Linear and non-linear optical studies of mono- and multilayer Langmuir-Blodgett films of N-docosanoyl para-nitroaniline

Second harmonic generation (SHG) and surface plasmon studies have been carried out on mono- and multilayer Langmuir-Blodgett (LB) films of an amphiphile containing a small chromophore (N-docosanoyl-4-nitroaniline). A quadratic variation in the second harmonic intensity was observed for Y-type multilayers (film thickness above 4 bilayers), but for the thinner films (film thickness less than 4 bilayers) the enhancement of the SHG intensity does not follow the quadratic dependence. The results indicate that the non-linear chromophores lie almost flat on the plane of the substrate with orientation preference in the dopping direction. The SHG behavior and the second-order non-linearity of the LB films are compared with those of 2-docosylamino-5-nitropyridine and N-docosyl-4-nitroaniline which also have a chromophore of similar size.