Electrooptical Effects in Ferroelectric Liquid Crystals

Much of this early effort dealt with modulator technology that is considered too slow (1-100 kilohertz) for high-speed applications such as optical interconnection and memory read/write. This includes modulators based on electrooptic effects in ferroelectric liquid crystals (ELCs) and in a ceramic containing lead, lanthanum, zinc, and titanium (PLZT). These electrooptic materials are bonded in some fashion to Si circuits to create hybrid SPAs. 

The current (third) period, which may be called a colonization, involves wide electrooptical investigations of novel effects in ferroelectric liquid crystals [9, 10] and a study of exotic materials like polymeric and lyotropic mesophases, blue phases in cholesterics, well-ordered smectics, and so on. For conventional (nematic and cholesteric) phases the accent was shifted to the optimization of the material properties for electrooptical devices, though novel phenomena like the supertwist effect [11] and a gamma of linear electrooptical effects [12-14] have also been discovered. 

It should be noted that phenomenologically this effect is analogous to the deformed helix electrooptical mode observed in ferroelectric liquid crystals where coupling of an electric field with the spontaneous (instead of flexoelectric) polarization is used [83, 84]. 

Concerning potential applications we considered only all-optical nonlinear effects. It should be mentioned that liquid crystals can be utilized in the so-called self-electrooptic-devices (SEED) too. In these devices there is an electrical feedback of the transmitted light to the layer. Such systems can be constructed by combining a photoconducting layer with a liquid crystal film. Ferroelectric liquid crystals are especially suitable for this purpose and may represent an alternative to the multiple quantum-well structures on which SEED-s are normally based. Whether all-optical effects will find a widespread application is still an open question. 

At present we do not know of any electrooptical effect which can compete in operation speed with ferroelectric liquid crystals. For instance, the record response time for nematic modulators, ever demonstrated [28], is only about 100 fJLS. 

This book was conceived as a renewed version of the earlier published original book, Electro-Optical and Magneto-Optical Properties of Liquid Crystals (Wiley, Chichester, 1983) written by one of us (L.M. Blinov). That book was first published in Russian (Nauka, Moscow, 1978) and then was modified slightly for the English translation. Since then new information on electrooptical effects in liquid crystals has been published. Novel effects have been discovered in nematics and cholesterics (such as the supertwist effect), and new classes of liquid crystalline materials, such as ferroelectric liquid crystals, appear. Recently, polymer liquid crystals attracted much attention and new electrooptical effects, both in pure polymer mesophases and polymer dispersed liquid crystals, were studied. An important contribution was also made in the understanding of surface properties and related phenomena (surface anchoring and bistability, flexoelectricity, etc.). 

The book is subdivided into three parts. The first three introductory chapters include consideration of the nature of the liquid crystalline state of matter, the physical properties of mesophases related to their electroop-tical behavior, and the surface phenomena determining the quality of liquid crystal cells giving birth to many new effects. The second part (Chapters 5-7) is devoted to various electrooptical effects in nematic, cholesteric, and smectic mesophases including ferroelectric compounds. Here major emphasis is given to explaining the physical nature of the phenomena. The last part (Chapter 8) is a rather technical one. Here recent applications of liquid crystalline materials in electrooptical devices are discussed. 

Table 8.7 shows that the parameters of the prototype light valve (CdS-nematic) are much worse than that of the a Si-FLC device. The operation speed of the latter comes closer to the solid electrooptical crystal modulator (PROM), but with a considerably higher resolution. Liquid crystal light valves on a Si-FLC operate using the Clark-Lagerwall mode [21], the electroclinic eflFect [22], or the deformed helix ferroelectric effect [24]. The operation speed in the two mentioned cases could be 10-100 times faster than mentioned in Table 8.7. 

In this book the authors present a complete and readily understood treatment of virtually all known phenomena occurring in liquid crystals under the influence of an electric field. In the first three chapters (Chapters 1-3) bulk and surface properties of liquid crystalline materials are discussed. The next two chapters (4, 5) are devoted to consideration of the electrooptical effects due to the formation of uniform and spatially modulated structures in nematics. In Chapters 6 and 7 the electrooptical properties of the cholesteric and smectic mesophases are presented, including a discussion of ferroelectric materials. Major emphasis is given to explaining the qualitative aspects of the phenomena and to portraying their physical basis. The prospects for the practical application of electrooptical effects are also discussed (Chapter 8). 

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