Quadratic Nonlinear Behaviour of Various Langmuir-Blodgett Molecules

The Langmuir-Blodgett (L-B) technique is well known as a method of building-up ordered arrays of organic molecules, and especially as a means of imposing a noncentrosymmetric structure from molecules that crystallize in a centrosymmetric space group(1–3). L-B films for use in quadratic nonlinear optics can be ordered in a statistically noncentrosymmetric lattice. The method involves the compression of a mono-layer of the organic molecule, spread on top of a water surface, into a two-dimensional solid, followed by the repeated dipping of the substrate to be coated into and out of the subphase. Monolayers may be deposited onto the substrate both on immersion and withdrawal, resulting in a centrosymmetric arrangement of layers (Y-type deposition), or just on immersion (X-type) or withdrawal (Z-type), in which case a noncentrosymmetric structure should be obtained. However, this kind of samples is not very stable and tends 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 stacked following a Y-type structure, the hydrophobic chain being grafted onto the acceptor group of the first species A and onto the donor group of the second species B (4,5). The quadratic nonlinear response, resulting from the addition of the individual nonlinearities of each moiety, is quite large, with the additional advantage of a high degree of order and a good stability of the multilayers.