Opal photonic crystals viewed along the ͓111͔ direction of the fcc structure have a threefold symmetry axis; however this microscopic symmetry is difficult to observe in optical measurements performed on macroscopic areas containing microdomains with different orientations. In this work polarized transmittance measurements on ͓111͔-stacked silica opals with single oriented microdomains, identified by field-emission scanning electron microscopy and laser-scanning confocal microscopy, demonstrate different optical response of twin structures with the two possible vertical stacking sequences. A detailed comparison with theory shows that microtransmittance experiments probe the photonic band structure along the ⌫-L-K and ⌫-L-U orientations of the Brillouin zone, respectively, thus giving conclusive evidence for macroscopic optical response related to the presence of a threefold ͑instead of a sixfold͒ symmetry axis in the photonic microstructure.
A chemical model is proposed to explain the passivation of carriers upon etching and their reactivation by the presence of gases in p + -type mesoporous silicon (m-PS). The model is based on the results obtained by means of FTIR, EPR, NMR spectroscopies and ab-initio calculations. This chemical description accounts for the behaviour of mPS when contacted with NO 2 or NH 3 . B atoms in subsurface location play a prominent role.
We report on the observation of self-alignment of nematic liquid crystals into colloidal photonic crystals, over distances much larger than the typical size of the voids between the spheres. We observe that the infiltrated structure possesses a unique optical axis that is determined by an intrinsic structural anisotropy of photonic crystal opals. We develop a simple model to describe this self-alignment based on the connectivity of the pores. The resulting structure constitutes a polarization dependent photonic crystal that can be controlled electrically.
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