The optical and structural properties of dense colloidal suspensions in the presence of long-range electrostatic repulsion are determined from both light and small-angle neutron scattering experiments. Short-range structural order induces an enhancement of the scattering strength while at the same time the total transmission shows strong wavelength dependence, reminiscent of a photonic crystal. Interestingly, the interplay between diffusive scattering and local order leads to negative values of the scattering anisotropy parameter. The tunable optical properties of these liquids furthermore suggest potential applications such as transparency switches or filters. DOI: 10.1103/PhysRevLett.93.073903 PACS numbers: 42.25.Bs, 42.70.Qs, 82.70.Dd When light is incident on a nonabsorbing material, its further propagation is strongly influenced by the microscopic structure of the material itself. For a bulk homogeneous medium, light is refracted according to Snell's law. Local variations in the dielectric properties lead to isolated scattering events that disperse the light beam. The scatterer density and cross section define the scattering mean free path l. As the number of scattering events increases, the transport of light becomes diffusive and the material appears turbid or ''white'' [1,2]. The relevant scattering length for diffusive light transport is the transport mean free path l . Both quantities are connected by the scattering anisotropy parameter g defined as the average of the cosine of the scattering angle g hcosi, l=l 1 ÿ g. Our current understanding of the diffusive transport is based on the knowledge of these key quantities. In the absence of positional correlations, l is usually equal to or larger than l [2 -5]. For instance, Mie particles (or human tissue [1]) scatter strongly in the forward direction (small scattering angles ) and hence g ' 1 while for Rayleigh scatterers g ' 0. Here we show that these common properties of diffusive transport can be manipulated by tuning the interaction between scatterers. By the appropriate control of the Coulomb repulsion between highly charged particles in suspension, we are now able to access the whole possible interval of g values (from forward scattering g ! 1 to the unusual case of strong backscattering g ! ÿ1).When mesoscopic variations of the dielectric constant can be neatly controlled over macroscopic distances, totally new, so-called photonic properties may appear [6]. At the core of the design of new photonic materials lies the intelligent way structures are assembled on length scales comparable to the wavelength of light. There are two main promising concepts to achieve lossless guidance and manipulation of light based on seemingly opposite principles: order or disorder. Photonic band gap materials are based on periodic structures predicted to inhibit light propagation completely [6]. In the case of disorder, light cannot propagate in the material due to recurrent interference called strong Anderson localization [7].Tailoring microstructures with an app...

Amorphous photonic materials based on dense assemblies of high refractive index spherical particles are presented. Light transmission through these photonic glasses shows a nontrivial wavelength dependence. The transmission spectra can be quantitatively reproduced by modeling the optical properties starting from their building blocks. Our results emphasize the relevance of including short range order correlations and an appropriate effective refractive index in the analysis of light transmission through amorphous photonic materials.

We formulate a quantitative description of backscattered linearly polarized light using an extended photon diffusion formalism taking explicitly into account the scattering anisotropy parameter g of the medium. From diffusing wave spectroscopy measurements the characteristic depolarization length for linearly polarized light is deduced. We investigate the dependance of this length on the scattering anisotropy parameter g spanning an extended range from -1 (backscattering) to 1 (forward scattering). Good agreement is found with Monte Carlo simulations of multiply scattered light

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We determine the structure of charge-stabilized colloidal suspensions at low ionic strength over an extended range of particle volume fractions using a combination of light and small angle neutron scattering experiments. The variation of the structure factor with concentration is analyzed within a one-component model of a colloidal suspension. We show that the observed structural behavior corresponds to a nonmonotonic density dependence of the colloid effective charge and the mean interparticle interaction energy. Our findings are corroborated by similar observations from primitive model computer simulations of salt-free colloidal suspensions.