Mapping of surface plasmon dispersion in thin Ag–Au layered composite films

Propagating surface plasmon polaritons (SPPs) at metal-dielectric interfaces allow extreme light confinement enabling important technologies such as label-free real-time sensing and nanoscale optical wave guiding. In this work, we experimentally and theoretically map the surface plasmon dispersion of thin films of sequentially deposited silver and gold based on the Kretschmann-Raether configuration using spectroscopic ellipsometry. The results show efficient excitation of SPP modes related to the symmetric and antisymmetric modes of an insulator-metal-insulator waveguide. Since the symmetric modes have a larger electric field inside the bimetallic layer than the antisymmetric modes, we show that the dispersion for both modes can be tuned independently by controlling the thickness ratio and deposition order of metals in the film. This behavior is fundamentally different than modes in a single film or a thin film alloy. Results are in agreement with the analytical eigenmode dispersion of these modes and with a full transfer matrix model. Our results provide a complete understanding of surface plasmon modes in thin bilayer or multilayer metallic films and their ability to influence the propagation of surface plasmon waves.