The Study On The Third-order Nonlinear Response And Raman Scattering Spectra Of Several Push-pull Type Azobenzene Molecules

For potential application in optical signal processing, optical limiting (OL) and optical devices, it is essential to search for nonlinear optical materials with large third-order optical nonlinearities and fast response time. Organic molecules doped with polymer thin films are attractive medium for application in optical electronic devices. In these system, dopants with excellent nonlinear optical capabilities are dispersed in glassy polymer matrices, which have advantages over the current inorganic crystal for ease of fabrication, low cost and large third-order nonlinear refractive index. It is also convenient to adjust their linear and nonlinear optical constants with the choice of dopant and its concentration. Recently, there has been growing interest in the third-order nonlinearity of azo-dye doped polymer materials for their largeχ(3) value, which are interesting for application in optical-limiting and optical switching application. Photoisomerization of azobenzene molecules makes it easy to modify linear and nonlinear polarizabilities of molecular as well as optical nonlinear refraction. The optical properties of azo-polymer can be controlled optically, which have intrigued the considerable interest of people . The push-pull type azobenzene organic nonlinear dyes represent one particularly promising class of organic nonlinear materials, whose structure consists of an azo group–N=N- between two benzene rings, and push-pull donor and acceptor groups on the opposite sides of the benzene rings.It has been known that noble metal nanoparticles possess a large cubic nonlinearity and ultrafast response time in the surface plasma absorption region. The surface plasma resonance (SPR) of noble metal particles in composites occurs in the UV-visble to near-IR region, depending on the metal species, shape and dielectric medium. The nanocomposite films provide a new method to improve the processibility and stability of nonlinear materials for applications in optical devices by incorporating noble nanoparticles into organic polymer. The large third-order nonlinearities of metal nanoclusters doped an oxide matrix composite films have been investigated widely. For most reports were carried out in the surface plasma absorption region, the Stegeman condition cannot be fulfilled because of their large linear absorption. There are few literatures reporting third-order nonlinearity of metal nanoparticles doped azobenzene molecules thin films induced by picosecond laser pulses. We reported the third-order nonlinearity of metal Ag nanoparticles doped methyl orange far from surface plasma absorption region for the first time. The mechanism responsible for the process of nonlinear refraction was discussed in term of the enhancement of local field effect, which was confirmed by SERS technique. At the end, the Stegeman conditions were evaluated and the results showed that this material would be valuable in the application of all-optical devices.For azobenzene doped polymer nonlinear optical materials, the mixing proportion is restricted greatly in the host-guest system. Its mixing weight ratio is generally below 10% because of phrase separate effect. The copolymer allows it to contain large mixing proportion of nonlinear group and the proportion can be controlled conviently. In view of this, we have studied third-order properties of azobenzene dispersed red 1 side-chain polymer, it demonstrates large nonlinear refraction under both 532nm and 1064nm excitations.This thesis mainly reported the third-order nonlinearity of three push-pull azobenzene molecules (methyl orange(MO), methyl red(MR) and p-Dimethylamino benzene arsenic Acid (PDBBA)) thin films, metal Ag nanoparticles doped methyl orange thin films and azobenzene side-chain polymer. The nonlinear origin was analyzed from the molecular structures of the three azobenzene molecules. At the same time, the Raman, IR and surface enhanced Raman spectra of two azobenzene (P-Aminoazobenzene (PAAB) and methyl orange) were studied. The paper was divided into five chapters and three parts:In the first section, i.e. chapter one, the current status of the development and practical applications of organic nonlinear optical materials andπconjugation azobenzene materials are introduced briefly. In addition, the current development of the Raman spetroscopy and surface enhanced Raman scatterring spectroscopy are also presented in this chapter. At the end, we introduce the current research status of DFT calculation of azobenzene molecules.In the second section, i.e. chapter two, three and four. Firstly, the third-order nonlinear optical properties of three azobenzene molecules MO, MR and PDBAA are studied under the ps pulse laser excitation with the wavelength 532 and 1064nm using Z-scan technique. Nonlinear refractive indices and nonlinear absorption coefficients are calculated according to the Z-scan experimental curves of the thin films. Moreover, the relation between the third-order nonlinear optical properties of the compounds and their molecular structures are analyzed by connecting the experimental results with molecular structures in chapter 3. In chapter 4, we have investigated the third-order nonlinearity of colloidal Ag nanoparticles doped methyl orange molecules composite films using the Z-scan technique with 38ps pulses at 532nm apart from the surface plasma resonance frequency of composite film. Large and negative third-order nonlinearity was observed. The mechanism responsible for the process of nonlinear refraction was discussed in term of the enhancement of local field effect, which was confirmed by using SERS technique. In chapter 4, the three-order nonlinearity of azobenzene side-chain polymer is investigated and detail analysis of nonlinear optical origin is discussed.In the third section, i.e. chapter 5. This chapter experimentally observed the normal Raman spectra (NRS) of the two azobenzene molecules PAAB and MO under CW 633 nm excitation and their infrared spectra. Also, we presented theoretical vibrational spectra of them calculated from density functional theory (DFT) method on the basis of B3LYP with 6-31 basis set. The calculated results show a very good agreement with the experimentally observed IR and Raman frequencies. We also report the SERS spectra of PAAB adsorbed on silver and copper layer, which offers a sensitive but simple method to detect the PAAB. The possible adsorbed states of PAAB molecule on the silver and copper surface are discussed. Also, the SERS spectrum of methyl orange on the silver mirror and its possible adsorbed state have been investigated.