Organic Superlattices for Nonlinear Optics: Alternating Langmuir-Blodgett Multilayers of Azobenzene Dyes

We report near-ideal quadratic enhancement of second harmonic generation (SHG) from alternating, non-centrosymmetric multilayers of two azobenzene dyes deposited on glass substrates by the Langmuir-Blodgett (LB) technique. In the first case we deposited layers of the azo dye 4-nitro-4'-N-(stearoyl) aminoazobenzene (NSAAB) diluted 2:1 with stearic acid alternating with layers of pure stearic acid. Dilution of the azo dye in stearic acid leads to an increase in the effective molecular hyperpolarizability β, due presumably to prevention of dye aggregation and/or tail ordering effects. By comparison to a quartz standard, we measure a β value of 2•4x10-28 esu for NSAAB in a 2:1 mixture with stearic acid. We have investigated multilayers containing up to 10 active layers of azo dye. The 10 active layer sample exhibits an SHG signal that is 82 times that of the single layer NSAAB film. On the other hand, a 13 layer Y-type (centrosymmetric) NSAAB/stearic acid film exhibits an SHG intensity of only 1•5 times the single layer value. A quadratic curve was fit to the data allowing variation of the initial NSAAB layer SHG intensity due to molecule-substrate perturbations. The data fits extremely well when the first layer intensity is taken to be 80% of its observed value. The absorption spectrum of a one layer LB film of 2:1 NSAAB/stearic acid exhibits a 40 nm blue shift of the absorption maximum of the azo chromophore. This extreme blue shift may indicate the presence of H-aggregates of the azobenzene dyes or strong chromophore-substrate interactions.