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-docosylamino-5-nitropyridine that were transferred by two different dipping techniques: verticaly-type and horizontal x-type depositions. The SHG behavior of horizontally transferred films is remarkably different from that of the vertically transferred films. Iny-type films the SHG is due mainly to the in-plane component χ⁽²⁾_yyy (which is along the dipping axis) of the second-order nonlinear optical susceptibility tensor and χ⁽²⁾_xxx negligible. In the case of x-type films, the SHG is due to the in-plane χ⁽²⁾ tensor components but χ⁽²⁾_yyy and χ⁽²⁾_xxx have comparable magnitudes. In both films the nonlinear optical chromophores lie almost flat with respect to the plane of the substrate. The cancellation of χ⁽²⁾_xxx 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.