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

The purely bound electromagnetic modes of propagation supported by asymmetric waveguide structures, comprised of a thin lossy metal film of finite width on a dielectric substrate and covered by a different dielectric superstrate, have been characterized at optical wavelengths. The dispersion of the modes with film thickness and width has been assessed and the effects caused by varying the difference between the superstrate and substrate dielectric constants on the characteristics of the modes have been determined. The modes are quite different from those supported by corresponding slab structures or similar finite-width symmetric waveguides. Unlike these limiting cases, the dispersion with film thickness can exhibit an unusual oscillatory character which is explained by a switching or swapping of the constituent interface modes. In addition, the four fundamental modes supported can evolve such that none has a diminishing attenuation with diminishing film thickness. This rather complex evolution of modes is unique to asymmetric finite-width structures. Under certain conditions, a long-ranging mode having a field distribution that is suitable to excitation using an end-fire technique can be supported. The long-ranging mode has a cutoff thickness below which it is no longer propagated, and its attenuation near cutoff decreases very rapidly, much more so than the attenuation related to the long-ranging mode in a comparable symmetric waveguide. Furthermore, its cutoff thickness is larger than that of the $s_b$ mode in the corresponding asymmetric slab waveguide, which implies that decreasing the film width increases the sensitivity of the mode to the asymmetry in the structure. This result is interesting and potentially useful in that the propagation characteristics of the mode can be affected by a smaller change in the dielectric constant of the substrate or superstrate compared with the $s_b$ mode guided by the corresponding slab structure.