  We report the observation of a Fano resonance between continuum Mie
scattering and a narrow Bragg band in synthetic opal photonic crystals. The
resonance leads to a transmission spectrum exhibiting a Bragg dip with an
asymmetric profile, which can be tunably reversed to a Bragg rise. The Fano
asymmetry parameter is linked with the dielectric contrast between the
permittivity of the filler and the specific value determined by the opal
matrix. The existence of the Fano resonance is directly related to disorder due
to non-uniformity of a-SiO2 opal spheres. Proposed theoretical "quasi-3D" model
produces results in excellent agreement with the experimental data.
