Second harmonic generation structural requirements

Self-assembly is essentially chemical fabrication. Like macroscale fabrication techniques, self-assembly allows a great deal of design flexibility in that it affords the opportunity to prepare materials with custom shapes or morphologies. The advantages of self-assembly include an increased level of architecture control and access to types of functionality unobtainable by most other types of liquid-phase techniques. For example, it has been demonstrated that materials with nonlinear optical properties (e.g., second harmonic generation), which require noncen-trosymmetric structures, can be self-assembled from achiral molecules. 

The first and third order terms in odd powers of the applied electric field are present for all materials. In the second order term, a polarization is induced proportional to the square of the applied electric field, and the. nonlinear second order optical susceptibility must, therefore, vanish in crystals that possess a center of symmetry. In addition to the noncentrosymmetric structure, efficient second harmonic generation requires crystals to possess propagation directions where the crystal birefringence cancels the natural dispersion leading to phase matching. 

The non-centrosymmetric ordering of benzylquinuclidinium structure directing agents inside the centrosymmetric IFR framework has been shown by single crystal X-ray diffraction and second harmonic generation measurements. The reasons behind this effect and the requirements of the zeolite structural architecture needed to show this ordering are discussed. 

It is seen that for high peak power applications a low refractive index is required. In the crystal case, to avoid SHG (second-harmonic generation), a centro-symmetric structure has to be searched for, because the so-called Miller rule (Miller 1964) gives for the allowed elements of a nonlinear susceptibility tensor for second-harmonic generation. 

Another line of research has been to assemble complex optical structures with exquisite precision examples are the work of Kuhn and Mobius on resonant energy transfer involving dye molecules" and second-harmonic generation, the principal requirement of which is a non-centrosymmetric structure, readily achievable using the LB technique. 

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