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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...