Second-order susceptibility Langmuir-Blodgett

To fit the experimental results, it is necessary to fix the overall phase. This can be done, for example, by defining h as a real quantity (/ / = 0). The values found for the coefficients /, g, and h can then subsequently be used to calculate the values of the components of the second-order susceptibility, X(2). This is done in detail for a Langmuir-Blodgett film of a poly(isocyanide) in the following section. Note that both phase and magnitude of all tensor components are relative values. The absolute phase cannot be determined... 

In this section, the experimental techniques described in the previous section are applied to the study of thin Langmuir-Blodgett films of chiral molecules and polymers. We will show in detail how the second-order susceptibility of chiral thin films can be analyzed and discuss the influence of chirality on the nonlinear optical response of these films. 

Detailed information about the components of the second-order susceptibility y2)(-2w to, w) can be obtained from second harmonic measurements on well-defined samples such as single crystals or oriented thin films, the latter obtained by procedures such as the asymmetric Langmuir-Blodgett deposition technique or electric-field poling of NLO chromophore-doped polymers.31 In the case of single-crystal samples, the second harmonic is... 

The requirement of non-centrosymmetry is not restricted to the molecular level, but also applies to the macroscopic nonlinear susceptibility, which means that the NLO molecules have to be organized in a non-centrosymmetric alignment. The first measurements of the macroscopic second-order susceptibility, have been performed on crystals without centrosymmetry [5]. However, many organic molecules crystallize in a centrosymmetric way. Other condensed oriented phases such as Langmuir-Blodgett (LB) films and poled polymers therefore seem to be the most promising bulk systems for NLO applications. 

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... 

A new approach to second-order nonlinear materials was reported by Verbiest et al. (56) in which chirality plays the key role. These authors investigated Langmuir-Blodgett films of chiral helicenes which lack features commonly associated with a high SHG response. The molecules adopt a helical structure on a solid support and this chiral supramolecular arrangement enhances the second-order NLO susceptibility by a factor of 30 when compared to the corresponding racemic mixture. An adequate description of the SHG response in a chiral system requires additional tensor elements. Experimental evidence was provided that those tensor elements which are only allowed in a chiral environment dominate the SHG response of the helicene system. 

Motschmann, H., Penner, T., Armstrong, N. and Enzenyil-imba, M., Additive second-order nonlinear susceptibilities in Langmuir-Blodgett multibilayers testing the oriented gas model, J. Phys. Chem., 97, 3933-3936 (1993). 

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... 

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