We discuss the necessary conditions for a two-level system in the presence of
an isotropic band edge to be transparent to a probe laser field. The two-level
atom is transparent whenever it is coupled to a reservoir constituted of two
parts - a flat and a non-flat density of modes representing a PBG structure. A
proposal on the reconstruction of the band edge profile from the experimentally
measured susceptibility is also presented.Comment: 15 pages, 3 figure