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5 May 2010 Light scattering in opal-based photonic crystals
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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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M. F. Limonov "Light scattering in opal-based photonic crystals", Proc. SPIE 7713, Photonic Crystal Materials and Devices IX, 771302 (5 May 2010);

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