Abstract

A thin metal film can support two (Fano) plasmon modes, of which one exhibits a symmetric and the other an antisymmetric field with respect to the plane bisecting the film. It has recently been shown¹ that, when the metal film is placed on a dielectric substrate, each of the two Fano modes evolves into two other distinct modes. However, the possible excitation of all four modes by realistic sources has not yet been explored. We therefore examine the field produced by an elementary (point or line) source placed in the film substrate geometry; any other arbitrary source can be described by a suitable superposition of such elementary sources. We then find that each of the two Fano modes along an isolated metal film accounts for a single viable mode when the film is placed on a substrate. Even though they are also solutions of the secular equation, the other two modes do not provide a distinct contribution to the total field due to an actual source. We therefore conclude that only two of the four plasmon modes may be experimentally observed in a film substrate geometry.

© 1985 Optical Society of America