Simultaneous observation of surface plasmons on both sides of thin silver films

Both prism and grating coupling have been used to optically excite surface plasmons on the two sides of thin silver films. This allows the optical per mittivity of the silver films to be determined simultaneously at the metal/ air and the (protected) glass/metal interfaces. It is found that there is a significant difference between the permittivities for the two interfaces w hich depends upon the thickness of the metal film. This difference is tenta tively attributed to the silver films being more porous at the air interfac e, although an alternative interpretation, based on possible changes in ele ctron density through oxygen absorption by silver, is not excluded.