The small-slope approximation method applied to a three-dimensional slab with rough boundaries

In this paper we present new results on the small-slope approximation method. We consider different three-dimensional structures like a randomly rough surface separating two different media and a slab delimited by one or two rough surfaces. We extend the small-slope approximation to the fourth order terms of the perturbative development, and give the expression of the cross-sections for the different polarization states. Numerical examples are treated for the studied structures and a comparison with the small-perturbation is discussed.