Electrooptical chiral smectics

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. 

The structure of the smectic A phase when it is composed of optically active material (i.e., smectic A ) remains the same as that for the achiral phase. The molecules are arranged in diffuse disordered layers, and there is no long-range periodic order. However, because of the molecular chirality, the environmental symmetry is reduced to [10]. As a consequence, when an electric field is applied to a chiral smectic A= phase there will be a coupling of the electroclinic susceptibility to the field and the long axes of the molecules will tilt with respect to the layer planes. The tilt angle, for relatively low applied fields, varies linearly with the field. This linear electrooptic phenomenon is called the electroclinic effect. 

On the other hand, it has been shown on LMWLCs that the well-known SmC, where the molecules are tilted with respect to the layer normal, is no longer the only possibility to obtain a fluid biaxial phase [63], As a consequence, a strict determination of the chiral smectic phase structure requires not only a careful analysis of the X-ray diagrams obtained on powder as well as on aligned samples, but also a study of the electrooptic response, which allows discrimination between the ferroelectric, the antiferro-electric, and the ferrielectric behavior. 

This is a chiral smectic A with symmetry Dqo. Its properties are similar to those of the achiral SmA. However, close to the transition to the smectic C phase, the chiral smectic A phase shows interesting pretransitional phenomena in the dielectric and electrooptical effects (the so-caUed soft dielectric mode and electroclinic effect). They will be discussed in Chapter 13. 

Beresnev, L., Chigrinov, V. G., Dergachev, D. I., Poshidaev, E. P., Funfschilling, J., and Schadt, M., Deformed helix ferroelectric liquid crystal display a new electrooptic mode in ferroelectric chiral. smectic C liquid crystals, Liq. Cry.st., 5, 1171-1177 (1989). 

Display devices based on nematic liquid crystals, notably small liquid crystal televisions, are used as SLMs due to their increased availability [1]. However, in most of the above applications the speed of operation is important, and nematic liquid crystals are too slow, so the emphasis here is on SLMs that use the faster electrooptic effects to be found in chiral smectic liquid crystals. We will start by looking at how these interact with light. 

Molecular ordering and electrooptic switching in chiral smectic phases are discussed in detail elsewhere (see Chap. VII, Sec. 9 of this Volume). In the case of ferroelectric switching in chiral smectic C phases, the direetor n is free to move about a cone of angles which is centered on the horizontal axis. Each molecule has a ferroelectric dipole P, which is perpendicular to its length. This is depicted in Fig. 3. 

In Fig. 69 we see how the quasi-book-shelf (QBS) smectic constitutes a retarder with a switchable optic axis. In the smectic A phase this axis can be switched conti-nously, but with small amplitude. The same setup is also used to study the DHM effect in short pitch chiral smectics and the flexo-electrooptic effect in the cholesteric phase, in both of which we achieve much higher tilts. Inserted between crossed polarizers, the retarder transmits the intensity... 

Approximately 20 different smectic phases have been identified up to now [3]. Eight among them consist of so-called tilted phases, Le.. the long axes of the moleciiles are tilted with respect to the layer normal (, Sb Sr. So. Sh, Si, Sk. and Sm). If these latter mesophases consist of chiral molecules, they in principle match the requiremeni for intrinsic ferroelectric polarization. In six of these tilled chiral phases (denoted herein by an asterisk), spontaneous polarization has been measured (Sr. Sr. , So St. Sy ). For technological application in electrooptical devices, the chiral smectic C phase Sc is prominent due to its lowest ordering a hence highest fluidity, making reorientation processes caused by electric fields very find. 

The coefficient y is rotational viscosity of the director similar to coefficient yi for nematics. In fact, it does not include a factor of sin cp and, in the same temperature range, can be considerably larger than the viscosity ytp for the Gold-stone mode. This may be illustrated by Fig. 13.10 the temperature dependence of viscosities y and have been measured for a chiral mixture that shows the nematic, smectic A and smectic C phases [15]. The pyroelectric and electrooptic techniques were the most appropriate, respectively, for the measurements of ya and ytp describing the viscous relaxation of the amplitude and phase of the SmC order parameter. The result of measurements clearly shows that y is much larger than y and, in fact, corresponds to nematic viscosity yj. 

The helix pitch value Ro is easily controlled by varying the concentration of a chiral dopant in a smectic C matrix. To provide variation of the helix pitch from 0.1 fim to 100 m and more, we should have a chiral dopant with a high twisting power and good solubility. At the same time, the chiral dopant should not depress the smectic C temperature range [11]. Moreover, the growth of polarization should be more pronounced than that of the rotational viscosity 7<, with an increasing concentration of the chiral dopant. Various types of chiral dopants are presented in [11], and some of them were used to reveal the novel electrooptical modes in FLCs [22]. 

On removal of the low frequency field, in thick cells, the texture relaxes back to focal conic with residual light-scattering properties. This has been described as the storage mode or memory effect in chiral nematic systems [162]. If a high frequency ac field is now applied (o)> co ), the dielectric torque restores the nonscattering planar texture and, just as in smectic A materials, this effect can be used in electrooptic storage mode devices [165], since we have a low frequency write and high frequency erase mechanism. 

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