  At the exit surface of a photonic crystal, the intensity of the diffracted
wave can be periodically modulated, showing a maximum in the "positive"
(forward diffracted) or in the "negative" (diffracted) direction, depending on
the slab thickness. This thickness dependence is a direct result of the
so-called Pendellosung phenomenon, consisting of the periodic exchange inside
the crystal of the energy between direct and diffracted beams. We report the
experimental observation of this effect in the microwave region at about 14 GHz
by irradiating 2D photonic crystal slabs of different thickness and detecting
the intensity distribution of the electromagnetic field at the exit surface and
inside the crystal itself.
