Multiple bragg diffraction and interference effects in 3D photonic crystal films

Shaping mechanisms of Bragg reflection spectra for thin photonic crystal films with the opal crystal lattice symmetry are studied making use of the dynamical diffraction theory generalized to the case of high dielectric contrast. It is shown that under the multiple Bragg diffraction conditions additional short-period oscillations of the reflection coefficient (the interference “comb”) can be manifested in the spectra of the photonic crystal films possessing high structural perfection. From the comparison of the Bragg reflection band shape with the energy spectrum of photonic crystal electromagnetic eigenmodes we conclude that the extra structure in the spectra is due to the interference of additional low-group-velocity modes.