Study On The Preparation And Properties Of Novel Optical Second-Order Nonlinear Optical Materials

With the rapid development of information technology, nonlinear optical (NLO) materials have been of great interest due to their potential applications in the fields of optical information processing, optical sensing, and telecommunications. The advantages of organic NLO materials in contrast to inorganic crystals include higher NLO susceptibility, ultra-fast response characteristics, easier processing into thin films and compatibility with microelectronic processes. It is still considered to be a challenge for researchers to prepare novel polymer lattices with macroscopically non-centrosymmetric structure in which chromophoric dipolar molecules are well arranged with orientation.In this desertation, the basic theories of NLO and molecular design are briefly introduced, and the species, the preparation as well as the research progress of second order NLO materials are also summerized. Based on the preliminary investigation, a novel chromophore was synthesized and the spectroscopic properties of it were investigated. The poled polymer films were prepared and the strip waveguide using it was fabricated successfully. A multilayer NLO assembly film was prepared. The main research results were as follows:1. A second-order NLO chromophore (DCDHF-2-V) with a novel electron acceptor was synthesized. The spectroscopic properities of the chromophore in the different polar solutions and polymer matrix were investigated by the absorption spectra and fluorescence emission spectra. It is found that the absorption and fluorescence maxima are largely red-shifted along with the increase of the solvent polarity, which means that the dipole moment of the DCDHF-2-V molecule is higher in the excited state than in the ground state. The absorption spectrum and fluorescence spectrum of the film is quite different from that of the chromophore in the solution. These spectral effects arise from the difference in the aggregate state of DCDHF-2-V molecules and photoisomerization process of DCDHF-2-V molecules in a PMMA matrix respectively. The fluorescence quantum yield and Stocks shift change along with the variation of the polarity of solvent. And the difference in the dipole moment between the ground and the excited states was obtained as 33.09×10^-30C·m. The second order polarizability value of DCDHF-2-V was estimated based on the quantum-mechanical two-level model and the calculated value was 3323.4×10^-40 m⁴/V at the wavelength of 1064 nm. The thermal behavior of the chromophore was investigated by thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC), respectively.2. Spin-coated films of poly methyl methacrylate (PMMA) doped with DCDHF-2-V were fabricated. The refractive index of the polymer film was measured by an ellipsometer. With the increase of DCDHF-2-V moleculars concentration, the absorbance of the film is also enhanced. The thermal stability of the polymer (DCDHF-2-V/PMMA) was investigated by thermal gravimetry and the kinetic parameters of the degradation processes were calculated. The alignment of the NLO chromophore of the polymeric thin film was carried out by corona poling method. Domain structures of NLO-chromophores for the poled film were obtained using atomic force microscopy (AFM). The surface of the film sample before poling is flat and this good quality film was dramatically changed after poling, resulting in numerous hills and valleys that were aligned the poling direction. Due to the rearrangement of the dipolar molecules, absorption of the polymeric film after poling is less than that of the film before poling. From the absorbance change, the order parameter of the poled film could be estimated, which is related to the poling efficiency. The second-order nonlinear coefficient d₃₃ was taken from the second harmonic generation (SHG) experimental setup. When the concentration of chromophore DCDHF-2-V is 15%, the d₃₃ value is maximum. The relation between the second-order nonlinear coefficients d₃₃ and d₁₃ for the poled polymer film was also discussed in detail. In the same way, another chromophore doped PMMA was investigated.3. The polymer P1 containing tricyanofuran units was synthesized by Friedel-Crafts reaction. Second-order NLO properties of the polymer were measured by SHG. Compared with the polymer (DCDHF-2-V/PMMA), P1 possesses competitive optical nonlinear stability.4. The non-linear optical films were fabricated by means of alternate electrostatic adsorption with positively charged diazoresin (DR) or with negatively charged azo polymer (PAZO). Regular film growth during adsorption was proved by means of UV-vis spectroscopy and quartz crystal microbalance (QCM). Upon the UV irradiation, accompanied with the transition of the ionic bonds between the layers to covalent bonds, the resulting multilayer films possess excellent environmental stability and high thermal stability. The SHG signal can retain 90% when the film is heated up to 200℃.5. The strip waveguide using the DCDHF-2-V/PMMA plolymer film was fabricated by semiconductor technology. The refractive index distribution and mode field distribution of the strip waveguide were simulated. These works give a solid foundation for the final realization of the prototype of polymer waveguide Mach-Zehnder modulator.