The influence of plasma waves on the dispersion of surface plasmons : experimental evidence

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Abstract
Surface plasmon excitation by attenuated total reflection in Otto geometry has been carried out in a wide spectral range between 1.9 and 5 eV. Samples were Al-films covered by thin (5 nm) continuous Ag-layers. Results were discussed in the framework of a hydrodynamic model of the conduction electrons both in the film and in the substrate using additional boundary conditions. The calculated and measured dispersion relation and reflectivity curves show that longitudinal plasma waves have to be taken into account. A new branch of the surface plamon dispersion is found experimentally between 4.2 and 4.6 eV. It is attributed to coupled plasma wave-surface plasmon modes localized in the tail of the electronic charge density at the interfaces