Langmuir-Blodgett technique symmetry

Any surface or interface breaks the inversion symmetry and is therefore a possible source of second-order effects. Owing to their surface sensitivity, second-harmonic generation measurements have developed into a very useful tool for probing the orientation of organic molecules in well-structured monolayers, such as those obtainable by the Langmuir-Blodgett technique (see Table 6). The surface susceptibility may in general be written as... 

To measure the force between lipid layers, a lipid is chosen, which at a certain concentration and temperature range forms an L phase. The L phase is a regularly spaced stack of lamellar fluid bilayers separated by water. From a symmetry point of view, the L phase can be considered a smectic-A (SmA) liquid crystal. The mean repeat distance between lipid bilayers in the L phase is measured by X-ray diffraction versus an applied osmotic pressure. Direct experiments have been carried out with the surface force apparatus. Therefore, the bilayers are formed either by spontaneous vesicle fusion [1254-1256] or by depositing two subsequent monolayers with the Langmuir-Blodgett technique [1255, 1257]. Atomic force microscope experiments, which have been carried out between two bilayers formed by spontaneous vesicle fusion, confirmed earlier results [1258]. 

The Langmuir-Blodgett deposition technique shows excellent promise for the assembly of films with noncentrosymmetric ordering and Coo symmetry. A system that was reported recently (3i) is shown in Figure 6.11. In this system, monolayers of the hemicyanine dye and the nitrostilbene... 

However, organic polymers, being amorphous, do not show second-order NLO effects. In order to employ them in second-order NLO measurements a polar order is induced by an external means such as electric field poling process or the Langmuir-Blodgett (LB) technique. Poled polymers and the majority of the LB films possess Cocv symmetry. Therefore, the second-order optical susceptibility tensor for SHG has only three independent nonzero elements, namely, zS ZzM Under Klienman symmetry [9] conjecture... 

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