Recent advances in the comprehension of the growth dynamics of colloidal crystal films opens the door to conscious
design of experiments aiming at fabricating lattices in which the density of intrinsic defects is minimized. Since such
imperfections have a dramatic effect on scattered light of wavelength smaller than the lattice constant, the evaluation of
the experimental optical response at those energy ranges, based on the comparison to rigorous calculations, is identified
as the most sensitive guide to accurately evaluate the progress towards the actual realization of defect free colloidal
crystals. The importance of the existence of a certain distortion becomes particularly relevant at the above mentioned
energy range. We have thoroughly analyzed the effect of fine structural features on the optical response to conclude that,
rather than the generally assumed FCC lattice of spheres, opal films are better approximated by a rhombohedral
assembly of distorted colloids. Interparticle distance of actual colloidal crystals coincides with the expected diameter for
spheres belonging to the same close-packed (111) plane but differs significantly in directions oblique to the [111] one.
We perform a theoretical study on the group velocity for finite thin artificial opal slabs made of a reduced number of
layers in the spectral range where the light wavelength is on the order of the lattice parameter. The vector KKR method
including extinction allows us to evaluate the finite-size effects on light propagation in the ΓL and ΓX directions of fcc
close-packed opal films made of dielectric spheres. The group is index determined from the phase delay introduced by
the structure to the forwardly transmitted electric field. We show that for certain frequencies, light propagation can either
be superluminal -positive or negative- or approach zero depending on the crystal size and absorption. Such
anomalous behavior can be attributed to the finite character of the structure and provides confirmation of recently
emerged experimental results.
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