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We present a new light scattering pattern in low-contrast opal-based photonic crystals (PhCs). The structure of real opals
is always imperfect because of the a-SiO2 particles being inherently inhomogeneous and nonuniform in size and average
dielectric permittivity. We found that opals possess all predictable properties of multi-component PhCs, which we define
as periodic structures consisting of inhomogeneous or multiple (three or more) components. By theory, by properly
tuning the permittivity of one of the components in ordered, low-contrast multi-component PhCs (for instance, of the
filler εf in an opal), one can produce selective disappearance of any non-resonant (hkl) stop band. A study of transmission
spectra of opals revealed that stop bands exhibit different (including resonant) behavior under variation of εf. Experiment
did not, however, substantiate complete disappearance of stop bands predicted by theory for an ordered PhC. In the
region of the predicted disappearance, a new effect has been observed, namely flip-over of the Bragg band, i.e.,
transformation of the Bragg dip into a Bragg rise. The flip-over effect, which has been studied in considerable detail in
the particular example of the (111) stop band, originates from the nonuniformity of a-SiO2 particles. This nonuniformity
leads to additional broad-band light scattering, the character of which is determined by Mie scattering. Thus, Mie
scattering is responsible for two components in opal transmission spectra, more specifically, narrow Bragg bands and
broad-band background. Their interference gives rise to formation of the Fano resonance, which in opal spectra becomes
manifest, first, in a Bragg band asymmetry, and, second, in the flip-over effect, i.e., transformation of a photonic stop
band into a photonic pass band.
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