Research
Optical methods based on the absorption and scattering of near-infrared light are becoming increasingly important for the non-invasive detection and imaging of brain function in humans since they combine minimal radiation loads with high temporal resolution, functional specificity and portability [1]. The contrast mechanism of conventional near-infrared spectroscopy (NIRS) is based on neurovascular coupling, i.e. the changes of total hemoglobin concentration and oxygen saturation which accompany electrical activation of specific cortical areas. However, the diffuse propagation of light due to the strong scattering by inhomogeities within tissue hampers the spatial resolution of near-infrared imaging. Using multiple source-receiver pairs allows for mapping of cortical activity with centimeter resolution through the intact scalp and skull. Recent developments using time-of-flight detection allows to reduce signal contaminations from superficial layers, leading to significantly enhanced sensitivity to functional signals from the cortex [2-4].
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When waves diffuse through a random medium, the mean free path is no longer the only length scale determining the physical situation. Due to interferences, additional effects will appear on the scale of the wavelength, lambda. A striking prediction made by Anderson in 1958 is that in fact diffusion should come to a halt in a medium where the mean free path is comparable with the wavelength. In the context of electronic systems, this has been used as an explanation for the transition from metallic to insulating behaviour with the addition of impurities. We study this phenomenon for the case of diffusing photons in a system made of particles with a high refractive index on the scale smaller than the wavelength of light. Here, localization can be observed without the complications induced by electron-electron, as well as electron-atom interactions, thus allowing a study of pure Anderson localization.
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Granular materials are inherent non-equilibrium materials and hence are model systems for the study of systems far from thermodynamic equilibrium. This gives rise to counterintuitive phenomena such as size-separation of binary mixtures or clustering transitions leading to compartmentalization of an initially well mixed collection of grains. We investigate the influence of gravitation on these phenomena by levitating the samples in a strong magnetic field gradient. In the dynamics of foams, the coarsening and coalescence of bubbles is usually strongly influenced by the drainage of the fluid through between the lipid membranes due to gravity. By levitating the foams it is again possible to study the intrinsic equilibration dynamics solely due to surface tension and external pressure.
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One can think of colloids as model systems for solid state physics: Using microscopic particles whose interparticle interactions can be controlled in strength and range we study the atomic processes during crystallization, melting, or near the glass transition. We take advantage of the fact that colloidal particles can be observed on all relevant time- and length scales using simple tools such as light scattering and microscopy. This allows novel investigations of structural and dynamic properties with ''atomic'' resolution. In addition, the motions of these particles can be easily manipulated by light forces.
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"Soft condensed matter" also includes biomaterials and living matter. Using modern optical spectroscopy (high resolution Brillouin scattering, ellipsometry, synchrotron radiation) we investigate low-frequency fluctuations of protein conformations in solution and in the crystalline state as well as elastic properties of biopolymers such as DNA. Particular interest is on the molecular features and the biologival relevance of the strong overstretching of DNA molecules.
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Colloidal suspensions and biological mocromolecules often appear turbid or coloured-opaque, since the incident light is scattered many times by particles or inhomogeneous structures resulting in diffusion of light waves. The random distribution of scatterers gives rise to complex interference patterns known in optics as speckle patterns. Speckle patterns of strongly scattering media show novel correlations over a wide angular range and the analysis of their temporal fluctuations provides information on the motions of the scatterers. This novel technique called "Diffusing Wave Spectroscopy" has found diverse applications, for example in particle sizing, in the on-line quality control of dispersions, in the determinatioin of flow velocity profiles or in medical imaging.
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Photonic crystals are materials patterned with a periodic variation of the refractive index creating a range of forbidden optical frequencies called a photonic band gap. A photonic band gap modifies fundamentally the propagation of light and emission processes. Based upon these effects novel integrated-photonic materials can be designed, opening various new areas of applications. We propose to synthesize photonic crystals composed of colloidal high index spheres with mono- and bimodal sphere size. For this purpose monodisperse spheres will be produced and characterized in a first step and then assembled into large 3D crystals.
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