Photopolymerizable metal nanoparticle-/semiconductor quantum dot-polymer nanocomposite materials for nonlinear optics 非線形光学のための光重合性金属ナノ微粒子-/半導体量子ドット⁻ポリマーコンポジット材料

博士論文

2011

This thesis presents experimental studies of Bragg grating formation in and the nonlinear optical properties of photopolymerizable nanocomposite materials dispersed with metal (Au or Pt) nanoparticles or semiconductor CdSe quantum dots (QDs). Such photopolymerizable nanocomposite systems are attractive because they are capable of forming multi-dimensional photonic lattice structures and hold promise for nonlinear optical applications. In the first place, the optical properties and possible applications of photonic nanocomposite optical materials, especially metal nanoparticle-/semiconductor quantum dot-dielectric nanocomposite materials, are described. The origins of their nonlinear optical properties are discussed and previous studies on their optical nonlinearities are summarized. Experimental techniques (Z-scan method and degenerate multi-wave mixing) for nonlinear optical measurements are explained. Based on the Gaussian decomposition method, a theoretical investigation is carried out to examine the characteristics of the closed-aperture Z-scan when a nonlinear medium possesses saturable absorption as well as third- and fifth-order optical nonlinearities simultaneously. In addition, the photopolymerization reaction processes and the principle of holographic assembly of nanoparticles in polymer are also explained. In the second place, experimental studies of Bragg grating formation in and the nonlinear optical properties of photopolymerizable nanocomposite polymer films dispersed with metal (Au or Pt) nanoparticles are given. The synthesis and the characterization of hyperbranched polymer (HBP)-metal (Au or Pt) nanoparticle complex are described. The recording and diffraction properties of 1D Bragg grating structure formed by holographic assembly of nanoparticles in HBP-metal (Au or Pt) nanoparticle complex-polymer nanocomposite films are presented. In addition, Z-scan measurements of the nonlinear optical properties of uniformly cured HBP-metal (Au or Pt) nanoparticle complexpolymer nanocomposite films are described. Saturable absorption is observed. Experimental results show that the real and imaginary parts of the measured third-order nonlinear optical susceptibility are positive (negative) and negative (negative) for nanocomposite films doped with HBP-Au (Pt) nanoparticle complex. The observed signs can be qualitatively explained by the Kramers?Kronig relations. Based on the Maxwell-Garnett model, the intrinsic third-order nonlinear optical susceptibilities of Au and Pt nanoparticles are estimated. In the third place, experimental studies of Bragg grating formation in and the nonlinear optical properties of photopolymerizable semiconductor CdSe QD-polymer nanocomposite films are presented. An aqueous synthetic method is used to obtain CdSe QDs, which are then extracted into ionic liquid monomers for their high dispersion. The fabrication of 1D Bragg grating structure with diffraction efficiency as high as 100% is given. The formed 1D Bragg grating structure is confirmed by an optical microscope and the confocal micro-Raman spectroscopy. Moreover, Z-scan and degenerate multi-wave mixing studies of the nonlinear optical properties of cured CdSe QD-polymer nanocomposite films are also described. It is found that the nanocomposite films exhibit the coexistence of negative third- and positive fifth-order optical nonlinearities. The intrinsic third- and fifth-order nonlinear optical susceptibilities of CdSe QDs are obtained. Furthermore, the observation of nonlinear Bragg diffraction from the recorded Bragg grating structure under intense laser pulses is also described. Finally, conclusions and future studies are given.