217 SECOND HARMONIC GENERATION IN ALTERNATE NON-LINEAR LANGMUIR-BLODGETT FILMS* I. LEDOUX, D. JOSSE, P. FREMAUX, J.-P. PIEL, G. POST AND J. ZYSS Centre National d'Etudes des Tdldcommunications. Laboratoire associd au CNRS 250, 196 avenue Henri Ravera, 92220 Bagneux Cddex (France) T. McLEAN, R. A. HANN, P. F. GORDON AND S. ALLEN Imperial Chemical Industries plc, Electronics Group, PO Box 11, The Heath, Runcorn, Cheshire ( U.K.) (Received July 26, 1987; accepted October 2, 1987) Second harmonic generation experiments are performed in alternate multi- layers of two non-linear molecules, the hydrophobic chain being grafted onto the acceptor group of the first species A and onto the donor group of the second species B. Two families of non-linear molecules are studied: p-aminobenzoic acid deriva- tives and diazostilbene derivatives. High ~(~2jvalues (around 5 × 10 -1° SI) and fl values (around 10-a7 SI) are obtained for the diazostilbenes. The variation in the second harmonic intensity with respect to the number N of active layers is interpreted in terms of the disorder appearing in the upper monolayer of the Langmuir-Blodgett film. 1. INTRODUCTION Non-centrosymmetric stuctures can be obtained in Langmuir-Blodgett (LB) films for quadratic non-linear optics such as monolayers 1'2, Z-type multilayers 3-5 or Y-type multilayers 6' 7, and second harmonic generation (SHG) experiments have given evidence of some large X t2~ values in Z-type multilayers of dye molecules 3-5. However, such samples are not quite stable and tend to relax towards disordered structures. An alternative method is the successive deposition of alternate layers ABAB... where A and B are two different molecular species. SHG has been demonstrated in alternate layers of o~-tricosenoic acid and a merocyanine dye 6'7. These films are called "passive-active" layers. More efficient structures were proposed, which alternate two different non-linear molecules, the hydrophobic chain being grafted onto the acceptor group of the first species A and onto the donor group of the second species B 8. Therefore, in such alternating structures, called "active- active" layers, the non-linear susceptibility results from the addition of the individual non-linearities of each species. Stable samples with high Zt2) values can be obtained using this deposition technique 9. In this work we have measured the Zt2) values of various structures, and elucidated the behaviour of the SH intensity I2~ * Paper presented at the Third International Conference on Langmuir-Blodgett Films, G6ttingen, F.R.G., July 26-31, 1987. 0040-6090/88/$3.50 © Elsevier Sequoia/Printed in The Netherlands