Maret, Georg


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Multiple light scattering from disordered media : the effect of brownian motion of scatterers

1987, Wolf, Pierre-Etienne, Maret, Georg

We have measured the time autocorrelation function of the light intensity multiply scattered from turbid aqueous suspensions of submicron size polystyrene spheres in directions near backscattering. It is found strongly non-exponential at short times revealing the very fast decay of coherence in extended scattering loops due to the thermal motion of the many spheres involved; the longest living decay time is found remarkably close to the single particle backscattering relaxation time even under conditions of interparticle interactions. These features are only weakly affected by the particular interference effect between time-reversed pairs of loops giving rise to the coherent backscattering enhancement. A simple argument is presented which accounts for these observations.

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Weak localization and coherent backscattering of photons in disordered media

1985, Wolf, Pierre-Etienne, Maret, Georg

Coherent backscattering of waves by a disordered scattering medium is responsible for weak localization.
We have directly observed this effect for the first time using visible light and concentrated aqueous suspensions of submicron-size polystyrene spheres. The scattered intensity is found enhanced by up to 75% within a narrow cone centered at the backscattering direction. As predicted by theory, the aperture of the cone is inversely proportional to the light mean-free path; the latter was controlled by the concentration of spheres. The importance of light polarization and particle size is discussed.


Nematic and cholesteric thermotropic polyesters with azoxybenzene mesogenic units and flexible spacers in the main chain

1982, Blumstein, Alexandre, Vilasagar, S., Ponrathnam, S., Clough, Stuart B., Blumstein, Rita B., Maret, Georg

Properties of linear polyesters based on azoxybenzene and 2,2′-methylazoxybenzene moieties with linear, flexible spacers based on mixtures of dodecanedioic acid (DDA) and methyladipic acid (MAA), chiral or racemic, of various compositions (system MAA/DDA-8 and MAA/DDA-9, respectively) have been described. Substitution of methyl groups in the 2,2′ or 3,3′ positions of the mesogenic core leads to soluble and relatively low-melting-point polyesters. The viscosity law for (MAA/DDA-9) polyesters in 1,1,2,2 tetrachloroethane gives an exponent 0.76, indicating well-sol-vated, coiled chain conformations in dilute solution. Calorimetric data show an increase in isotropization entropy ΔSNI with increasing average length of the spacer. This suggests a nonrandom conformation of the spacer in the nematic melt with a degree of order superior to that of low-molecular-weight analogs. X-ray data obtained with an oriented nematic glass quenched from the nematic melt of DDA-9 subjected to a magnetic field of 10–12 T also support the extended-chain model in the nematic phase of DDA-9. Oriented fibers can be produced by subjecting nematic melts of polyesters 8 and 9 either to magnetic fields of high intensity or to shear fields. The x-ray data obtained from these fibers also support the extended-chain model. Cholesteric systems do not orient in the magnetic field of 10–12 T. The study of mesophases of systems 8 and 9 indicates a dramatic influence of the position of the ester group on the stability of the mesophase in the azoxybenzene polyesters. The results are interpreted in terms of geometric factors influencing the colinearity of the mesogenic core and of the extended spacer.