Plasmonic coupling and how standard ellipsometry can feel surface plasmon

Interacting electromagnetic modes are known to hybridize, and their dispersions split instead of crossing, resulting in an energy gap between them. Such a coupling exists between localized plasmon of nanoparticles and surface plasmon of a supporting metallic substrate. Here, a gold film with gold nanoparticles deposited on the surface was investigated by ellipsometry in the standard configuration with measurements at external reflection and in Kretschmann geometry with measurements at internal reflection. Dispersion relations obtained from measurements in both configurations coincide and demonstrate expected splitting. Surface plasmon on a clean surface do not couple with external light, as such interaction would violate the law of momentum conservation. However, coupling with localized plasmons of deposited nanoparticles allowed excitation of surface plasmon by external light, as confirmed by the results of measurements in the standard ellipsometric configuration.