Calculation of linear and nonlinear optical response of model Langmuir-Blodgett films

Linear and nonlinear optical response are calculated for lattice models of Langmuir-Blodgett films. Molecules are treated as a set of five point submolecules, when interactions outside a layer are negligible. Molecular polarizabilities and hyperpolarizabilities are treated as axially symmetric with an anisotropy of 5:1. Local fields, linear susceptibilities, refractive indices, and quadratic and cubic nonlinear susceptibilities are calculated. All the properties depend markedly on molecular tilt away from the normal to the film, but very little on distortions from axial packing in the film. One-dimensional approximations to the hyperpolarizabilities tend to underestimate significantly the magnitude of the nonlinear susceptibility coefficients. Cascading terms quadratic in the first hyperpolarizability augment the cubic susceptibility substantially from the direct terms linear in the second hyperpolarizability. Extensions to more realistic models are outlined.