M. Megens

Light sources inside photonic crystals

We have measured the optical fluorescence spectra of dye incorporated in high-quality photonic crystals made from colloids. The spectra reveal a stopgap that is due to Bragg reflection with strikingly reduced attenuation compared with plane-wave transmission. The modified attenuation is independent of the position of the sources in the sample and is brought about by diffuse scattering from defects near the surface. In the presence of a photonic bandgap, the diffuse component would disappear. Thus we have found a simple, unambiguous probe for the presence of photonic bandgaps.

Enhanced backscattering from photonic crystals

We have studied enhanced backscattering of both polystyrene opals and strongly photonic crystals of air spheres in TiO 2 in the wavelength range of first and higher order stop bands. The shape of the enhanced backscattering cones is well described by diffusion theory. We find transport mean free paths of the order of 15 mm both for opals and air spheres. Close to the stop band the cone width is decreased due to internal reflections generated by the photonic band structure. Widening occurs due to attenuation of the coherent beam by Bragg scattering. We present a model that incorporates these effects and successfully explains the data. q 2000 Elsevier Science B.V. All rights reserved.

Synchrotron small angle X-ray scattering of colloids and photonic colloidal crystals

Synchrotron small-angle X-ray scattering (SAXS) experiments have been performed at the ESRF on colloidal dispersions. The samples are optically multiply scattering and are aimed at so-called photonic applications. Scattering from dilute suspensions yields the particle form factor, which shows many oscillations. From these results, the radius, size distribution and information about the internal structure of the particles in situ can be determined, in contrast to most studies on colloids. Using the form factor, structure factors can be extracted from the diffraction patterns of the colloidal crystals. The structure factors clearly reveal sharp Bragg peaks. The crystal structure, lattice parameter and average orientations were determined as a function of height in sedimented samples. This provides important information for the interpretation of optical experiments.

Transmission and diffraction by photonic colloidal crystals

We have performed optical transmission and synchrotron small-angle x-ray scattering (SAXS) experiments on colloidal crystals with optical refractive index ratios as large as possible over a wide range of volume fractions. These conditions push colloidal crystals into the regime where strong coupling of photonic crystals with light occurs. The optical transmission spectra reveal minima corresponding to stop gaps on the edges of the Brillouin zone of the photonic band structures. The positions of the optically measured stop gaps agree well with lattice spacings measured by SAXS. The stop gap in the 111 direction of crystals of polystyrene in water has a width of up to 5% of the gap frequency as a function of volume fraction, in agreement with theoretical band-structure calculations. A maximum of the relative width confirms the notion that the strength of the interaction between photonic crystals and light has an optimum as a function of volume fraction. The detailed structural information from SAXS data greatly assists in the interpretation of optical experiments on photonic crystals.

Orientational relaxation times of Rhodamine 700 in glycerol-water mixtures

We determined orientational relaxation times for rhodamine 700 dye in glycerol-water mixtures using time-resolved fluorescence depolarization. It appears that the orientational relaxation time varies linearly with the viscosity of the solvent between 1 and 60 cP, in accordance with the Perrin-Stokes-Einstein model with stick boundary conditions. Previously others have found that for two anionic dyes in glycerol-water and a cationic dye in glycerol-ethylene glycol mixtures, the orientational relaxation time becomes less sensitive to the viscosity at very high viscosities (>25 cP at least). We discuss the influence of dye and solvent on the relation between orientational relaxation time and viscosity, which suggests that the relaxation time as a function of viscosity can be scaled on a common curve.