Higher order Bragg diffraction by strongly photonic fcc crystals: Onset of a photonic bandgap

We have measured angle-resolved reflectivity of fcc photonic crystals made of air spheres in titania (TiO₂), with lattice constants between 830 and 860 nm, that strongly interact with light. At normal incidence, we observe three new Bragg peaks at frequencies of 14800, 16700 and 18100 cm^-1 in the range of 2nd order diffraction. The peak frequencies have negligible dispersion over more than 30°angular range, unlike usual Bragg diffraction. The number, center frequencies and angular range of the peaks agree well with stop gaps in photonic bandstructures, computed fora model wherein the TiO₂ is distributed in shells about close-packed and interconnected air spheres. It appears that higher-order Bragg diffraction in strongly photonic crystals is caused by a complex coupling of many Bloch waves that results in dispersionless bands. The computed stop gaps persist for stronger photonic crystals with larger filling fraction or increased dielectric contrast. The 16700 cm^-1 peak is the precursor of a complete photonic band gap. (C) 2000 Elsevier Science B.V.