precludes faster than light information transfer by SPP waves and is in agreement with relativistic causality. A second important feature concerning Fig. 2 A is that the air/gold dispersion curve is located inside of the light cone for glass defined by the equation K 2 = ( ω/c ) $^{2}$ϵ$_{glass}$ where K is a real light wave vector and ϵ$_{glass}$ = 2 . 25. Writing K = [ K$_{x}$, 0 , K$_{z}$ ] the wavevector of a TM (i. e., p polarized) plane wave propagating away from the interfaces into the glass side one see that SPPs propagating at the air/metal interface can radiate into the glass substrate if the condition is approximately fulfilled. Here we neglected the role of the imaginary part in Eq. 3. Similarly one can deduce that none of the SPPs propagating at the two interfaces can radiates into the air side. In order to have a more complete analysis one must actually consider the problem with two coupled interface 0/1 (glass/metal) and 1/2 (metal/air) supporting SPP waves and separated by a small distance D . The two interfaces are coupled by evanescent SPP waves tunnelling through the metal slab. Such a mathematical problem can only be treated numerically by resolving an implicit equation. As for the single interface this equation can be defined directly from Maxwell Equations [11]. However it is much easier and convenient for the following to remark with Raether [1] that Eq. 2 and consequently Eq. 3 are obtained by finding the zeros of the numerator in the Fresnel reflectivity coefficient for a TM wave coming from the dielectric side: with k$_{j}$ = ± √ (( ω/c ) $^{2}$ϵ$_{j}$ − k 2 $_{x}$). Actually Raether [1] reasoned with the denominator of the Fresnel coefficient due to different conventions for the signs of the wave vectors k$_{j}$ . However it is remarkable that the result is the same at the end of the calculations. Identically one can thus define the Fresnel coefficient for a TM wave reflected by the slab 0-1-2 [1, 12, 17]: and find the zeros of the numerator, i. e., one can solve the implicit equation 7 K$_{x}$ ≈ ± Real[( ω/c ) √ ϵ$_{0}$ϵ$_{1}$ ϵ$_{0}$ + ϵ$_{1}$ ] (11) R p 0 , 1 = ( k$_{0}$/ϵ$_{0}$ − k$_{1}$/ϵ$_{1}$ ) ( k$_{1}$/ϵ$_{1}$ + k$_{0}$/ϵ$_{0}$ ) , (12) R p 0 , 1 , 2 = R p 0 , 1 + R p 1 , $_{2}$e 2 ik 1 D 1 + R p 0 , $_{1}$R p 1 , $_{2}$e 2 ik 1 $_{D}$, (13) R p 0 , 1 + R p 1 , $_{2}$e 2 ik 1 D = 0 , (14)