W. M. K. P. Wijekoon

Second-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 χyyy(2) (which is along the dipping axis) of the second-order nonlinear optical susceptibility tensor and χxxx(2) is negligible. In the case of x-type films, the SHG is due to the in-plane χ(2) tensor components but χyyy(2) and χxxx(2) have comparable magnitudes. In both films the nonlinear optical chromophores lie almost flat with respect to the plane of the substrate. The cancellation of χxxx(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.

Second-harmonic generation studies of Langmuir-Blodgett monolayers of indodicarbocyanine and hemicyanine dyes and stilbazium salts

Second harmonic generation and UV-visible spectroscopic measurements have been performed on a series of Z-type Langmuir-Blodgett monolayer of indodicarbocyanine, hemicyanine and stilbazium salts. Optical spectra of these monolayer indicate the presence of hypsochromically shifted aggregates on the silica surface. The extent of aggregation is found to be largely dependent on the nature of the counter-ion as well as on the conditions of the monolayer preparation. The SHG intensity of dye aggregates are much smaller compared to that of monomeric dye molecules. In the case of amphiphillic counter-ion containing stilbazium salt compression speed of the monolayer has very little effect on the second harmonic signal and on the absorption spectrum, presumably due to the suppression of aggregation. It seems that the contribution from nonlinear optically active amphiphillic counter-ion to the SHG intensity is not that significant.