Study On The Aggregate Influences On The Nonlinearities Of Silver Nanoparticles

Noble metal nanoparticles have attracted significant attention because of their good chemical stability, unusual optical and electronic properties, and their potential applications in optoelectronic and photonic technologies. Their linear optical characteristics are recognized as being dominated by the surface plasmon resonance (SPR), which is sensitive to the boundary conditions. So, the shapes and sizes of these particles would provide important control over their linear and nonlinear responses. Parallel to the linear optical properties, their nonlinear optical properties have been reported on fewer occasions. In this dissertation, the aggregate influences on the second-order and third-order optical nonlinearities of silver nanoparticles have been studied. The dissertation is mainly comprised with the following parts.The Hyper-Rayleigh scattering experimental system has been established. The optical route of HRS is relatively comprehensible, but the HRS signal is easily disturbed. Therefore, the experimental condition is restricted. In practice, the excited way of off-focus is applied to avoid some unwanted nonlinear processes, such as stimulated Raman scattering, stimulated Brillouin scattering and dielectric breakdown etc. By using pNA, the quadratic dependence of the HRS signal with the incident light intensity, the linear dependence of the HRS signal with the material concentration, and scattering intensity as a function of the wavelength around 532 nm were checked to access the HRS system.A highly monodisperse 10.5 nm diameter silver nanoparticle has been synthesized by following the Lee and Meisel method. According to the inner reference method (IRM), the particle'sβis determined by HRS. The second-order polarization process has been discussed.The HRS has been employed to determine the second-order nonlinear optical response from the silver nanoparticle and its aggregates, which are induced by the presence of KNO₃. Through size distribution and TEM measurements, the morphology of aggregated silver nanoparticles has been observed as a chain-like aggregation. The HRS results reveal that there is an optimum size for the aggregates to yield the maximum HRS response. The dramatically enhanced second-order nonlinearity was explained by the enhanced electromagnetic (EM) field near the surface of thesilver nanoparticles as they approaching. The enhanced local EM field participates to the nonlinear polarization through surface and bulk contributions.The silver aggregates could also be caused by the addition of pyridine, and these two kind aggregates are obviously different. The different enhancement between the aggregates was considered with the diversity of separated distance between silver nanoparticles.The HRS signals from three volume silver nanoparticles with aggregate effects have been measured. With the size increase, the HRS enhancement is decreased. The observed size dependence of the second harmonic generation from the aggregates is explained by assuming the surface contribution and phase delay effect.The models of induced dipolar accumulation and interaction among the dipolar fields are established to analysis the above phenomena, and good agreements have been found between experimental results and theoretical simulations.The aggregate influence on the nonlinear refraction of silver nanoparticles has been observed by Z-scan technique. The nonlinear refraction of the silver aggregates is enhanced by 4 times at the optimum size of 120 nm. This phenomenon is explained by the local field effect.Using the Z-scan technique and theoretical calculation, the local field effect on the nonlinear refraction of Fe₂O₃ nanoparticles has been studied, and a consistence is reached.