Electrooptic devices, applications

In addition to conventional applications in conducting polymers and electrooptical devices, a number of recent novel applications have emerged. Switching of DNA electron transfer upon single-strand/double-strand hybridization fonns the basis for a new medical biosensor teclmology. Since the number of base pairs of length 20... 

Dendrimers are a special class of arborescent monodisperse nanometer sized molecules that have been used in the synthesis of Au NPs as surface stabilizers or nanoreactor/templates for nanoparticle growth. Moreover, these hybrid nanomaterials have great potential for application in different fields such as sensors, imaging in cells, electrooptical devices, catalysis, drug delivery agents, and so on. 

Knowledge of the electrooptic behavior of the FLCPs is of the utmost importance for display device applications. One relevant parameter in this respect is the response time. As for the spontaneous polarization, the determination of the response time requires a uniformly aligned sample. The test cell is placed between crossed polarizers so that one tilt direction is parallel to the direction of one polarizer. The electrooptic effect is achieved by applying an external electric field across the cell, which switches the side chains from one tilt direction to the other as the field is reversed. A photodiode measures the attenuation of a laser beam when the cell is switched between the two states. Generally, the electrooptical response time is defined as the time corresponding to a change in the light intensity from 10 to 90% when the polarity of the applied field is reversed ( 10-9o)-... 

Transparent. For applications in which a transparent electrode is required metals are completely unsuitable. Metals, because of their very small Eg, can absorb all wavelengths of visible light and are, as a result, opaque in the visible part of the electromagnetic spectrum. Only metal layers <1 pm will be transparent. A ceramic used as a transparent electrode is indium tin oxide (ITO). A typical electrode composition is 90In203-10Sn02. Transparent electrodes are important for many electrooptic devices. 

Ferroelectric materials have numerous microelectronics applications, including capacitors (7,2), nonvolatile memory devices (2-4 electrooptic devices 1,4) and many others (5). The ferroelectrics described in this paper are Pb(ZrxTii x)03 (PZT), BaTi03, and YMn03. Here we investigate the use of a photochemical method for the direct deposition of these complex materials. 

A special class of materials that is also ferroelectric are electrooptic ceramics. Materials such as lanthanum-modified lead zirconate titanate (PLZT) produce excellent electrooptic devices. These polycrystalline ceramics exhibit voltage-variable behavior—that is, they can be switched from optically transparent to opaque by the application of voltage. Most of these devices, which are used for shutters, modulators, and displays, are processed by hot pressing to full density. Experiments in many laboratories are being carried out to tape-cast these materials into thin sheets. The main problem encountered to date has been the ability to sinter to full density. The use of nanosized powders has helped in this regard. The ability to tape-cast large sheets could open a wide variety of applications for these materials. 

For every nonlinear optical effect, one would expect that there is a measurement technique to characterize it. Not all of NLO effects, however, are subject to measurements that are convenient or informative for comparative purposes or device applications. This section highlights a few of the more common test methods for NLO organic molecules and polymers, and provides references for more detailed explanations of these techniques. Significant omissions here are techniques based on the linear and quadratic electrooptic effect, which are discussed in the article Electrooptical Applications. 

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. 

The interaction of liquid crystals with neighbor phases (gas, liquid, solid) is a very interesting problem relevant to their electrooptical behavior. The structure of liquid crystalline phases in close proximity to an interface is different from that in the bulk, and this surface structure changes boundary conditions and influences the behavior of a liquid crystal in bulky samples. The nematic phase is especially sensitive to external agents, in particular, to surface forces, and the majority of papers devoted to the surface properties of mesophases have been carried out on nematics. In addition, the nematic phase is of great importance from the point of view of applications in electrooptical devices. Thus, in this chapter, we will concentrate on surface properties of nematics, though the properties of the other phases will not be skipped either. 

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. 

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. 

Due to their unique mechanical, thermal, optical, chemical, and electrical properties, the other refractory semiconductors are anticipated to find applications in thermoelectric, electrooptic, piezoelectric, and acousto-optic devices as well as protective coating, hard coatings, and heat sinks. The refractory semiconductors described in Chapters 15-27 are progressing year by year and differ from material to material. These chapters cover the issues related to crystal growth microstructure defects doping electric, thermal, and optical properties and device applications, while identifying common themes in heteroepitaxy and the role of defects in doping, compensation, and phase stability of unique classes of materials. 

Among the many applications of LB films, the creation or arrangement of colloidal particles in these films is a unique one. On one hand, colloidal particles such as 10-nm silver sols stabilized by oleic acid can be spread at the air-water interface and LB deposited to create unique optical and electrooptical properties for devices [185]. 

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