Electrooptical Cells

FIGURE 3.15. Configuration of electrooptical cells of (a) sandwich type, (b) planar type, and (c) sandwich structure with interdigital electrodes. 1—Glass plates 2—electrodes 3—dielectric spacer. 

FIGURE 3.16. Illustration of the mechanisms for (a) planar orientation by an obliquely evaporated thin film of metal, and (b) homeotropic orientation by a monolayer of surfactant molecules. 1—Substrate 2—obhquely evaporated film 3—molecules of the surfactant material 4— nematic liquid crystal molecules. 

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On the other hand, the interest towards this field is accounted for by the possibility to create polymeric systems, combining the unique properties of low-molecular liquid crystals and high molecular compounds, making it feasible to produce films, fibers and coatings with extraordinary features. It is well-known that the utilization of low-molecular thermotropic liquid crystals requirs special hermetic protective shells (electrooptical cells, microcapsules etc.), which maintain their shape and protect LC compounds from external influences. In the case of thermotropic LC polymers there is no need for such sandwich-like constructions, because the properties of low-molecular liquid crystals and of polymeric body are combined in a single individual material. This reveals essentially new perspectives for their application. 

Fig. 10.18 Electrooptical cells of sandwich (a) and planar type (b) and the structures with in-plane interdigitated electrodes (c)...

Fig. 11.27 Converse flexoelectric effect (a) Structure of the electrooptical cell, (b) Distribution of the director angle over the cell thickness pictured by lower straight lines for zero (solid line) and finite (dot line) anchoring energies, respectively. The upper curves show spatial dependence of two principal refraction indices no (dash line) and (z) (solid line)...

It is very spectacular that the electrooptical cell shown in Fig. 12.10 can provide very high and spectrally tunable optical contrast between the field -off and -on states. To this effect, we install the cell between two polarizers and each of them should precisely be oriented at particular angles. Using variable optical anisotropy the spectral band of high contrast may be done either very narrow and tunable (for large An) or very wide for white light applications (small An). 

Similar materials could be obtained by an emulsification method [253]. Nematic liquid crystal is emulsified into an aqueous dispersion of a water-insoluble polymer colloid (i.e., latex paint). An emulsion is formed which contains a droplet with a diameter of a few microns. This paint emulsion is then coated onto a conductive substrate and allowed to dry. The polymer film forms around the nematic droplets. To prepare an electrooptical cell a second electrode is laminated to the PDLC film [253]. In the phase separation and solvent-casting methods the chloroform solutions of liquid crystal and polymer are also used [254, 255]. The solution is mixed with the glass spheres of the required diameter to maintain the desired gap thickness and pipetted onto a hot (140 °C) ITO-coated glass substrate [255]. After the chloroform has completely evaporated another ITO-coated glass cover is pressed onto the mixture and then it is cooled down. Structural characteristics of the PDLC films are controlled by the type of liquid crystal and polymer used, the concentration of solution, the casting solvent, the rate of solvent evaporation, perparation temperature, etc. [254]. 

In this section we will consider another type of nonuniform liquid crystal structures in nematics. These structures are created by a spatially nonuniform electric field, and have nothing in common with the modulated orientational and electrohydrodynamic patterns discussed above which, in fact, were created as a result of self-organization. A spatially nonuniform electric field exists in an electrooptical cell in many important cases such as, photosensitive liquid crystal cells [152-154], spatial light modulators with matrix addressing [152], liquid crystal defectoscopy of surfaces [155], liquid crystal microlens [156], etc. By analyzing the liquid crystal electrooptical behavior in a nonuniform field we can estimate different characteristics of the layer, in particular, sensitivity (i.e., the intensity of the optical response at a given voltage), spatial resolution, etc. 

The boundary conditions at the walls of an electrooptical cell have a strong influence on a layer of cholesteric liquid crystal enclosed between these walls. With layer thicknesses d greatly above the equilibrium pitch Pq> the... 

FIGURE 7.6. Different methods for measurements of the rotational viscosity 7< (a) dynamics of transmission I t) in the electrooptical cell (b) the repolarization current ip versus time t and (c) the repolarization current for the special (triangular) form of field E t). 

Figure 37. Schematic diagram of a twisted nematic electrooptic cell for (a) zero voltage and (b) a voltage above threshold, V,h(TN). Note that some chiral nematic mesogens remain anchored in a planar arrangement on the alignment surface, which then provides the coupling for the field-off decay back to the twisted structure. The weak chiral nature prevents back flow.

Figure 38. Schematic operation of the White-Taylor dye guest-host chiral nematic electrooptic cell. In (a) for zero applied field the axis of each focal-conic domain is random in the x, y plane, as therefore is the dye, using homeotropic surface alignment. In (b) the texture is planar for the zero field state and therefore the dye spirals around the z direction. In (c) the focal conic (a) or planar (b) transition to homeotropic nematic has taken place above the threshold voltage V,], (WT). The black ellipses represent the dyes in the chiral nematic matrix.

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

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. 

Figure 4.6 Working principle of a twisted nematic (TN) cell in the normally white" configuration (left), and the change of transmission with increasing applied voltage (right). In the cell configuration sketched above the threshold voltage (V,, ) for the electrooptical response corresponds to approximately V90 for 90% of maximum transmission.

GaAs Semiconducting devices electrooptics (includes solar cells)... 

The photoinduced phenomenon where the change in refractive index is proportional to the second order term of applied voltage is called an electrooptical Kerr effect. Kerr cells of liquid or solid transparent media can be used together with polarizers to transmit or block light, depending on whether an electric field is applied or not. Such light switches have many uses in laser technology. 

Liquid crystals are unique molecular materials because of their anisotropic nature and molecular dynamics [1-4]. Over the last three decades, these materials have been developed as advanced materials for electrooptical applications such as display devices. Liquid crystals also have close relationships to biomolecular systems [5]. Cell membranes form dynamic and anisotropic molecular states, which possess liquid-crystalline behavior. Recently, liquid-crystalline complexes of DNAs and liposomes have been considered as potential systems for gene therapy [6]. The design of liquid crystals by using a variety of structures and interactions may lead to wider applicability of mesomorphic materials. 

Melendez J., de Castro A. J., Lopez F., and Meneses J., Spectrally selective gas cell for electrooptical infrared compact multigas sensor. Sens. Actuators A, 46(47), 417-421, 1995. 

The thickness of the organic layer or, in multilayer devices, of the organic layers, is as a rule in the range between 10 and a few 100 nanometers. The electrooptics of organic devices is thus also a nanotechnology. The optimisation of the contacts and the layer thicknesses is - along with the intrinsic materials parameters - of central importance for the efficiency of the devices, i.e. for the luminous yield of electroluminescent devices or for the electric power of photovoltaic cells. The devices must therefore be optimised by both controlled variation of the layers and layer thicknesses and by comparison with simulations. 

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)-... 

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