Anomalous light propagation, finite size-effects and losses in real 3D photonic nanostructures

The propagation of light through 3D ordered photonic nanostructures is strongly affected by real aspects of actual crystals, especially at the spectral range where the wavelength of light is on the order of the lattice parameter. We perform a theoretical study on anomalous light propagation in finite thin artificial opal slabs made of a reduced number of layers. The vector KKR method we use accounts for the finite character of the structure and for different sources of losses. We show that an interplay between finite-size effects and losses engineers group velocity at this high energy range. Only depending on the crystal size and extinction, light propagation can either be superluminal (positive or negative) or approach zero for certain frequencies. The numerically calculated group index is in good agreement with experimental results some of which remained partially unexplained.