Two-photon Nonlinear Optical Properties Of Organic Functional Materials, And Ultrafast Dynamics Research

Along with the great developments of the optical science and optical electronics in recent decades, the materials with excellent nonlinear optical properties have currently attracted considerable attention and show a variety of potential applications. Among the various classes of nonlinear optical materials, two-photon absorbing materials have been the subject of growing interest because of their potential application in many fields, including two-photon fluorescence imaging, two-photon up-conversion laser, optical limiting and, three-dimensional data storage. With the wide utilization of the femtosecond laser system and the great achievement in the field of the ultrafast spectroscopy, ultrafast processes of the novel material can now be investigated in the picosecond and femtosecond domains. Because the ultrafast processes can reflect a lot of significant information about the interaction between light and matter, the research results of these processes are meaningful for the synthesizing and optimizing the new materials, as well as exploring the novel devices. Therefore, the study of nonlinear optical properties and ultrafast dynamics of the materials with intense two-photon absorption (2PA) becomes 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 experiment to systematically study the nonlinear optical properties and the ultrafast response of several series of organic compounds. Basing on the comparison and analyses of the experimental data, we get the better understanding of the relationship between the molecular structure and the optical properties.The main results and conclusions are as follows:1. We successfully measured the time-resolved fluorescence anisotropy throughthe optical Kerr gate technique for the first time. The molecule with the tri-branched structure shows the fastest anisotropy decay and smallest residual anisotropy value. The fast anisotropy decay is a signature of strong intramolecular interactions which can also enhance the nonlinear optical effects. From the z-scan measurement, we find that the ratio of 2PA cross sections of tri-branched molecule to that of monomer is about 5.2. The relaxation behaviors of the fluorescence were measured by optical Kerr gate technique to study the solvent effect. The results show that the relaxation processes of the sample solution will be accelerated in the solvent with larger polarity. This conclusion is also confirmed by the femtosecond pump-probe experiment.3. We find that the tri-branched polymer whose monomer has D-Ï€-D structure will exhibit more excellent two-photon absorption properties than that of the polymer combined with three D-Ï€-A monomer. Substitution of the donor or acceptor is a useful way to improve the two-photon absorption ability of the porphyrin derivatives.