Organic electro-optic crystals

We recognize that there are applications in two- and three-dimensional waveguides (12,13) which do not have the same criteria of phase-matching as in simple crystals or that one may just as well be interested in screening these materials for the related electrooptic performance by the simple SHG powder method. (It has been shown for several organic materials that although the electro-optic and SHG x tensors are in principle unequal, due to dispersion and due to the possible contribution of atomic and molecular distortions... 

Figure 2 shows the tradeoffs between crystal and polymer organic NLO materials for device applications. Although cither type of materials could in principle be used for both applications, crystals are best for second harmonic generation, and poled polymers are best for electro-optic waveguide devices such as modulators and switches. 

Figure 2. Tradeoffs between polymer and crystal organic nonlinear optical materials. EO refers to applications for electro-optic waveguide devices such as modulators and switches. SHG refers to applications for frequency doubling of moderate and low power laser sources. A + indicates favored, - indicates disfavored, 0 indicates neither favored nor disfavored, and x indicates not relevant.

Most reports over the past 4 years have dealt with the manipulation of display-related parameters such as electro-optic response and alignment, but increasingly also with thermal effects, pattern formation, nanoparticle-liquid crystal compatibility (i.e., enhancing the stability of dispersions), and to some degree with nanoparticle organization. 

Monodisperse spherical colloids and most of the applications derived from these materials are still in an early stage of technical development. Many issues still need to be addressed before these materials can reach their potential in industrial applications. For example, the diversity of materials must be greatly expanded to include every major class of functional materials. At the moment, only silica and a few organic polymers (e.g., polystyrene and polymethylmethacrylate) can be prepared as truly monodispersed spherical colloids. These materials, unfortunately, do not exhibit any particularly interesting optical, nonlinear optical or electro-optical functionality. In this regard, it is necessary to develop new methods to either dope currently existing spherical colloids with functional components or to directly deal with the synthesis of other functional materials. Second, formation of complex crystal structures other than closely packed lattices has been met with limited success. As a major limitation to the self-assembly procedures described in this chapter, all of them seem to lack the ability to form 3D lattices with arbitrary structures. Recent demonstrations based on optical trapping method may provide a potential solution to this problem, albeit this approach seems to be too slow to be useful in practice.181-184 Third, the density of defects in the crystalline lattices of spherical colloids must be well-characterized and kept below... 

The EO effect is a second-order nonlinear optical (NLO) effect. Only non-centrosymmetrical materials exhibit second-order NLO effects. This non-centrosymmetry is a condition, both at the macroscopic level of the bulk arrangement of the material and at the microscopic level of the individual molecule. All electro-optic modulators that are presently used by telecom operators are ferro-electric inorganic crystals. The optical nonlinearity in these materials is to a large fraction caused by the nuclear displacement in the applied electric field, and to a smaller fraction by the movement of the electrons. This limits the bandwidth of the modulator. The nonlinear response of organic materials is purely electronic and, therefore, inherently faster. 

ICP thin films were used as driving electrodes for polymer-Dispersed Liquid-Crystals (PDLC) display devices. Liquid-crystalline-based display devices, which are commonly made of a liquid-crystal compound sandwiched between two substrates coated with a conducting layer of indium tin oxide (ITO), whose substitution with ICP electrodes could improve the optical and mechanical properties of the display devices. On the way to all-organic displays, PDLC sandwiched between two plastic substrates coated with ICP layers are promising devices for paper-like displays for electronic books which require flexibility, lightness, and low-power consumption. The electro-optical characteristics (transmission properties, drive voltages and switching times) of the PDLC devices depend on the nature of the ICP substrate used [13]. 

Singer, K. D., Lalama, S. L., Sohn, J. E., and Small, R. D. Electro-optic Organic iVlatermls. Chapter II-8, in Nonlinear Optical Properties of Organic Molecules and Crystals, Volume L Edited by D. S. Chemla and J. Zyss, Academic Press, Orlando, 1987. 

A wealth of information about photolumescent materials, their properties and conditions for their optimum preparation is found in [257]. In combination with glass substrates coated with transparent electrodes, the electro-optical properties of liquid crystal films, which themselves emit no light, offer some interesting possibilities [258, 259]. Liquid crystals have the mobility of liquids and the optical properties of solids. The molecular structure lies between the liquid and solid state. Liquid crystals are organic compounds of relatively long molecules compared with their diameter, and often contain polar groups and multiple bonds. 

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