Investigations On Third-order Optical Nonlinearities Of Metal And Semiconductor Nanoparticles

Metal nanoparitcal composite films and semiconductor quantum dots have large third-order nonlinear susceptibilities and fast time responses due to quantum size effect, dielectric confinement effect, surface effect, localized field effect, which make them be important nonlinear optical materials. Composite thin films comprising Ag and Au nanoparticles embedded in Bi₂O₃,TiO₂ or Al₂O₃ and semiconductor quantum dotsα-FeOOH, CdSe andγ-MnS were studied by the Z scan technique in this thesis. The linear and nonlinear optical properties were investigated. The paper is composed of following contents.The first chapter describes the development and significance of studies on nanoparticles. The second chapter introduces the Z-scan technique which is one of the important methods for measuring nonlinearities. In the third chapter, the nonlinear properties of the metal nanoparticle composite films of Ag:Bi₂O₃, Au:TiO₂ and Au:Al₂O₃ are studied both experimentally and theoretically. In the last chapter, the nonlinear properties of semiconductor quantum dots of a-FeOOH, CdSe andγ-MnS are studied from experiment and relative theories.Some new phenomena have been observed from studies. The main results are briefly listed as follows:1. For the first time, Ag:Bi₂O₃ composite films were measured by the Z-scan technique with picosecond laser pulse. The effects of Ag concentration,probing wavelength and preparation condition were discussed. The peak-valley configuration of this Z scan transmittance curve indicated a negative(self-defocusing) nonlinearity. When detected by a wavelength near surface plasmon resonance(SPR) peak, the third-order nonlinear susceptibilityχ(3) showed an enhanced result, in which SPR contributed enhancement by one order. SPR enhancement and appropriate metal concentration made nonlinearities enhanced by two orders. The theoretical calculation kept consistent with the experimental results. (Manuscript has been accepted by《Acta Phys. Sin.》(in Chinese) and will be published on July, 2007)2. For the first time, Au:Al₂O₃ and Au:TiO₂ composite films were measured by the Z scan technique with picosecond laser pulse. The effects of Au size,probing wavelength and preparation condition were discussed. The peak-valley configuration of this Z-scan transmittance curve indicated a negative(self-defocusing) nonlinearity. The third-order nonlinear susceptibility increased as the nanopatical size and substrate dielectric constant increased, in which the quantum size effect dominated. Surface plasmon resonance and large metal particles or the substrates with large dielectric constant all made the third-order susceptibilityχ(3) increased by two orders. The theoretical calculation kept consistent with the experimental results. ( This work has been delivered to《Acta Phys. Sin.》(in Chinese) and is being examined)3. For the first time, the nonlinearities ofα-FeOOH quantum dots (Q-dots) were measured by the Z scan technique with picosecond laser pulse. The effects of Q-dot size and probing wavelength were discussed. The probing wavelength and Q-dot size had large effect on the nonlinear response. Under the condition of large Q-dot size and resonance enhancement larger than band gap, the third-order susceptibility values of the samples were larger, in which quantum size dominated. The reverse saturated absorption also came to existence. Resonance enhancement and large quantum dot madeχ(3) increased nearly by 16 times. The experimental results kept consistent with the theoretical calculation. (Manuscript of work has been finished.) 4. For the first time, the nonlinearities of CdSe Q-dots were measured by the Z scan technique with picosecond laser pulse. The effects of Q-dot size and probing wavelength were discussed. The probing light and quantum size had large effect on the nonlinear response. Under the condition of large quantum dot size and resonance enhancement larger than band gap, the third-order susceptibility values of the samples were larger. The reverse saturated absorption also came to existence. Resonance enhancement and large Q-dot madeχ(3) increase nearly by 20 times. The experimental results kept consistent with the theoretical calculation. (Manuscript of this work has been finished)...