Interplay of Order and Disorder in Self-assembled Photonic Crystals

Yurii A. Vlasov
Ioffe Institute of Physics and Technology, St. Petersburg, Russia

Self-organized synthetic opals possessing a face centered cubic fcc lattice are promising for fabrication of three-dimensional photonic crystal with a full photonic band gap in the visible. The fundamental limiting factor of this method is the large concentration of lattice defects and, especially, planar stacking faults, which are intrinsic to self-assembling growth of colloidal crystal. On the other hand this makes synthetic opals an ideal model system for the studies of the effects of disorder on photonic band structure.

We have numerically analyzed the mechanisms of light localization in such a periodic-on-average system with variable amount of disorder. It is known that photonic Bloch states can become strongly localized near the bandedges in a disordered photonic crystal. We show that Bloch states are disrupted and the new localization regime establishes when local fluctuations of the bandedge frequency caused by randomization of refractive index profile becomes as large as the bandgap width. This results in strong inhomogeneous broadening of the photonic stopbands in experimental reflection and transmission spectra. Transmission experiments performed on opal photonic crystal have shown the exponential decay of light throughout the gap region, which is ascribed to building up of this second regime of light localization.

We have experimentally studied the influence of a special type of defects - stacking faults, which are always present in self-assembled crystals - on the photonic band structure of opals by means of optical transmission, reflection and diffraction along different crystallographic directions. We found that stacking faults are responsible for large broadening and a doublet-like structure of the stop-bands along directions other than [111] direction of growth. As a result the inhomogeneously broadened stop-bands for different directions and polarizations are overlapped over considerable range of frequencies. This effect, however, does not result in diminished photonic DOS as in the case of an omnidirectional photonic band gap, but rather in a spatial confinement of light in large volumes of the crystal.

This work was done at Solid State Optics Department of Ioffe Institute of Physics and Technology in collaboration with Vasiliy Astratov, Mikhail Kaliteevskii, Mikhail Limonov and Alexander Kaplyanskii.

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