Broadband coherent backscattering spectroscopy of the interplay between order and disorder in three-dimensional opal photonic crystals

We present an investigation of coherent backscattering of light that is multiple scattered by a photonic crystal by using a broadband technique. The results significantly extend on previous backscattering measurements on photonic crystals by simultaneously accessing a large frequency and angular range. Backscatter cones around the stop gap are successfully modeled with diffusion theory for a random medium. Strong variations of the apparent mean free path and the cone enhancement are observed around the stop band. The variations of the mean free path are described by a semiempirical three-gap model including band structure effects on the internal reflection and penetration depth. A good match between theory and experiment is obtained without the need of additional contributions of group velocity or density of states. It is found that the internal reflection contribution does not improve the semiempirical fits. We argue that the cone enhancement reveals additional information on directional transport properties that are otherwise averaged out in diffuse multiple scattering.