Dispersions of electroabsorption susceptibilities: application to a polymeric Langmuir-Blodgett film

Abstract

The relation between the dispersions of nonlinear optical susceptibilities χ(n)(ω;ω,0,…,0) and χ(n−1)(ω;ω,0,…,0) (for electro-optic process) of organic polymeric materials is studied based on the premises that the microscopic polarizabilities α, β, γ, δ, ... are derivatives of their next lower order polarizabilities, with respect to the effective internal field F across the polarizable π-conjugated systems, and that these derivative relationships are still valid when dynamic (frequency-dependent) processes are considered. Electroabsorption spectroscopy has been used to determine the dispersion of χ113(2) and χ1133(3) of a polyamic acid salt Langmuir-Blodgett film containing covalently bonded azobenzene NLO chromophores. The results are consistent with our prediction, and confirm the validity of the derivative relationships proposed by Marder et al. [Science 265 (1994) 632; J. Am. Chem. Soc. 116 (1994) 10703]. This LB film also gives appreciable values of both χ113(2) (26 pm/v) and χ1133(3) (2 × 10−11 esu).