2ND HARMONIC-GENERATION IN ANISOTROPIC LANGMUIR-BLODGETT-FILMS OF N-DOCOSYL-4-NITROANILINE

Langmuir-Blodgett (LB) films of N-docosyl-4-nitroaniline have been made and their nonlinear optical properties studied by second harmonic generation (SHG) measurements. A significant enhancement of the intensity of the second harmonic of the 1.064-mu-m YAG was observed when a two layer Y-type film rather than a monolayer was used. Also, the dependence of the second harmonic signal on the polarization of the fundamental beam suggests that the nonlinear chromophores are lying nearly flat with respect to the substrate and are oriented along the dipping direction resulting in a noncentrosymmetric structure. Both of these observations are not common for Y-type LB films and the usual assumption of C(infinity nu) symmetry is therefore not valid. The results make us suggest that these LB films possess C(s) and C2-nu symmetry for mono- and multilayers, respectively. Theoretical expressions for the envelope functions of the SHG for these symmetries are derived and compared with the experimental results.