New Ferroelectric Liquid Crystal Polymers for Nonlinear Optics Applications
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more traditional inorganic crystals for some nonlinear optical applications. For instance, a recent paper by Meerholt et al? recounted the measurement of nearly 100% diffraction efficiency in a photorefractive polymer. The NLO response of inorganic materials results from collective band structure effects. Contrastingly, for organic materials, each individual molecule has its own degree of NLO response, and the overall NLO response of a material thus depends heavily on the degree of orientation of the molecules. An additional complication arises when second-order NLO responses are desired, since they require noncentrosymmetric materials. Thus either the material must be thermodynamically noncentrosymmetric, or some external force must be applied to make it noncentrosymmetric. The latter alternative leads to materials that are not thermodynamically stable, and the former alternative still requires specific control of the supermolecular structure. There are several approaches to making X(2) organic materials. The "conventional" approach involves the poling of organic polymers. Poled polymers for NLO are typically mixtures of dyes and polymers. The dye molecules are oriented by applying a large electric field across the material while cooling the polymer into a glass. The resulting materials are kinetically rather than thermodynamically stabilized, and can relax over time to their most stable states, a process that is accelerated by heating. They often show large decays in a matter of minutes, and from 20-70% decay in as little as two days. 4 This is unfortunate, since in addition to having high X(2), a useful secondorder NLO material must have thermal and temporal stability. Fortunately, there are alternatives to poled polymers. Ferroelectric liquid crystals (FLCs) are made up of chiral (noncentrosymmetric) molecules which spontaneously self-assemble into materials wherein the relative orientation of the molecules is coherent over large distances. In FLCs, useful magnitudes of X(2) result from constructive polar orientation of molecules in the material. The orientation of the NLO chromophores in FLCs is thermodynamically stable, so it does not degrade over time. For most NLO applications solids are preferable to liquids. FLC polymer glasses, wherein the order present in a high temperature FLC phase is frozen into a solid, are most attractive and are therefore the focus of this study. FLC polymers (FLCPs) possess the added advantage that in the high temperature liquid crystal phase, it is possible using electric fields to "pattern" the material with structures as small as 1 Pm 3 or as large as 1 [tm x 100 cm 2 . This type of spatial patterning is important for some interesting potential applications such as quasi phase-matched second harmonic generation and other types of guided-wave optics. Once cooled into the glass, any structures patterned into the FLCP should be permanent and stable. The FLCP may thus be considered a piece of plastic with permanent optical integrated circuits imprinted into it - certainly an attra
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