Optical Nonlinearities In Organic Multipolar And Dipolar Molecules

Organic molecules are promising candidates for high-performance NLO materials due to their possible applications in a variety of optoelectronic and photonic devices. Guidelines for the optimization of the second-order and third-order nonlinearity of a molecule have been improving, but the understanding is far less developed than for application need. Most of the attempted chromophores are designed as one-dimentional charge transfer molecules with an extended conjugation in a single direction. Multi-polar molecules were explored in this work, Compared to classical one-dimensional molecules, two-dimensional multi-polar chromophores have appeared to show promising properties.On the other hand, Z-scan technique is a extensively utilized experimental tool for studying optical nonlinearities in a wide class of materials. The technique relies on the fact that the light intensity vaties along the axis of a convex lens and is a maximum at the focus. By moving the sample through the focus, the intensity dependent absorption is measured as a change of the transmission through the sample. The nonlinear refraction is determined by the spot size variation at the plane of a finite aperture detector combination, because the sample itself acts as a thin lens with varying focal length as it moves through the focal plane.Six 3,6-di-acceptor-substituted carbazole azo chromophores were designed in this work, in which two hyperpolarizable bridges were directly connected to form A-π-D-π-A multipolar system. The carbazole-cored chromophores were synthesized and characterized by NMR, IR, UV. The third-order NLO properties of the carbazole chromophores were investigated using single-beam Z-scan technique with picsosecond laser pulses at 1064nm. Results indicate that larger third-order polarizabulitiesγcan be readily obtained in such carbazole chromophores because of increasing of strength of withdrawing group and molecular conjugation length with two aromatic bridges in the two-dimensional conjugated system. Therefore, the increase in the obtainedγvalues considerably arises from the conjugation path of the delocalized electrons for large third-order nonlinear optical effects.Six azo diol chromophores were synthesized through multi-step azo-coupling reaction and characterized by NMR, IR, UV. The measurements of second-order hyperpolarizabilities were performed using Z-scan technique. Results indicate that larger third-order polarizabilitiesγcan be readily obtained in such chromophores because of increasing of strength of withdrawing group, heteroaromatic rings and molecular conjugation length. Especially two-dimensionally chromophore A6 with two donor-acceptor conjugation path exhibits large hyperpolarizability and excellent transparency.Second-order polarizabilities of ten ferrocenyl derivatives which possess donor-acceptor conjugated structures were determined with DC electric-field-induced second-harmonic generation (EFISH) experiments. The relation between the molecular structure and nonlinear optical property were discussed issues concerning donor-acceptor strength, conjugation length, linkage and planarity were included. Five charge-transfer azo diol and azo aldimine were designed and synthesized.The second-order polarizabilities of the chromophores were determined by hyper-Rayleigh scattering (HRS). The decomposition temperature was determined with differential scanning calorimetry, and the absorption spectra were measured. The nonlinear optical properties of chromophores were discussed.The second-order polarizabilities of multipolar and unipolar chromophores were calculated via solvatochromic methods.The nonlinear optical properties of these compounds were discussed. For most organic chromophores the two-level models was a good approximatio. As a consequence the dominate component of the second-order polarizabilityβijkis referred asβCT. From two-level model yields the simple expression for the second-order polarizability.