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The development of important applications related to optical telecommunications and optical signal processing rely increasingly on nonlinear optical phenomena underlying these optical functions such as frequency conversion and modulation, amplification and emission, multiplexing and directional switching, optical logical gates, and others. This will depend on the design, elaboration and availability of new materials displaying enhanced nonlinearities to be subsequently integrated into waveguiding devices. Organic materials are noteworthy candidates in this context, provided their microscopic and macroscopic properties are properly optimized with respect to the type of application required. Among the key-factors in the development of these materials are the quasi-unlimited variety of molecular structures accessible by organic synthesis, the possibility to connect specific nonlinear properties to other kinds of functionalities which are of crucial importance for the development of commercial devices (e.g., compatibility with other materials used in integrated optical circuits such as semi-conductors), and their exceptionally large second-order, non-resonant nonlinear response. Such performances are strongly dependent on a better understanding of the ‘molecular engineering’, rules, correlating molecular structures to their nonlinear optical properties towards optimization of the microscopic second order polarizabilities β and third-order polarizabilities γ. We will present in the following some examples of the most recent developments of highly efficient nonlinear molecules, ranging from purely one-dimensional structures such as polyenes to fully three-dimensional systems, taking full advantage of the tensorial character of the microscopic or macroscopic susceptibility tensors.

The synthesis of a series of push-pull systems [donor (D)-acceptor (A)], associating the 1,3-dithiol-2-ylidene moiety (D) to various (A) fragments through polyolefinic linkages of various lengths, is described. Design optimization of these NLO phores is via systematic determination of the molecular first hyperpolarizabilities b by the EFISH method. Selected compounds of this series, displaying the highest b values, are then chemically functionalized in order to promote their covalent grafting to polymeric backbones.

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