Two-photon Absorption Properties Of New Organic Functional Materials, And Ultrafast Dynamics

Along with the progresses of the optical science and optical electronics in the past decades, the materials with excellent nonlinear properties are currently of considerable significance and they show a variety of potential applications. Two-photon absorption has shown great potential applications in biology, physics, chemistry, medicine, micro-electronic technology and many other fields due to its unique optical response, nonlinear property, 3D processing capabilities, and high spatial resolution. At present, two-photon technique has been applied in high-density data storage, single-molecule detection, medical diagnosis, three-dimensional microfabrication and so on. Of course, the materials with excellent properties are the basis for wider application. Therefore, to get the materials with large two-photon absorption cross section as well as strong two-photon fluorescence has become one of the most attractive topics. With the maturity of the femtosecond laser system and the great achievement in the field of the ultrafast spectroscopy, ultrafast processes of the novel materials can be investigated in the picosecond and femtosecond domains. Due to the fact that the ultrafast processes can reflect a lot of significant information about the interaction between light and matter, the investigation of these processes are meaningful for the synthesizing and optimizing the new materials, as well as exploring the novel devices. Therefore, the study on the nonlinear optical properties and ultrafast dynamics of the materials with intense two-photon absorption ability, are become one of the frontier and hot topics in the optical science.We employed the UV-Vis and fluorescence spectroscopy, z-scan technique,femtosecond pump-probe and time-resolved fluorescence experiments tosystematically study the nonlinear optical properties and the ultrafast response of aseries of organic compounds, and we analyzed the relationship between the structureand the optical properties. The main innovative results and conclusions are as follows:1) We successfully measured the time-resolved fluorescence and anisotropy throughthe optical Kerr gate method. The fluorescence relaxations of multi-branchedmolecule show great differences at different detection wavelength, and theanisotropy relaxation time is only 720fs for the molecule possessing mostbranches, while the relaxations do not change with the detection wavelength inmolecules with few branches. The results show that it is the strongintra-molecular coupling within the multi-branched molecules which will enhance the two-photon absorption ability. The conclusions were confirmed from the z-scan experimental results.2) By using z-scan technique, we measured the two-photon absorption cross section of these molecules and studied the solvent effect. The largest value of two-photon absorption cross section for a given molecule was observed in the most polar solvent. The two-photon absorption cross section for the largest molecule in DMF is seven times larger than that in THF. The mechanism of the enhancement mainly comes from the strong interactions among branches. For the smallest molecule, the intra-molecular interaction is weak, thus the solvent effect is not remarkable.3) The pump-probe experimental results indicate that the exciton migration in oligomers has a strong choice of direction, they can only happen along the direction of backbone. Introducing strong electron donor, electron acceptor or an expansion are all good methods for improving two-photon absorption ability.4) Through the comparative study of two metal phthalocyanine derivatives, we found that the central metal atom in phthalocyanine has a very important influence on the energy relaxation. When a metal atom with open-shell structure (eg. cobalt) was put in theπcenter, the non-paired electron will couple with theπorbital and cause the splitting of energy level, thus speeding up the excited state relaxation as well as reducing the fluorescence emission.5) We have also studied three kinds of dithienylethene derivatives which are known as good candidate of photochromic materials. We found that the molecules containing electron donor show better two-photon absorption ability in comparison with others. After the irradiation of UV light, the two-photon absorption ability of the close-circle materials is enhanced.