We made large, highly ordered structures consisting of crystals of macropores in titania (TiO 2 ), by template-assisted growth. The crystals were characterized by synchrotron smallangle X-ray diffraction, X-ray absorption, wide-angle X-ray diffraction, scanning electron microscopy, optical microscopy, optical reflectivity, and Raman spectroscopy. Care was taken to make well-ordered templates, by slowly growing colloidal crystals from lightly charged polystyrene latex particles and carefully drying them to form opals. Solid material was deposited in the opal template by precipitation from an alkoxide hydrolysis. Subsequently, the samples were heated to 450 °C to form anatase TiO 2 and to remove the latex template, which resulted in a macroporous crystal, inverse opal, or air-sphere crystal. The macropores were close-packed and interconnected by windows, and small additional voids were located at interstices between the pores. The macropores were arranged on a face-centered cubic lattice with domains of more than 750 × 350 × 250 unit cells. The size polydispersity of the air spheres and mean-squared displacements from the lattice sites were studied for the first time. Both quantities were small, thus quantifying the excellent long-range order. The lattice parameter was shrunk by about 30% relative to the template, irrespective of the radii of the pores, while the long-range order persisted. The surfaces of the macropores appeared to be rough up to length scales of ∼10 nm, the sizes of the largest TiO 2 crystallites, with a roughness distribution following a power law. The volume fraction of the solid backbone was determined for the first time in situ by X-ray absorption and was found to be between 5 and 12 vol % TiO 2 . The wavelengths of the optical Bragg reflections were proportional to the radii of the air spheres. By the use of the measured TiO 2 fraction, the refractive index of the solid matrix was found to be 2.7 ( 0.4, in good agreement with the refractive index of massive anatase TiO 2 . We briefly explored the assembly of macropores in rutile-TiO 2 and in rock salt. It was concluded that macroporous crystals are highly effective three-dimensional photonic crystals.

We have investigated the fluorescence from R6G dye molecules embedded in fcc photonic crystals with a large range of lattice parameters. Both polystyrene opals and alumina inverse opals are studied, allowing us to compare direct and inverted structures. We observe clear stop bands in the fluorescence spectra, whose center positions, widths, and depths are analyzed and compared to stop bands from reflectivity measurements. In the frequency range of first-order stop gaps, the measured stop band centers and widths agree well with theoretical predictions. The depths are interpreted in terms of the mean free path (disorder) and the Bragg attenuation length (order). We observe intriguing enhanced emission at the blue side of the stop bands, which is attributed to the escape of diffuse light from the photonic crystal (related to both order and disorder). We perform the first experiments in the range of second-order stop gaps, which is the regime where the photonic band gap is anticipated. We observe complex multiple-Bragg features that correlate favorably with reflectivity peaks.

Broadband Fivefold reduction of vacuum fluctuations probed by dyes in photonic crystals Koenderink, A.F.; Bechger, L.; Schriemer, H.P.; Lagendijk, A.; Vos, W.L.

We present the first experiments that demonstrate strong angle-independent modification of spontaneous emission spectra from laser dyes in photonic crystals, made of inverse opals in titania. We show that both the fluorescence quantum efficiency and weak disorder play a key role in interpreting the experimental data. We compare the angle-independent emission spectra of dye in photonic crystals with spectra from such crystals with much smaller lattice spacings, for which emission is in the long wavelength limit. The ratio of emission power spectra shows inhibition of emission up to a factor $ 5 over a large bandwidth of 13% of the first order Bragg resonance frequency. The inhibition shifts to increasing wavelength with the lattice parameter, confirming the photonic nature of the phenomenon. The center frequency and bandwidth of the inhibition agree with the calculated total density of states, but the measured inhibition of the vacuum fluctuations is much larger. This result is confirmed by experiments using different dyes. We likely probe the strongly modulated local photonic density of states, due to the spatially nonuniform distribution of dye molecules over the unit cell.

Germanium is electrodeposited in a template formed from a dried suspension of silica spheres. The germanium completely fills the pores of the silica matrix. The semiconductor, as deposited, is amorphous but can be crystallized by annealing. Selective dissolution of the silica template gives a macroporous germanium-air sphere matrix, which offers interesting possibilities for photonic applications.