Optical Properties of Nematics for Applications

T. Ohtsuka, H. Ohnishi, H. Takatsu [ Datareview in this book 11.1 Optical properties of nematics for applications ]... 

Optical properties of nematics for applications TABLE 2 continued... 

From the point of view of physics, LCs are partially oriented fluids that exhibit anisotropic optical, dielectric, magnetic, and mechanical properties. The most important property of LCs is the reorganization of their supramolecular structures on external stimuli such as electric and magnetic fields, temperatnre, and mechanical stress, which lead to changes in their optical properties. In particular, electric tiled-induced control of optical properties of LCs (electro-optical effects based on the Freedericksz transition ) is at the heart of the multi-billion dollar liquid crystal display (LCD) industry. Most current LCD technologies rely on nematic " and to a lesser extent on ferroelectric LCs, while the recently discovered bent-core and orthoconic LCs still require significant investment into fundamental research and development. These and other applications and technologies continne to drive the search for new liquid crystal materials, and provide impetus to continue fundamental studies on new, often exotic, classes of compounds. 

Apart from high strength materials formed from nematic polymer fibres, most applications of nematic liquid crystals depend on their anisotropic optical properties. As a consequence the refractive indices of nematics are of prime importance in the development of materials for applications. The refractive indices are determined by the molecular polarisability coupled to the mientational order of the mesogens in the liquid crystal phase, so refractive indices can provide a direct probe of the order parameter. Furthermore the optical properties of liquid crystal films are frequently used to determine phase behaviour, identify phase types through characteristic optical textures or explore the properties of defects, and such experiments rely on the anisotropy in the refractive index of the material. The first tool of a liquid crystal scientist is the polarising microscope, which emphasises the importance of optical properties in general and refractive indices in particular to the stufy of liquid crystals. 

There are as yet ho accepted standard materials for discotic nematic liquid crystals. It is expected that the birefringence of these materials will be negative, and they have already found application as optical compensation films for liquid crystal displays [23]. In order to illustrate an example of the optical properties of discotic nematic liquid crystals, we give in TABLE 10 measurements of the ordinary and extraordinary refractive indices of hexakis[(4-octylphenyl)ethinyl]benzene (Tm = 100 C). These are taken from [24] and were measured for a wavelength of 589 nm. 

By casting films from suspensions of cellulose microcrystals, cellulose films with the optical properties of chiral nematic liquid crystals can be prepared. The films can be tailored to give different colors of reflected light by altering the salt content of the suspension for a given source of cellulose and set of hydrolysis conditions. Possible areas of application include optical variable films and ink pigments for security papers [81]. 

This technique allows one to distinguish between hole and electron transport and measure this across a broad range of fields and at different temperatures. As the optical properties of liquid crystals are commonly studied in liquid crystalline cells, which have electrodes, the cells make ideal ToF samples. It is worth noting that application of an electric field across a liquid crystalline cell may disrupt the orientation of the molecules in the mesophase particularly in low-viscosity, high-temperature nematic phases. It is necessary to verify, using polarised microscopy, that such switching is not occurring in samples used for mobility measurements. 

Polarized light is the must powerful tool for investigating liquid crystals, all of which exhibit characteristic optical properties. A smectic liquid crystal transmits light more slow ly perpendicular to the layers than parallel to them. Such substances are said to be optically positive. Nematic liquid crystals are also optically positive, bui their action is less definite than that of smectic liquid crystals. However, the application of a magnetic field to nematic liquid crystals lines up their molecules, changing their optical properties and even their viscosity. 

---