Linear and nonlinear optical properties of Langmuir-Blodgett multilayers from chromophore-containing maleic acid anhydride polymers

Organized polymer films prepared from amphiphilic side-chain polymers were investigated with respect to their application in passive or active photonic devices, The polymers were synthesized by means of polymer analogous reaction of azo chromophores with a maleic acid anhydride polymer which bears saturated hydrophobic side chains. The resulting polymer contains, besides hydrophilic groups and hydrophobic side chains, azo chromophores with high second-order hyperpolarizability which are attached to the main chain via a spacer. in order to increase the hydrophobic character of the chromophore, a fluorinated moiety was chosen as acceptor of the chromophore, Langmuir-Blodgett (LB) technique was applied for preparation of multilayer structures with distinct supramolecular architecture from these polymers. They an characterized by high anisotropy and a special combination of linear and nonlinear optical (NLO) properties. The polymers were deposited both in Y-type and Z-type LB layers. The Y-type layers of the azopolymer show high birefringence and dichroism but small nonlinear response which originates from the first deposited layer only. Multilayers of the azopolymer deposited in Y-type configuration no characterized by high internal order which can be concluded from a pronounced Bragg-peak and Kiessig-fringe patterns as obtained from gracing incidence X-ray scattering. Experiments to deposit the active polymer directly in Z-type configuration by passing a blank water compartment before layer deposition in a double compartment LB-trough led to the result that well-ordered, anisotropic films are obtained only if the number of deposited monolayers is limited to a value of about 30. The layers deposited in Z-type manner show low order. This can be concluded from the fact that only a weak Bragg-peak is observed in the X-ray scattering curves, and Kiessig-fringes are very weak or missing. The same conclusion can be drawn from ellipsometric and second-harmonic generation (SHG) experiments. A second kind of Z-type multilayers was prepared by alternate deposition of the active azopolymer and of another maleic acid anhydride polymer without chromophores. They show similar linear optical properties like Y-type layers but due to the noncentrosymmetric internal structure, a NLO second-order response grows proportional to the square of the sample thickness. Again, thick layers of the azopolymer prepared by alternate deposition LB-technique were prepared, which can be used as active waveguide for second-order NLO application. (C) 1999 Elsevier Science S.A. All rights reserved.