  We demonstrate a mesoscopic self-collimation effect in photonic crystal
superlattices consisting of a periodic set of all-positive index 2D photonic
crystal and homogeneous layers. We develop an electromagnetic theory showing
that diffraction-free beams are observed when the curvature of the optical
dispersion relation is properly compensated for. This approach allows to
combine slow light regime together with self-collimation in photonic crystal
superlattices presenting an extremely low filling ratio in air.
