Saturation effect in the optical response of Ag-nanoparticle fractal aggregates -: art. no. 035438

The enhancement and extinction spectra for fractal aggregates consisting of monodisperse nanosized silver particles are studied theoretically. We find the iterative solution to the coupled-dipole equations in the quasistatic approximation, taking into account a contribution of the nonlinear susceptibility chi((3)) to the intensity-dependent dielectric function. Using a modified version of Broyden's method, the self-consistent problem was solved and convergence of the solutions 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 obtained previously within the linear theory. Our 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.