Plasmon-polariton waves guided by thin lossy metal films of finite width: Bound modes of symmetric structures

Abstract

The purely bound electromagnetic modes of propagation supported by symmetric wave guide structures comprised of a thin lossy metal film of finite width embedded in an infinite homogeneous dielectric have been characterized at optical wavelengths. The modes supported are divided into four families depending on the symmetry of their fields. In addition to the four fundamental modes that exist, numerous higher order ones are supported as well. A nomenclature suitable for identifying all modes is discussed. The dispersion of the modes with film thickness and width has been assessed and the effects of varying the background permittivity on the characteristics of the modes determined. The frequency dependence of one of the modes has been investigated. The higher order modes have a cutoff width, below which they are no longer propagated and some of the modes have a cutoff thickness. One of the fundamental modes supported by the structure exhibits very interesting characteristics and is potentially quite useful. It evolves with decreasing film thickness and width towards the transverse electromagnetic (TEM) wave supported by the background (an evolution similar to that exhibited by the ${s}_{b}$ mode in symmetric metal film slab wave guides), its losses and phase constant tending asymptotically towards those of the TEM wave. Attenuation values can be well below those of the ${s}_{b}$ mode supported by the corresponding metal film slab wave guide. Low mode power attenuation in the neighborhood of 10 to 0.1 dB/cm is achievable at optical communications wavelengths, with even lower values being possible. Carefully selecting the film's thickness and width can make this mode the only long-ranging one supported. In addition, the mode can have a field distribution that renders it excitable using an end-fire approach. The existence of this mode renders the finite-width metal film wave guide attractive for applications requiring short propagation distances and two-dimensional field confinement in the transverse plane.