2ND-HARMONIC GENERATION STUDIES OF DIFFERENCES IN MOLECULAR-ORIENTATION OF LANGMUIR-BLODGETT-FILMS FABRICATED BY VERTICAL AND HORIZONTAL DIPPING TECHNIQUES

Second-harmonic generation (SHG) studies have been carried out on Langmuir-Blodgett films of 2-docosyl-amino-5-nitropyridine that were transferred by two different dipping techniques: vertical y-type and horizontal x-type depositions. The SHG behavior of horizontally transferred films is remarkably different from that of the vertically transferred films. In y-type films the SHG is due mainly to the in-plane component chi(yyy)(2) which is along the dipping axis) of the second-order nonlinear optical susceptibility tensor and chi(xxx)(2) is negligible. In the case of x-type films, the SHG is due to the in-plane chi(2) tensor components but chi(yyy)(2) and chi(xxx)(2) have comparable magnitudes. In both films the nonlinear optical chromophores lie almost f lat with respect to the plane of the substrate. The cancellation of chi(xxx)(2) in y-type films is due to the preferential orientation of molecular domains along the dipping axis. In horizontally transferred films, molecular domains seem to be randomly distributed on the surface.