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We have measured the absorption spectrum, the emission spectrum, the emission lifetime, and the photostability of fluorescein isothiocyanate (FITC) incorporated inside colloidal silica spheres as a function of the dye concentration in the spheres, while minimizing scattering effects. Six batches of stable, monodisperse particles were synthesized with FITC up to high densities of 0.03 M. At dye concentrations above 0.001 M, we observe a large red shift of 10 nm in the absorption and the emission spectra, as well as a strong reduction of the lifetime. At the same time, the photostability of the dye is considerably improved. These effects are caused by an increased energy transfer between the dye molecules as their concentration increases. Several excitation quenching models are examined, namely annihilation quenching, surface quenching, and a fractal distribution of quenchers. None of the models that assume a homogeneous distribution of FITC provide a sufficient explanation of the observed effects. It is suggested that the dye molecules tend to form clusters during the synthesis of the colloidal spheres. It is concluded that colloids with a low dye concentration are useful for photonic applications, whereas high dye concentrations are interesting for optical experiments in colloid science.

PACS. 32.70Cs -Oscillator strengths, lifetimes, transition moments. PACS. 32.80Pj -Optical cooling of atoms; trapping. PACS. 42.50Lc -Quantum fluctuations, quantum noise, and quantum jumps.Abstract. -We show rigorously that the coefficient for spontaneous emission of an atom placed in a dielectric is proportional to the local radiative density of states -that is only a part of the local density of the eigenmodes of the Maxwell equations. Spontaneous emission is inhibited if the atom is located at a position where this local radiative density is small, even if the total density of states is not vanishing. This radiative density of states can be obtained without having to perform a full quantum calculation of the radiation-matter system. We demonstrate this principle by solving numerically a scalar model for a dielectric that consists of a lattice of resonating dipoles.

Dynamic light scattering studies on the sol-gel transition of a suspension of anisotropic colloidal particles Kroon, M.; Wegdam, G.H.; Sprik, R. Published in:Physical Review E DOI:10.1103/PhysRevE.54.6541 Link to publicationCitation for published version (APA): Kroon, M., Wegdam, G. H., & Sprik, R. (1996). Dynamic light scattering studies on the sol-gel transition of a suspension of anisotropic colloidal particles. Physical Review E, 54, 6541-6550. https://doi.org/10.1103/PhysRevE.54.6541 General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We present a dynamic light scattering study on the sol-gel transition of a suspension of disk-shaped colloidal particles in water. We obtain the static and fluctuating part of the scattered intensity, the fraction of frozen-in density fluctuations, and the intermediate scattering function from a local time-averaged measurement of the intensity correlation function and the scattered intensity. The sol-gel transition is marked by a drastic change in the static part of the scattered intensity. The intermediate scattering function shows a stretching of the translational correlation time over more than five orders of magnitude. In the gel phase the function shows a power-law decay, with a concentration dependent scaling exponent. Our results show strong similarities with the scenarios given by the mode coupling theory of the structural glass transition. ͓S1063-651X͑96͒04212-2͔ PACS number͑s͒: 82.70. Dd, 78.35.ϩc, 82.70.Gg, 64.70.Pf I. INTRODUCTIONStructural relaxation in amorphous systems is an area of much current interest. Many studies have been devoted to the sol-gel transition in systems based on polymers ͓1-3͔, natural gelatine ͓4͔, and gels based on spherical colloids ͓5-7͔. Here we report on the sol-gel transition in a system of charged disks with an aspect ratio of 25, suspended in water. In such systems the orientational degrees of freedom play a crucial role not only in the dynamics but also in the static structure of the gel. Molecular dynamics simulations on hard disk systems reveal a rich phase diagram with nematic and cubatic liquid crystalline phases ͓8͔. However, before these liquid crystalline phases can be formed the system enters a glassy phase or gel. Current opinion favors the ''house of cards'' structure for the ge...

Strong effects of photonic band structures on the diffraction of colloidal crystalsVos, W.L.; Sprik, R.; van Blaaderen, A.; Imhof, A.; Lagendijk, A.; Wegdam, G.H. Published in:Physical Review. B, Condensed Matter Link to publication Citation for published version (APA):Vos, W. L., Sprik, R., van Blaaderen, A., Imhof, A., Lagendijk, A., & Wegdam, G. H. (1996). Strong effects of photonic band structures on the diffraction of colloidal crystals. Physical Review. B, Condensed Matter, 53, 16231-16235. General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. The influence of photonic band structures on optical diffraction has been studied with colloidal crystals with large refractive index ratios up to 1.45 and polarizibilities per volume as large as 0.6. It is found that the apparent Bragg spacings are strongly dependent on the wavelength of light. The dynamical diffraction theory that correctly describes weak photonic effects encountered in x-ray diffraction also breaks down. Two simple models are presented that give a much better description of the diffraction of photonic crystals. ͓S0163-1829͑96͒06523-X͔

We studied the spontaneous emulsification and droplet growth mechanism in trans-anethol/water/ethanol solutions, also known as the beverage ouzo, using dynamic light scattering spectroscopy. This simple ternary mixture is a generic example of a system that forms microemulsions spontaneously when brought into the two-phase region. The volume fraction of the dispersed phase was found to profoundly affect the growth rates of the droplets, which is a new finding that has not been predicted by the Lifshitz-Slyozov-Wagner theory. Time-dependent measurements show that the droplet growth is governed by Ostwald ripening (OR), and no coalescence was observed. Furthermore, the OR rates increase with increasing oil concentration at low alcohol content. We attribute this behavior to enhanced droplet-droplet interactions. At high ethanol concentrations, we found that the measured rates decreased as the oil concentration increased. The OR growth mechanism completely correlates with changes in droplet size. The kinetics of droplet growth shows that the ripening has a saturation limit at a droplet radius of about 1.5 mum. Thus, formed emulsions remain stable for months.

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