COUPLED SURFACE-PLASMONS IN STRUCTURES WITH THIN METALLIC LAYERS

If the negative-permittivity region in a layered structure is very thin ( APP 1STH 300 A for Ag at 6328 A excitation), the two surface plasmons at its interfaces couple in its interior to yield a coupled-surface-plasmon mode whose propagation constant increases with decreasing layer thickness. These modes were observed by measuring reflectance versus angle of incidence through a high-index prism for TM-polarized light. For very thin Ag films, the reflectance minimum diverges from the phase-matching condition, as determined from independently measured refractive indices and thicknesses. This divergence apparently arises from separation of the pole and the zero in the reflectance versus propagation vector as the Ag thickness is decreased.