We have studied enhanced backscattering of both polystyrene opals and strongly photonic crystals of air spheres in TiO_2 in the wavelength range of first and higher order stop bands [1]. The shape of the enhanced backscattering cones is well described by diffusion theory. We find transport mean free paths of the order of 15 micron both for opals and air spheres. Close to the stop band the cone width is decreased due to internal reflections generated by the photonic band structure. Widening occurs due to attenuation of the coherent beam by Bragg scattering. We present a model that incorporates these effects and successfully explains all data.
[1] A.Femius Koenderink, Mischa Megens, Gijs van Soest, Willem L. Vos and Ad Lagendijk, accepted by Phys. Lett. A