Linear and nonlinear optical properties of Ag/Au bilayer thin films

The linear and nonlinear optical properties of Ag/Au bilayer metallic thin films with a total thickness of around 20 nm and with different Ag/Au mass-thickness ratios were studied. This study shows that the spectral dispersion of the effective refractive index of bilayer films can be tuned by controlling the mass-thickness ratio between Au and Ag. Improvement of the figure-of-merit for potential plasmonic applications and linear optical filters in the visible spectral range are reported and discussed. The nonlinear optical properties of bilayer metal films studied using femtosecond white-light continuum pump-probe experiments are also shown to be tunable with this ratio. The nonlinear change of optical path length is extracted from the pump-probe data and agrees with simulated values derived from a combination of the two-temperature model, describing the ultrafast electron heating dynamics, and a physical model that describes the dielectric permittivity of Au as a function of electron and lattice temperature.