Determination of surface complex nonlinear optical susceptibilities and molecular orientational distribution functions using resonant surface second-harmonic generation

Abstract

Using the resonant optical surface second-harmonic generation (SHG), we have determined the relative values of the complex nonlinear optical (NLO) components $({\ensuremath{\chi}}_{\mathrm{zzz}},$ ${\ensuremath{\chi}}_{\mathrm{zxx}},$ and ${\ensuremath{\chi}}_{\mathrm{xxz}}$) at isotropic interfaces ${(C}_{\ensuremath{\infty}v})$ of a polymer with SHG active side chains. The introduced configuration of the SHG experiment was a polarizer--rotating quarter wave plate--sample--analyzer. It was shown that this configuration gives information on complex NLO coefficients without using the Kleinmann symmetry. For the experiments, we measured resonant surface SHG from the air-polymer and the substrate-polymer interfaces of a thick polymer film. By theoretically fitting the SHG data, we unambiguously determined the nonlinear susceptibility components at the both interfaces of the polymer film. Moreover, unbiased molecular orientational distribution functions (ODFs) at both interfaces were also determined using the modified maximum entropy method. The obtained ODFs were found to be quite different from the previous ones obtained by assuming the Kleinmann symmetry, indicating the important role of the imaginary part of \ensuremath{\chi}'s played when determining ODFs.