Plasmonics of media with negative material parameters
An important and unique feature of materials with negative material parameters is their ability to support a variety of surface electromagnetic modes. These surface modes have electromagnetic ﬁelds that have maximum amplitude at the surface of the medium with negative material parameters and the ﬁelds decay exponentially inside both the bulk of the medium and in vacuum (or the positive medium) outside. This feature is well known and extensively studied in the case of metals or plasmas, which have negative dielectric coeﬃcients, and the surface modes are called surface plasmon (Ritchie 1957, Raether 1986). On a metal surface, the surface plasmons are essentially collective excitations of electrons with the charges displaced parallel to the (real part of the) wave vector on the surface of the metal. The interior of the metal is, however, shielded from these electromagnetic ﬁelds and the wave amplitude decays into the bulk of the metal as in a regular conducting medium. These surface plasmon modes have also been termed Zenneck waves or Sommerfeld waves in the context of the ionosphere. Fig. 7.1 qualitatively represents a surface plasmon mode on the surface of a metal. These charge density waves ﬂow on the surface, they scatter oﬀ obstacles on the plane, reﬂect and refract oﬀ interfaces between two surfaces: thus they can literally be considered twodimensional entities that exist on the surface. Surface plasmons on a plane surface cannot directly interact with propagating radiation in vacuum and are coupled mainly through scattering events (surface roughness, Bragg scattering in the case of periodic scatterers such as a diﬀraction grating, etc.). There are also localized surface plasmons that can be conﬁned to the surface of a sphere, a cylinder or, in general, any scatterer made of a negative dielectric medium. These plasmons can be directly excited by an incident plane wave of light.