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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 

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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 

