BIOphysics & SOFT Matter Department of Ultrafast Optics and Nanophotonics

Institut de Physique et Chimie des Matériaux de Strasbourg

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Wave Spectroscopy When a colloidal suspension is concentrated enough, it becomes turbid: any photon going through the suspension exits with a random direction. If this suspension is illuminated with a coherent light, one observes a speckle pattern. When particles move in the solution, the speckle pattern fluctuates. In turbid enough suspensions, it may be considered that a photon experiences a large enough diffusion events from particles, and one can, from the measurement of the fluctuation of the speckle pattern, deduce the dynamics of the particles in solution with a spatial resolution of the order of 1 nm on a temporal time scale ranging from 10 ns up to several second. When the origin of this motion is Brownian motion, one can deduce the size of the particles. Inversely, if their size is known, the visco-elastic properties of the suspending medium on such time and spatial scale, are determined, on characteristic length scales of the order of a few nm and at frequencies up to 100 MHz.