Noncentrosymmetric order

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... 

What is also very interesting from the practical point of view is that such an alternate illumination of the zwitterionic chromophore during the corona poling leads to a significant increase of x susceptibility. The observed increase of 12 intensity by a factor of three (Fig. 42) corresponds to an improvement of x susceptibility (and correspondingly of the E-O coefficient r) by 70%. Also, the temporal stability of the induced noncentrosymmetric order in alternately illuminated thin films is significantly better than in untreated films (an increase of the decay time constant by a factor of 5 [174]). 

Electrostatic layer-by-layer (LBL) self-assembly techniques based on oppositely charged poipelectrolytes can be useful to create stable noncentrosymmetric order in thin films. Using this interesting technique, thermodynamically stable noncentrosymmetric multilayer films can be prepared without any need for poling. Tripathy et al. reported the fabrication stable multilayer films of epoxy-based side chain azo polymers for second-order nonlinear optics.The second-order NLO coefficients of the five-bilayer LBL films of these polymers were found to be comparable to those of spin-coated poled films. A schematic view of the procedure to fabricate polyelectrolyte-based LBL films is shown in Fig. 7. 

Ci6H33))-2, 2 6, 2"-terpyridine have been investigated, measuring their SHG at 1.064 pm incident wavelength. Rather low values were unexpectedly obtained, probably due to a scarce noncentrosymmetric ordering of these rather bulk chromophores [94]. 

Dalton, L.R., A.W. Harper, and B.H. Robinson. 1997. The role of London forces in defining noncentrosymmetric order of high dipole moment-high hyperpolarizability chromophores in electrically poled polymeric thin films. Proc Natl Acad Sci USA 94 4842 847. 

Noncentrosymmetric order induced by electric field poling or by molecular self-assembly processes must be stable at the elevated temperatures encountered in device fabrication and operation. Conservatively, this means stability at temperatures on the order of 100 C in general, with precise requirements determined by the particular device application pursued and the device fabrication protocols followed. The requirement of stability at elevated temperatures frequently mandates additional lattice hardening to stabilize induced noncentrosymmetric order. 

As is a third-rank tensor, it is effective only for noncentrosymmetric media. Therefore, the medium used for the second-order nonlinear optical effect must have a noncentrosymmetric ordering of the NLO dipoles, possibly by spontaneous ordering or by electric field poling (39). In contrast, order is not required for third-order nonlinear materials (e.g., conjugated polymers), but the of a given material is given by (40) ... 

A noncentrosymmetric ordered crystal structure whidi prevents averaging out of the... 

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