Optically nonlinear Langmuir–Blodgett films: linear electro-optic properties of monolayers

The linear electro-optic properties of Langmuir–Blodgett monolayers of an homologous series of hemicyanine betaine dyes are determined by a novel measurement technique developed by us. The dielectric constant changes Δ∊ are seen as field-induced modulations in the mode-coupling conditions in prism-coupled surface-plasmon spectroscopy. The magnitude, sign, and phase of χ(2)(−ω; ω, 0) are obtained for monolayers of these nonlinear dyes. We report that, for one of the series, monolayers are deposited having χ(2)(−ω; ω, 0) comparable with that of the previously studied hemicyanine dye but possessing imaginary dielectric constant components lower by more than 1 order of magnitude. This is ascribed to a reduction in optical scattering in the new films.