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We report on the photonic band gap phenomenon in the visible range in a three-dimensional dielectric lattice formed by close-packed spherical silica clusters. The spectral position and the spectral width of the optical stop band depend on the direction of light propagation with respect to the crystal axes of opal, and on the relative cluster-to-cavity refraction index n. Manifestations of the photonic pseudogap have been established for both transmission and emission spectra. The stop band peak wavelength shows a linear dependence on n. Transmission characteristics of the lattice have been successfully simulated by numerical calculations within the framework of a quasicrystalline approximation. ͓S1063-651X͑97͒13805-3͔

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ricevuto il 6 Settembre 1995) PACS 71.35 -Excitons and related phenomena (including electron-hole drops). PACS 78.66.Hf-II-VI semiconductors. PACS 42.25.Bs -Wave propagation, transmission and absorption.