are able to form uniform pretilt angles in the range of 0°–90° reliably and controllably. The requisite conditions for having such uniform alignment properties are investigated. It will be shown that there is an optimal size range for the nanostructures such that the anchoring energy can be strong and the alignment pretilt angle is uniform. This range is reasonably large and achievable. Three techniques for the generation of these nanostructured surfaces are reviewed. The first two techniques can be classified as self-assembly of nanodomains, whereas the third one is deterministic and consists of nanoimprinting of the nanostructures. The last technique can be used to produce large pretilt angles over a large area uniformly. Several applications of such large pretilt angles are discussed. Major applications include fast liquid crystal modes and bistable displays based on nematic liquid crystals.

Photoalignment is emerging as an important manufacturing technology. This chapter reviews the recent progress in the application of the photoalignment process to the fabrication of various liquid crystal devices. In particular, new results on high-speed ferroelectric liquid crystal modes based on photoalignment are presented. Fast liquid crystal modes are essential in many applications such as field sequential color displays. The application of optically rewritable devices based on realignable azodyes is also reported here. Finally, a variety of photonic devices that can be fabricated using photoalignment, including switchable optical fibers, photonic switches, liquid crystal lenses, switchable retarders, photonic gratings, and q-plates, have been documented in this chapter.

The application of UV-curable liquid crystals: the light scattering LCD using polymer network liquid crystals (PNLC), the effect of in situ photopolymerization of liquid crystalline monomers in aligned liquid crystals such as polymer stabilization, polymer-stabilized V-shaped ferroelectric LCD (PSV-FLCD), and the various kinds of retardation film with high quality and good thermal stability, are reviewed and discussed in this chapter.

Bistable surface-stabilized ferroelectric (SSFLC) devices have generated a great deal of interest, but defects can sometimes degrade their performance. In this chapter, it is shown that by considering the alignment layers used for bistable SSFLC devices, defect-free alignment can be achieved for all pretilt angles. In the case of low pretilt angles, it is important, as was shown by Kobayashi, to use an alignment layer with low azimuthal anchoring energy. In the case of low and intermediate pretilt angles, it is important to control the surface topography of the alignment layers. Furthermore, alignment layers that improved bistable matrix display performance can be realized in the case of high pretilt angles.

A novel display mode based on surface-polymer-assisted vertically aligned (SPA-VA) liquid crystal is reported. The SPA-VA comprises a nematic liquid crystal with negative dielectric anisotropy and polymer nano-protrusions localized at the surfaces of electro-optical cells formed by photo-initiated polymerization of a small amount of a reactive monomer. With optimum monomer concentration, polymer density, and morphology, this polymer stabilization alignment technique enables fast electro-optical switching in VA liquid crystal cells.

Vertically aligned liquid crystal displays (VA-LCDs) have now been well developed and are widely used in TVs, monitors, notebooks, and mobile devices due to its many advantages over other LC modes. In particular, the contrast ratio in the normal direction is very high, and by adopting a multi-domain technique, a wide viewing angle is also possible. In contrast with some other LC modes, yields are higher because of the possibility of rubbing-free mass production, and there is the additional possibility of creating transflective displays. In this section, various VA-LCD modes and their respective advantages as well as the principal configuration of transflective displays are discussed.

In mid-1990s, the birth of the liquid crystal display (LCD) desktop monitors heralded the dawn of wide viewing angle technologies and future advancements in this field. We review the initiation and advancement of the patterned vertical alignment (PVA) technology as an approach that is different from other wide viewing angle technologies.

In-plane switching (IPS) liquid crystal display (LCD) is known to display high image quality comparable to that of the cathode ray tube (CRT). Nevertheless, the device has several drawbacks, such as high operating voltage, low transmittance, and high sensitivity in the process margin. The fringe-field switching (FFS) LCD overcomes such drawbacks of the IPS device, while maintaining wide viewing angle. Now, it has become a major device, applied to all types of LCD applications and produced by the majority of companies. This chapter reviews the history, switching principle, main electro-optic characteristics, and future of IPS and FFS devices.

The fabrication of 160° wide-viewing-angle, high-contrast-ratio liquid crystal displays (LCDs) was a major achievement in LCD development. In this chapter, progress in WVA LCD technology is reviewed, with some highlights of my personal excitements of the milestones achieved in LCD development, in this important and exciting period of LCD technology developments.

An outstanding route for controlling the diffraction of light by using light is exploited. To this end, we report on the fabrication and characterization of diffraction gratings based on a polymeric microstructure including a light responsive material, which enables an all-optical control of the diffraction of light. The all-optical switching of the grating between highly diffractive (more than 80% efficiency) and non-diffractive transparent states is realized by a (visible) photochemical phase transition between nematic and isotropic phases based on the photoisomerization of the azobenzene guest molecules. The switching, which is reversible and repeatable, is detected through the change in the diffraction efficiency of a low power probe light (λ = 633nm) triggered by a visible (λ = 532nm) pump irradiation. A detailed photophysical and all-optical characterizations is reported.

Born as a result of a lucky encounter between two rising disciplines and two scientists, the OLC conferences provide great opportunities and pleasant settings for scientists and engineers active in the field of optics of liquid crystals to discuss their recent results, review current research trends, and meet the top experts in the multi-disciplinary field comprising complex materials and optical systems. The legacy of OLC meetings constitutes a breeding ground for new ideas, new collaborations, and new friendships.

In this chapter, various methods for measuring liquid crystal cell parameters such as cell thickness, twist angle, and pretilt angle are presented. These factors play a very important role in the operation of liquid crystal displays because the electrooptical properties of liquid crystal displays are affected significantly by these LC cell parameters.

Three-dimensional (3D) display is expected to emerge as the dominant next-generation display technology for showing images more realistically on screen. Although, observers could perceive 3D images with their naked eyes using autostereoscopic 3D technologies, degradation in image quality still occurs. In this chapter, various liquid crystal lens (LC-lens) technologies for improving the issues are illustrated, followed by the future developments of LC-lens on 3D applications.

Recent results on the interaction of liquid crystals with the angular momentum carried by light beams are reviewed, with particular attention focused on the exchange of the orbital angular momentum of light. A novel liquid-crystal–based device, the q-plate, is also introduced and some of its applications in both classical and quantum optics are presented.

Three types of fast-response liquid crystal (LC) materials and devices: nematic LC, polymer network liquid crystal (PNLC), and polymer-stabilized blue phase liquid crystal (BPLC) are investigated and their corresponding physical mechanisms described. For nematic LCs, thin cell gap approach, elevated temperature operation, and overdrive and undershoot driving schemes can be employed to achieve fast response. While in PNLC, the strong anchoring force from polymer network assists the LC molecules to relax back when the applied voltage is removed. Sub-millisecond response time could be obtained through the dramatically increased elastic constant. However, the tradeoff is the increased voltage. Polymer-stabilized BPLC is based on Kerr-effect–induced isotropic to anisotropic transition. With short pitch length, strong polymer network, and low viscosity through temperature effect, a device with microsecond response time is demonstrated. These LC materials and devices hold promise for next-generation display and photonics applications.

We present several novel liquid crystal display (LCD) concepts enabling fast switching of the liquid crystal molecules in LCDs, with the response times being in the microseconds range for most of the cases. Surface-localized polymeric network and field-induced flexoelectric polarization are among the concepts that are employed in LCDs for obtaining a fast electro-optic response. Proper assembling and electronic driving of two parallel liquid crystal cells, arranged in a double-cell configuration, is another promising concept which enables field control of both switching times, and rise and fall time, thus enabling fast switching between bright and dark state of the double cell operating as a liquid crystal light shutter. We discuss these concepts in terms of device performance and their suitability for application in LCDs.

A three-dimensional (3D) display provides one or more physiological depth cues for 3D information. An integral imaging method using binocular disparity as a depth cue has been extensively studied for the realization of an autostereoscopic display through the flat-panel-type implementation. For the 3D display based on a liquid crystal (LC) panel, an integrated elastomer substrate possessing a lenslet array, together with spacers for the uniform cell gap, has been developed to be used as the top substrate of the LC panel. The integrated top substrate, being capable of eliminating the difficulty of the precise alignment of a lenslet array on the LC panel, can be fabricated through the imprinting process of an elastomeric material, polydimethylsiloxane. In addition to the advantages of being simple and light weight, it is expected that the elasticity of the integrated elastomer substrate would lead to a new scheme of developing 3D displays.

Polymer-dispersed liquid crystals (PDLCs) have been the subject of extensive research and development for the last 30 years. The classic PDLCs were light scattering films formed by either microencapsulation or phase separation, and the resulting PDLC films offered great promise as switchable windows and displays. While the classic PDLC has found applications only in a few niche markets, they opened up an entire field of research combining polymers and liquid crystals. A deeper understanding of the basic science of PDLC formation and operation led to the development of holographic PDLCs and the use of low-concentration polymer networks to stabilize and modify the performance of liquid crystal devices. Researchers are now exploring spinning as a possible solution to combine polymers and liquid crystals into fibers. Combining the properties of polymers and liquid crystals has been a powerful tool to modify the properties and performance of both materials. Therefore, research in this field is promising in the foreseeable future.

Polymer-stabilized liquid crystals are composites where the polymer networks are dispersed in the liquid crystals. The polymer networks are anisotropic and mimic the structure of the liquid crystals during the polymerization. They can stabilize the liquid crystals in desired states and improve the performance of existing liquid crystal displays as well as create liquid crystal devices.

Simulations demonstrate that extremely high contrast ratios can be achieved through self-compensation in twisted VAN structures, obviating the need for external trim compensators. The twisted structure also suppresses the formation of defects at the pixel level that are associated with slow response. A practical example is given for an LCoS imager, which can achieve a theoretical contrast ratio exceeding 700,000 with f/2.4 projection optics.

Transmissive liquid crystal displays (LCDs) are not readable under strong ambient light when used outdoors. Moreover, high power consumption is inevitable in transmissive LCDs because a backlight unit is turned on at all times. On the other hand, reflective LCDs are not readable in dark environments since they rely on ambient light. In this chapter, we explain how to design a transflective LCD capable of achieving good visibility in both outdoor and indoor environments. We introduce several types of transflective LCDs that can overcome the drawbacks of reflective and transmissive LCDs.

Liquid crystal displays (LCDs) are extensively used in many consumer and professional products. Low power consumption is essential in portable devices to extend the life of dry cells or to reduce the frequency of recharging the cells that are the main source of power in portable products. Low power consumption demands a multi-pronged approach, beginning with identifying the right choice of an electrooptic effect for the LCD to selecting the appropriate design, materials, processes, and the methods employed to drive the display. Electronic means of achieving low power consumption is the main focus of this chapter. Simple methods to reduce power consumption by reflective-type passive matrix LCDs are reviewed first, followed by a discussion on universal waveforms to reduce power consumption of active matrix LCDs as well. Other methods to reduce power consumption of active-matrix LCDs are reviewed next. Most LCDs come with a backlight and power consumption of backlight is much higher than the LCDs that act as light modulators. The chapter ends with a review of methods to reduce power consumption of light sources in LCD.

The following sections are included:

• PHOTOGRAPHS

• Index