Wl. Vos et Hm. Van Driel, Higher order Bragg diffraction by strongly photonic fcc crystals: onset ofa photonic bandgap, PHYS LETT A, 272(1-2), 2000, pp. 101-106
We have measured angle-resolved reflectivity of fcc photonic crystals made
of air spheres in titania (TiO2), with lattice constants between 830 and 86
0 nm, that strongly interact with light. At normal incidence, we observe th
ree new Bragg peaks at frequencies of 14800, 16700 and 18100 cm(-1) in the
range of 2nd order diffraction. The peak frequencies have negligible disper
sion over more than 30 degrees angular range, unlike usual Bragg diffractio
n. The number, center frequencies and angular range of the peaks agree well
with stop gaps in photonic bandstructures, computed for a model wherein th
e TiO2 is distributed in shells about close-packed and interconnected air s
pheres. 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 photon
ic crystals with larger filling fraction or increased dielectric contrast.
The 16700 cm(-1) peak is the precursor of a complete photonic band gap. (C)
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