Fourier optics heuristics for diffraction at infinity by an index discontinuity in a one-dimensional slab

We study the far-field reflected diffraction pattern of an index discontinuity in a thin one-dimensional slab illuminated by a plane wave and show that a time-saving modeling technique based on plane wave expansion approaches fairly well the Maxwell-based rigorous models. This method is simple to implement, and it furthermore allows a good understanding of the optical phenomena involved in the propagation of light through the slab.