Third-Order Nonlinear Optical Characteristics Of Organic And Organometallic Materials

With the advent of photonic age, the theoretical and experimental researches on all-optical switching, all-optical computing and all-optical communication have become more and more important. As a key information media among them, the application of materials with excellent third-order nonlinear optical (NLO) properties has also received more interests. Compared with inorganic crystals, third-order NLO-active organic and organometallic molecules have become prominent candidates to be developed as novel all-optical devices for their high nonlinear optical susceptibility, fast nonlinear optical response time, low dielectric constant, and good tailorability. Unfortunately, the studies on the molecular design and material processing of organic and organometallic third-order nonlinear optical materials are still extensively and intensively preliminary.In this thesis, two series of organic and organometallic molecules, polymethine cyanines endcapped with different heterocyclic groups, and carbazole derivatives coordinated with carbonyl chromium moieties, were well-designed and synthesized. Based on the "molecular and material engineering" approach, the systematic and thorough investigations on the structural dependences of their third-order nonlinear optical properties were carried out from both theoretical and experimental point of views, and some interesting results have been unraveled as the following:(1) Five polymethine cyanines endcapped with different heterocyclic groups and three carbazole derivatives coordinated with carbonyl chromium moieties were designed and synthesized, and their molecular structures were characterized by element analysis, mass spectra, infrared spectra, and nuclear magnetic resonance, and the detailed crystallographic data of two carbonylchromium - complexed carbazole derivatives were successfully acquired by X-ray diffraction.(2) The linear optical properties of the above-mentioned compounds were studied by UV-visible absorption and fluorescence spectroscopy, and their molecular electronic structure and the distribution of their energy levels were revealed. The results indicate that they all possess good optical transparency in visible region. For carbonylchromium - complexed carbazole derivatives, the characteristic absorption bands originated from metal-to-ligand charge transfer transition can be clearly observed.(3) The nonlinear optical parameters, including their third-order nonlinear susceptibilities, their second-order hyperpolarizabilities and their nonlinear refractive indexes, were systematically measured by pico-second degenerate four-wave mixing technique, and the off-resonant y values of about 10^-29 esu were obtained in the polymethine cyanines, which was attributable to the introduction of heterocyclic fragments.(4) The dependences of third-order optical nonlinear on the molecular structures of polymethine cyanines were thoroughly analyzed by introducing the simplified free electron model and the time-dependent perturbation theory. It suggested that the molecular y values can be effectively improved by lengthening the polymethine chain, the substitution of heavy atoms, and increasing the molecular permanent dipole moment by increasing the electronegative difference between the endcapped heterocyclic groups.(5) The coordination of chromium center was observed to dramatically improve the molecular third-order optical nonlinearities in the carbonylchromium - complexed carbazole derivatives, and it could be attributable to the metal-to-ligand charge transfer transition with the low energy gap. Based on the time-dependent perturbation theory and density functional theory, it could be further deduced that the different molecular conformation would induce the different spatial distribution of the molecular electronic wave function, and therefore result in the different third-order optical nonlinearities, which provides an effective guideline for the rational molecular design towards novel organometallic nonlinear optical chromophores;(6) A series of well-ordered J-, H-, and Herringbone-type aggregate ultrathin films of three polymethine cyanines were successfully achieved by spin-coating technique. Their structural and morphological characters were confirmed by UV-visible spectra, infrared spectra, small-angle X-ray diffraction, atomic force microscopy, and ellipsometry. Their third-order optical nonlinearities were also measured by forward degenerate four-wave mixing technique, and the off-resonant enhanced third-order nonlinear optical susceptibilities were observed. Their mechanisms were elaborated based on one-dimensional linear oscillator model and collective electronic oscillator (CEO) theory, and it implied that there was a strong correlation between their third-order NLO behaviors and optically driven aharmonic oscillations of photoexcitons confined in molecular aggregates, and the freedom and coherence of the photoexciton motion were augmented in the ordered molecular self-organization processes and have the positive contributions to the largely enhanced third-order optical nonlinearities.(7) A H-aggregate ultrathin film of carbonylchromium - complexed carbazole derivative was also prepared by spin-coating technique. Its structure was basically characterized by UV-visible and fluorescence spectra. The considerable third-order optical nonlinearity (X⁽³⁾= 4.21×10^-12 esu) of the film at the off-resonance wavelength (532 nm) was first observed by forward degenerate four-wave mixing technique. The research on the physical origin of the corresponding third-order optical nonlinearity is in progress.