Zero-(n)over-bar-bandgap in photonic crystal superlattices

We demonstrate that photonic superlattices consisting of a periodic distribution of layers of materials with positive index of refraction and photonic crystal slabs that, at the operating frequency, have negative effective index of refraction present a photonic gap that corresponds to the frequency at which the spatial average of the refractive index distribution, taken over the unit supercell of the superlattice, vanishes. We also show that, unlike the Bragg gaps, the frequency of this zero-(n) over bar gap is invariant to the geometrical scaling of the superlattice or the direction of wave propagation in the superlattice. (c) 2006 Optical Society of America.