Liquid Crystals. Iam-Choon Khoo

, also precesses around . If, by some external field, the helical structure is unwound and points in a fixed direction, as in Figure 1.12d, then will point in one direction. Clearly, this and other director axis reorientation processes are accompanied by a considerable change in the optical refractive index and other properties of the system, and they can be utilized in practical electro‐ and opto‐optical modulation devices. More detailed discussions of smectic liquid crystals are given in Chapter 4.

      1.3.4. Functionalized and Discotic Liquid Crystals

Through various chemical synthesis techniques as well as nanotechnologies, an entire class of novel or so‐called functionalized liquid crystals have emerged [11–13]. Figure 1.14 shows, for example, the shuttlecock‐shaped liquid crystal formed by incorporating fullerene C60 to various crystals and liquid crystals reported by Sawamura et al. [11]. Others have investigated a special class of liquid crystals comprising disc‐like molecules, discotic liquid crystals, that possess interesting and useful semiconducting properties suitable for optoelectronic applications [12, 13].

1.4. MIXTURES, POLYMER‐DISPERSED, AND DYE‐DOPED LIQUID CRYSTALS

      In general, temperature ranges for the various mesophases of single constituent liquid crystals are quite limited. Therefore, while many fundamental studies are still conducted on such liquid crystalline materials, industrial applications employ mostly mixtures, composites, or specially doped liquid crystals with large operating temperature range and tailor‐made physical and optical properties.

Figure 1.14. A shuttlecock‐shaped liquid crystal formed by incorporating fullerene C60 into various liquid crystals was reported [11].

There are many techniques for modifying the physical properties of a liquid crystal. At the most fundamental level, various chemical groups such as bonds or atoms can be introduced to modify the LC molecule. A good example is the cyanobiphenyl homologous series nCB (n = 1,2,3…). As n is increased through synthesis, the viscosities, anisotropies, molecular sizes, and many other parameters are greatly modified. Some of these physical properties can also be modified by substitution. For example, the hydrogen in the 2, 3, and 4 positions of the phenyl ring may be substituted by some fluoro (F) or chloro (Cl) group [14].

      Besides these molecular synthesis techniques, there are other ways to dramatically improve the performance characteristics of liquid crystals. In the following sections, we describe three well‐developed methods, focusing our discussion on nematic liquid crystals as they exemplify the unique characteristics of liquid crystals widely used in optical and photonic applications.

      1.4.1. Mixtures

A large majority of liquid crystals in ubiquitous devices are eutectic mixtures of two or more mesogenic substances. A good example is E7 (from EM Chemicals), which is a mixture of four liquid crystals (cf. Figure 1.15). The optical properties, dielectric anisotropies, viscosities of E7 are very different from those of the individual mixture constituents.

Figure 1.15. Molecular structures of the four constituents making up the liquid crystal E7 (from EM Chemicals).

Creating mixtures is an art, guided, of course, by some scientific principles. One of the guiding principles for making the right mixture can be illustrated by the exemplary phase diagram of two materials with different melting (i.e. crystal → nematic) and clearing (i.e. nematic → isotropic) points, as shown in Figure 1.16. Both substances have small nematic ranges (T_i–T_n and ). When mixed at the right concentration [2], however, the nematic range () of the mixture can be orders of magnitude larger. To date, there is an assortment of nematic liquid crystals, for example, with an operating temperature range from sub freezing (<−30 °C) to well over 100 °C that found their way in ubiquitous display devices (Figure 1.16).

Figure 1.16. Phase diagram of the mixture of two liquid crystals.

      If the mixture components do not react chemically with one another, their bulk physical properties, such as dielectric constant, viscosity, and anisotropy, are some weighted sum of the individual responses. Since optical and other parameters (e.g. absorption lines or bands) are largely dependent on the electronic responses of individual molecules, they generally follow such a simple additive rule. Other physical parameters such as viscosities, transition temperature, and elastic constants are highly dependent on intermolecular forces and therefore follow more complex physio‐chemical rules (see e.g. [2, 13]).

      1.4.2. Dye‐doped Liquid Crystals

An obvious effect of introducing dye molecule to liquid crystals is to increase the absorption of a particular liquid crystal at some specified wavelength region. In particular, dye molecules with absorption anisotropy, or those that undergo conformation changes such as trans–cis isomorphism or produce photo‐charges, are often used for photonic applications [15–17]. For example, dichroic dye molecules that are more absorptive for optical field polarization parallel than perpendicular to its long axis are often used for the guest–host effect as their oblong shape makes them compatible for dispersing in the host nematic liquid crystals without disturbing the order. These dichroic molecules can then be oriented and reoriented by an external field applied to the host NLC to switch the transmission of the cell (cf. Figure 1.17); such dichroic dye‐doped liquid crystals have been utilized to demonstrate optical diode action [15] in the transmission of polarized light.

      If the dye molecules undergo some physical changes such as trans–cis isomorphism or produce space charges following photon absorption, they could give rise to nonlinear optical effects [16]; others [17] have shown that dye molecules deposited on the cell windows can be optically aligned as