content_image/356843.jpg content_image/356841.jpg content_image/356840.jpg content_image/356838.jpg content_image/356839.jpg
for a SPP wave propagating along the x direction. The choice of the sign convention con- necting the z and x components of the wave vector is a priori arbitrary and must be done only on a physical ground. Indeed, due to ohmic losses in the metal we expect an expo- 

nentially decaying SPP wave propagating along the interface. This condition implies the relation k ′ x · k ′′ x ≥ 0 [11]. This inequality is actually always fulfilled since from Eq. 3 one deduces 2 ′′ 

<img src='content_image/356843.jpg'>

<img src='content_image/356841.jpg'>

which is indeed positive because ϵ ′′ 1 > 0. By writing k$_{zj}$ = k ′ j + ik ′′ j one additionally obtains the relation 

<img src='content_image/356840.jpg'>

This relation fixes the sign conventions since the wave must also decay exponentially when going away from the interface in both media. More precisely one gets 

<img src='content_image/356838.jpg'>

<img src='content_image/356839.jpg'>

The product k ′ 1 · k ′′ 1 is positive if ϵ ′ 1 ≥ −| ϵ$_{1}$ | $^{2}$/ (2 ϵ$_{0}$ ), a fact which is indeed true for silver and gold interfaces with air or glass in most of the visible optical domain. However small negative 

