Strong dispersive effects in the light-scattering mean free path in photonic gaps

We have performed measurements of the scattering mean free path (l(s)) in photonic crystals with different and controlled amounts of disorder. In the most perfect crystals, 1 order of magnitude chromatic variation in l(s) for just 3% shift around the band gap (27 nm in wavelength) is obtained. It is argued that the l(s) dispersion is governed by both the total density of states and the group index in the incident direction, with this last quantity being responsible for the large dispersion of l(s).