| Organic materials with large two-photon absorption (TPA) cross-sections, delta, are of interest due to potential applications in optics, materials fabrication, and biology. These applications exploit the advantages of a two-photon process compared to a one-photon process, in particular, the improved penetration depth, reduced scattering, and higher spatial selectivity in three dimensions. Also, TPA can be used to investigate the nature of electronic states that are one-photon forbidden in a compound with high symmetry.;In this dissertation, we discuss structure -- non-linear optical (NLO) properties relationships in chromophores with enhanced third-order poralizability and/or two-photon absorption response, on the basis of highly correlated quantum-mechanical methods. Chapter 1 will introduce the general linear and non-linear optical processes in organic chromophores, and present the major computational methodologies (including ab initio Hartree-Fock and Density Functional Theory as well as the semi-empirical Intermediate Neglect of Differential Overlap methods) we have used to compute the NLO properties. Our main results are discussed in the following chapters. In Chapter 2, we focus on first- and third-order polarizabilities in cyanine and alkyne-cyanine compounds that have been recently synthesized. In Chapter 3, we discuss the TPA response as a function of the electronic coupling between porphyrin moieties in porphyrin dimers. Finally, Chapter 4 is devoted to a study of TPA in squaraine and cyanine compounds, and presents and analysis of the vibronic origin of the TPA response in some of these molecules. Our general conclusions and outlook are given in the final Chapter. |
