the dispersion is close to the asymptotic light lines: we speak about Zenneck surface modes by opposition to Fano and evanescent modes existing close to the interband [13, 16]. An FIG. 2: Dispersion relations of SPP waves propagating along a air/gold (thin dashed curve) and glass/gold (thick dashed curve) interface. (A) Real part of the dispersion relations showing the evolution of k ′ x with ω/c = 2 π/λ . The light cones corresponding to an optical wave propagating in air and glass (i. e., in the bulk medium) are represented by a thin and thick continuous line respectively. The back bending at λ = 520 nm correspond to the interband resonance. (B) Imaginary part of the dispersion relations showing the evolution of L$_{SPP}$ with ω/c = 2 π/λ . The damping is higher for the glass/gold interface. important feature occurs close to this interband transition since the slope in Fig. 2A is diverging. This means that the group velocity defined by v$_{g}$ = ∂ω/∂k ′ x is infinite. One can qualitatively deduce that there is however no paradox with causality by remarking that the propagation length is approaching zero at such wavelengths. Actually the SPP life time τ$_{SPP}$ defined by v$_{g}$ · τ$_{SPP}$ ≈ L$_{SPP}$ is tending as well to zero. This is a clear signature of the absence of significant SPP propagation in the interband spectral region. This fact actually 6