Experimentele en theoretische bepaling van de eerste hyperpolariseerbaarheid.

The field of nonlinear optics and nonlinear optical applications has become one of the fastest growing areas of research for the last four decades, impacting upon many other disciplines, such as medicine, signal processing, communications, computing, etc. Although the Kerr-effect was discovered in 1875, the birth of modern nonlinear optics should be associated to the first observation of second harmonic generation by Franken etal. in 1961, soon after the invention of the laser in 1960.Although the first nonlinear optical effects were observed in crystalline structures, they were soon extended to organic materials and later tosolid state heterostructures. In particular, the area of organic nonlinear optics has been extremely active in the past two decades.Organic materials might be preferred for many reasons. First, the origin of the nonlinear optical response in organic system lies in the electronic polarizability of the electrons at the molecularlevel. The intrinsically electronic nature of the response implies a very fast response for non resonant optical excitations, a great advantagefor organic photonic devices over some of the inorganic analogues. Organic materials are also easier to produce, more versatile and potentiallycheaper. Yet, the biggest advantage of organic materials is the virtually infinite number of organic molecules that can be synthesized. The great versatility of organic synthesis means that a wealth of potential strategies for the optimization of the nonlinear response are available. The number of medium size organic molecular structures is immense. A recent study showed that the number of organic structures that could be considered as drug candidates is greater than Avogadro's number. There is a strong need for a theoretical and experimental framework that provides clear guidance in the search for optimal structures.The quantum limit analysis, originally used to show how the quantum sumrules impose limits on the first and second hyperpolarizabilities, became the first step towards the development of such a theoretical and experimental framework. By establishing the existence of the quantum limits,it became possible to determine the performance of a molecular design motif. In this work, we present further results that extend the use of the quantum limits to provide a better understanding of the molecular nonlinear response. With the introduction of the dipole-free expressions for the molecular susceptibilities and the ``Three-level ansatz'' the problem of consistency has been simplified and reviewed. An overview of the theory of nonlinear optical response is presented The theory of the nonlinear optical response is based on the work of Butcher and Cotter, and the qualitative approach reviews the treatment of Boyd, after Owyoung, while the calculations of the local field follows the description of Kuzyk. The expressions for the molecular polarizations were first obtained by Orr and Ward, using quantum perturbation theory, and are used to express the molecular susceptibilities in terms of quantum mechanical quantities.Hyper-Rayleigh scattering, the experimental technique that was used to characterize most of the molecular compounds included in this study, is presented. The generalized Thomas-Kuhn sum rules are introduced, in the same manner they were presented in my Master's dissertation. The dipole-free expressions for the first and second hyperpolarizability are introduced . The dipole-free expression for the first hyperpolarizability was originally developed by Mark G. Kuzyk, and here the dipole-free expression for the second hyperpolarizability is presented for the first time. The off-resonance quantum limits for the first hyperpolarizability are derived, and the existence of a gap between the fundamental limit and measurements is studied. The results of this work were first published in the Journal of Chemical Physics. Two different hypothesis that could explain the existence of the quantum gap are analyzed.We present the results of a study of ionic chromphores, a material thathas been published in the Journal of Chemical Physics. Also, the quantum limits analysis are extended to study the third-order nonlinear response of organic molecules, using the same structure as in its publication format. The results are discussed and summarized and some perspectives are presented.Finally, for completeness, the results of our most recent publication in the Optics Letters journal, are presented as an epilogue.status: publishe