content_image/373929.jpg content_image/373926.jpg content_image/373927.jpg
with SPP electromagnetic modes in this layered system is given by 

<img src='content_image/373929.jpg'>

<img src='content_image/373926.jpg'>

The coefficients α, β, γ are obtained by considering the boundary conditions and one finds 

<img src='content_image/373927.jpg'>

As an illustration we show in Fig. 4 the evolution of the real part of the magnetic field across a 50 nm thick gold slab sandwiched between the glass substratum and the air superstratum for an optical wavelength λ = 800 nm . As visible the SPP field is located in the vicinity of the air/gold interface and is evanescent on the air side. This is clearly reminiscent of our former analysis of SPPs propagating at the single air/metal. In addition however the wave is leaking radiatively (i. e., propagatively) into the glass substrate. However from Eq. 18 

