Scattering by quasi-gratings and space-frequency coupling

The phenomenon of scattering by rough surfaces is interpreted and analyzed as an interaction process between the incident wave and the Fourier spectrum of the surface. Perturbative and rigorous numerical results are given for the simplest nonperiodic surface, the quasi-grating, which has only two components in its Fourier spectrum. Three different kinds of coupling between the space frequencies of the surface are distinguished. Finally, the rigorous computation of the intensity scattered by a grating having harmonics given by the first three terms of a Fibonacci series is reported. It shows that enhancement or reduction of the scattered intensity in a given direction may occur when many combinations of space frequencies of the surface can generate this direction.