(a) Schematic of propagating SPPs on a metal-dielectric interface. Dispersion of SPPs propagating along a metal-air interface for (b) real frequency and complex wave vector, and (c) complex frequency and real wave vector. The flat asymptote dashed line represents the non-retarded surface plasmon solution. The straight solid line represents the dispersion curve of free-space propagating light. Reprinted from reference [18] with permission.