Three-dimensional photonic crystals: opals
Opals are three-dimensional (3D) photonic crystals consisting of dielectric spheres arranged in a fcc lattice. Artificial opals can be prepared by self-assembling of, e.g., silica or polystyrene spheres in a colloidal solution. These are the so-called direct opals, which possess a photonic band gap along the  or G-L direction and are interesting for the study of light propagation, diffraction, scattering, focusing etc in a 3D geometry. Inverse opals can be prepared by filling the silica template by a high refractive-index material (usually Si or TiO2) and removing the template, yielding a structure which in principle supports a complete photonic band gap.
Our research is concerned with experimental and theoretical studies of the optical properties of opals in the low energy region (close to the fundamental stop band) as well as in the high-energy region (close to the second-order stop band and above), where Bragg diffraction effects related to high-order crystalline planes come into play. Opals preparation and their infiltration by luminescent polymers is also pursued. On the experimental side, we studied optical properties by angle-resolved reflectance and related the presence of weak structures at large angles of incidence to the occurrence of Bragg diffraction effects. We determined the effective refractive index (real and imaginary parts) as a function of frequency by means of white-light interferometry and demonstrated the occurrence of slowing-down as well as superluminal propagation effects within the first- and second-order stop bands. Also, we demonstrated the presence of a three-fold (instead of a six-fold) symmetry axis in angle-resolved transmittance from micro-domains in thin opal films and related this macroscopic symmetry to the distinction between GLK and GLU orientations in the microscopic structure. On the theoretical side, we developed an accurate simulation method of the optical properties based on a Fourier-modal (scattering matrix) formalism in which the spheres are discretized into cylindrical slices.
L.C. Andreani, A. Balestreri, D. Comoretto, M. Galli, J.F. Galisteo-Lopez, M. Liscidini, F. Marabelli, M. Patrini
MIUR-FIRB 2003-2005 "Miniaturized systems for electronics and photonics"
A. Balestreri, "Optical properties of opal-based photonic crystals" (University of Pavia, 2007)
Optical response with three-fold symmetry axis on oriented microdomains of opal photonic crystals,
L. C. Andreani, A. Balestreri, J. F. Galisteo-López, M. Galli, M. Patrini, E. Descrovi, A. Chiodoni, F. Giorgis, L. Pallavidino, and F. Geobaldo,
Slow to superluminal light waves in thin 3D photonic crystals,
J. F. Galisteo-López, M. Galli, A. Balestreri, M. Patrini, L. C. Andreani, and C. López,
Optical properties and diffraction effects in opal photonic crystals,
A. Balestreri, L.C. Andreani, and M. Agio,
Effective refractive index and group-velocity determination of three-dimensional photonic crystals by means of white-light interferometry,
J.F. Galisteo-López, M. Galli, M. Patrini, A. Balestreri, L.C. Andreani, and C. López,