Plasmon-polariton waves guided by thin lossy metal films of finite width: Bound modes of asymmetric structures - art. no. 125417

The purely bound electromagnetic modes of propagation supported by asymmetr ic waveguide structures, comprised of a thin lossy metal film of finite wid th on a dielectric substrate and covered by a different dielectric superstr ate, have been characterized at optical wavelengths. The dispersion of the modes with film thickness and width has been assessed and the effects cause d by varying the difference between the superstrate and substrate dielectri c constants on the characteristics of the modes have been determined. The m odes are quite different from those supported by corresponding slab structu res or similar finite-width symmetric waveguides. Unlike these limiting cas es, the dispersion with film thickness can exhibit an unusual oscillatory c haracter which is explained by a switching or swapping of the constituent i nterface modes. In addition, the four fundamental modes supported can evolv e such that none has a diminishing attenuation with diminishing film thickn ess. This rather complex evolution of modes is unique to asymmetric finite- width structures. Under certain conditions, a long-ranging mode having a fi eld 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 lone-ranging mod e in a comparable symmetric waveguide. Furthermore, its cutoff thickness is larger than that of the so mode in the corresponding asymmetric slab waveg uide, which implies that decreasing the film width increases the sensitivit y 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 subs trate or superstrate compared with the so mode guided by the corresponding slab structure.