Liquid crystals optical properties

Liquid crystals— Electric properties. 2. Liquid crystals--Optical properties. I. Chigrinov, V. G. (Vladimir G.)... 

IC Khoo. Liquid crystals Physical Properties and Nonlinear Optical Phenomena. New York Wiley, 1995. 

Cogswell (1985) expressed it in the following words "To make the connection from the basic material properties to the performance in the final product, industrial technologists had to learn a new science". It is more or less so, that - for liquid crystal polymers -properties like stress history, optical and mechanical anisotropy, and texture seem to be independent variables this in contradistinction to the situation with conventional polymers. 

The turbid liquids which were thus obtained were found to possess not only the usual properties of liquids (such as the property of flowing and of assuming a perfectly spherical shape when suspended in a liquid of the same density), but also those properties which had hitherto been observed only in the case of solid crystalline substances, viz. the property of double refraction and of giving interference colours when examined by polarised light the turbid liquids are anisotropic. To such liquids, the optical properties of which were discovered by 0. Lehmann, the name liquid crystals, or crystalline liquids, was given." Since the term crystal implies the existence of a definite space lattice, which is not found in the case of liquid crystals, it is perhaps better to use the term anisotropic liquids, ... 

I. C. Khoo, Liquid Crystals Physical Properties and Nonlinear Optical Phenomena Wiley Interscience, New York 1994 I. C. Khoo and S. T. Wu, Optics and Nonlinear Optics of Liquid Crystals World Scientific Singapore 1993. 

Static force [45,50,51,57] and friction properties [45,46,51,57-59] have been extensively studied in thermotropic nematics. The surface anchoring conditions were symmetric, with both mica sheets inducing either planar or homeotropic alignment. Bare mica naturally induces planar anchoring on cyanobiphenyls such as 5CB the liquid crystal optical axis orients at abont 7t/6 with respect to the slow optical direction of a (birefringent) freshly cleaved mica sheet [60]. A homeotropic ahgnment can be obtained by adsorbing a mono-layer of surfactant [45,50] on mica. 

I. C. Khoo, Liquid crystals, physical properties and non-linear optical phenomena, (John Wiley Sons, New York, 1995). 

In the present volume we discuss a relatively new and rapidly developing branch of the field, namely nonlinear optical effects in liquid crystals. Optical studies have always played a significant role in liquid crystal science. Research of optical nonlinearities in liquid crystals began at the end of the sixties. Since then it became a powerful tool in the investigation of symmetry properties, interfacial phenomena or dynamic behaviour. Furthermore, several new aspects of nonlinear processes were demonstrated and studied extensively in liquid crystals. The subject covered in this book is therefore of importance both for liquid crystal research and for nonlinear optics itself. 

As this class of electrohydrodynamic phenomena is observed in the isotropic phase as well, the mechanism of its excitation is also called isotropic. Roughly speaking, a liquid crystal optically develops and makes visible isotropic instabilities, and the corresponding optical patterns reflect specific properties of the liquid crystal. 

Yang F, Zoriniants G, Ruan L, Sambles JR. Optical anisotropy and liquid-crystal alignment properties of rubbed polyimide layers. Liq. Cryst. 2007 341433-1441. 

Khoo IC (1981) Optically induced molecular reorientation and third-order nonlinear optical processes in nematic liquid crystals. Phys Rev A 23(4) 2077-2081 Khoo IC (1982a) Nonlinear light scattering by laser- and dc-field-induced molecular reorientations in nematic-liquid-crystal films. Phys Rev A 25(2) 1040-1048 Khoo IC (1982b) Theory of optically induced molecular reorientations and quantitative experiments on wave mixing and the self-focusing of light. Phys Rev A 25(3) 1636-1644 Khoo IC (1995) Liquid crystals physical properties and nonlinear optical phenomena. Wiley, New York... 

The recent work of Krigbaum on main-chain and Finkelmann, Ringsdorf, etc., on side-chain polymer liquid crystals has generated much interest in the potential of these systems for use in electro-optic devices. The combination of polymeric specific and monomeric liquid crystal specific properties leads to an interesting range of potential materials for new display devices. The majority of the research over the last five years has concentrated on synthesis and the establishment of the basic property-structure relationships. However in the last year or so papers have started appearing where the electro-optic properties of some of these materials have been examined." ... 

Parameters (ii)-(vii) depend on the dielectric, mechanical and optical properties of the mesogens. To optimize a dis compromise between different molecular characteristics is often required and mixtures of liquid crystals are usually commercial displays. 

Blinov L M 1983 Electro-optical and Magneto-optical Properties of Liquid Crystals (Chichester Wiley)... 

Simoni F 1997 Nonlinear Optical Properties of Liquid Crystals and Polymer-Dispersed Liquid Crystals (Singapore World Scientific)... 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Electro-optic The liquid crystal plastics exhibit some of the properties of crystalline solids and still flow easily as liquids (Chapter 6). One group of these materials is based on low polymers with strong field interacting side chains. Using these materials, there has developed a field of electro-optic devices whose characteristics can be changed sharply by the application of an electric field. 

As a result of the unique molecular electronic properties and geometrical structure, liquid crystals also tend to be optically nonlinear materials meaning... 

Many technological applications of liquid crystals, as in electro-optic display devices, are based on multicomponent mixtures. Such systems offer a route to the desired material properties which cannot be achieved simultaneously for single component systems. Mixtures also tend to exhibit a richer phase behaviour than pure systems with features such as re-entrant nematic phases [3] and nematic-nematic transitions possible. In this section, we describe simulations which have been used to study mixtures of thermotropic calamitic mesogens. 

In the previous sections, we have seen how computer simulations have contributed to our understanding of the microscopic structure of liquid crystals. By applying periodic boundary conditions preferably at constant pressure, a bulk fluid can be simulated free from any surface interactions. However, the surface properties of liquid crystals are significant in technological applications such as electro-optic displays. Liquid crystals also show a number of interesting features at surfaces which are not seen in the bulk phase and are of fundamental interest. In this final section, we describe recent simulations designed to study the interfacial properties of liquid crystals at various types of interface. First, however, it is appropriate to introduce some necessary terminology. 

Liquid crystals form a state of matter intermediate between the ordered solid and the disordered liquid. These intermediate phases are called mesophases. In the crystalline state the constituent molecules or ions are ordered in position and orientation, whereas in the liquid state the molecules possess no positional and orientational ordering. Liquid crystals combine to some extent the properties of both the crystalline state (optical and electrical anisotropy) and the liquid state (fluidity). 

Electro-Optical Properties of Bimetallic Nanoparticle-Doped Liquid Crystal Displays... 

An LCD is a ubiquitous electronic display. Now, it is widely distributed among human daily life, like mobile phones, TV, and personal computers. The LCD has, however, a drawback, i.e., slower response than a plasma display or an electroluminescene display. Recently we have first succeeded in combination of a nanoparticle technology with the LCD technology, which realized fast response of the LCD [45,235,236]. Thus we have found a phenomenon, i.e., a frequency modulation of the LCD doped with metallic nanoparticles. Since the frequency modulation, or electro-optic property depends on the kind of metals, we have prepared AgPd bimetallic nanoparticles protected with a typical liquid crystal molecule, 4-cyano-4 -pentylbiphenyl (5CB) to investigate the electro-optic property [45,235,236]. 

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