Liquid crystal display geometry

The switching of the director in the surface stabilised ferroelectric liquid crystal cells (SSFLC) [8] has briefly been discussed in Section 13.1.2. Due to its importance for ferroelectric liquid crystal displays we shall discuss this effect in more detail. The geometry of a planar cell of thickness d is shown in Fig. 13.1.2. Now, the helical structure is considered to be unwound. We are interested in the field and time behaviour of the director or c-director given by angle cp(r), and this process is considered to be independent of z and y- coordinates. The smectic C equilibrium tilt angle 9 is assumed constant. 

The most important multicolour display technology in current use involves liquid crystal displays (LCDs), which overtook CRTs in popularity around the mid-2000s. Liquid crystals (LCs), commonly referred to as the fourth state of matter, are materials that are intermediate in character between the crystalline solid and liquid states.Unlike normal isotropic liquids in which the molecules essentially adopt a randomised orientation, liquid crystals show some time-averaged positional orientation of the molecules. In this sense, they resemble solid crystalline materials, although they retain most of the properties of liquids, notably the ability to flow. They are formed most commonly from molecules with rod-like geometry, which are referred to as calamitic. These molecules may orientate in various ways to form different types of LC phases [mesophases). There are three main mesophase types smectic, nematic and chiral nematic. In the smectic mesophase, as illustrated in Figure 11.1(a), the molecules are arranged in raft-like layers with their molecular axes parallel. These layers can pass over each other as the material flows. In the... 

Liquid Crystal Displays (LCD) contain a thin layer of liquid crystal material sandwiched between two substrate glasses containing electrode patterns (e.g. indium oxide) These patterns are made by a photolithographic procedure, followed by etching of the electrode material. Screen printing is also used, but only for large geometries. This technique was discussed in Sect. 5.3. As LCD s have become very popular in such products as watches and calculators, this field is an important consumer of photoresist materials. 

J.L. West, The challenge to new applications to liquid crystal displays, in Liquid Crystals in Complex Geometries Formed by Polymer and Porotis Networks (edited by G.P. Crawford and S. Zumer), Taylor and Francis, London, 1996, pp. 255-264. 

Brief explanation of the geometry of the liquid crystal displays (upper row), and their addressing method by time multiplexing (lower row). 

The realization of this device geometry was first applied in 1980 in the surface-stabilized ferroelectric liquid crystal display and provided much faster switching times than the nematic devices of the time (<0.1 ms) however, the main drawback of the smectic device has been the stability of liquid crystal alignment within the pixels. Nematics are very fluid-like, and after a deformation, they rapidly revert to their previous uniform state of alignment (think about what happens when you press on your laptop screen). Smectics are much more viscous and unfortunately do not self-repair when deformed. 

Gold(III) usually displays a square planar geometry, typically observed in d8 metallic complexes such as palladium(II), platinum(II), rhodium(I) and iridium (I), for which an enormous number of liquid crystals have been described [3-5], mainly as orthometallated compounds. However, only a gold(III) metallomesogen has been published. Since the first gold mesogen was reported in 1986, many other compounds have been described. 

In the past few decades the technological possibilities and interests have boosted research in systems in highly restricted geometries in almost every field of physics — recently down to lengthscales close to or even below the molecular level. In the field of liquid crystals, the importance of electro-optical applications which incorporate ordered liquid materials [1-3] has focused the research on LC systems with high surface-to-volume ratio [4]. In order to provide mechanically stable applications, liquid crystals are dispersed in polymers, stabilized by a polymer network, fill the cavities in porous materials, etc. [5,6]. The major technological interest concerns the scattering, reflective and bistable displays, optical switches, and others. 

To conclude, the research on directed assembly of Sm LCs in complex geometries has just begun. The advances on how to control FCDs in thin films could lead to a new paradigm of liquid crystal soft lithography for micro- and nanofabrications, which in turn will extend applications beyond displays and... 

Now we consider a TN display whose geometry is shown in Figure 3.3 [11,12]. The TN liquid crystal is sandwiched between two polarizers. The x axis of the lab frame is chosen parallel to the liquid crystal director at the entrance plane. The angles of the entrance and exit polarizers are a, and a , respectively. The Jones vector of the incident light is... 

When a bistable TN display is optimized, the transmittance of one of the stable states should be 0 and the transmittance of the other stable state should be 1. The parameters of the display are the twist angles (f). In + (p) of the stable states, the angle a,- of the entrance polarizer, the angle of the exit polarizer, and the retardation F of the liquid crystal. As discussed in Chapter 3, the transmittance of a uniformly twisted nematic display in the geometry shown in Figure 3.3 is... 

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