| This thesis presents a theoretical study of the third-order susceptibility, chi (3), for Ag dielectric composites. The results reveal the critical role of saturation for optical transitions between discrete states of conduction electrons in such metal quantum dots.; The calculated size dependence of chi(3) for individual Ag nanoparticles is in good agreement with published experimental results, in contrast to the previously proposed theoretical approach. Saturation effects are responsible for a decrease of the local field enhancement factor that is of particular importance for surface-enhanced phenomena, such as Raman scattering and nonlinear optical responses.; The enhancement and extinction spectra for fractal aggregates consisting of monodisperse nanosized Ag particles were also studied. For that, an iterative solution to the coupled-dipole equations in the quasistatic approximation was needed. The solution was found taking into account the contribution of the nonlinear susceptibility, chi(3), to the intensity-dependent dielectric function. Using a modified version of Broyden's method, a self-consistency problem was solved, and convergence of the solution for the dipole moments was achieved even in the case of "weak" hysteretic behavior. The nonlinearity partly suppresses the giant enhancement of the local electrical fields predicted within the linear theory. Calculations for fractal aggregates consisting of silver particles with radii of 3 and 5 nm show strong saturation effects of the optical transitions, which, in the case of 5 nm particles, are accompanied by hysteretic effects. |
