Mesoscale optical phenomena occur when light interacts with a number of different types of materials, such as biological and chemical systems and fabricated nanostructures. As a framework, mesoscale optics unifies the interpretations of the interaction of light with complex media when the outcome depends significantly upon the scale of the interaction. Most importantly, it guides the process of designing an optical sensing technique by focusing on the nature and amount of information that can be extracted from a measurement. Different aspects of mesoscale optics are addressed in this dissertation which led to the solution of a number of problems in complex media. Dynamical and structural information from complex fluids—such as colloidal suspensions and biological fluids—was obtained by controlling the size of the interaction volume with low coherence interferometry. With this information, material properties such as particle sizes, optical transport coefficients, and viscoelastic characteristics of polymer solutions and blood were determined in natural, realistic conditions that are inaccessible to conventional techniques. The same framework also enabled the development of new, scale-dependent models for several important physical and biological systems. These models were then used to explain the results of some unique measurements. For example, the transport of light in disordered photonic lattices was interpreted as a scale-dependent, diffusive process to explain the anomalous behavior of photon path length distributions through these complex structures. In addition, it was demonstrated how specialized optical measurements and models at the mesoscale enable solutions to fundamental problems in cell biology. Specifically, it was found for the first time that the nature of cell motility changes markedly with the curvature of the substrate that the cells iv move on. This particular work addresses increasingly important questions concerning the nature of cellular responses to external forces and the mechanical properties of their local environment. Besides sensing of properties and modeling behaviors of complex systems, mesoscale optics encompasses the control of material systems as a result of the light-matter interaction. Specific modifications to a material’s structure can occur due to not only an exchange of energy between radiation and a material, but also due to a transfer of momentum. Based on the mechanical action of multiply scattered light on colloidal particles, an optically-controlled active medium that did not require specially tailored particles was demonstrated for the first time. The coupling between the particles and the random electromagnetic field affords new possibilities for controlling mesoscale systems and observing nonequilibrium thermodynamic phenomena

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Le développement actuel des piles à combustible SOFC fonctionnant à température intermédiaire suppose l'optimisation des méthodes de synthèse et de mise en forme pour les matériaux nouveaux développés au cours des dernières années. En effet, les propriétés électrochimiques de ces dispositifs sont étroitement liées aux caractéristiques des poudres de départ ainsi qu'à la microstructure des électrodes (ou de l'électrolyte) après leur mise en forme. Une amélioration significative des dites propriétés peut être obtenue par la nanostructuration des matériaux. Dans ce contexte, ce travail de thèse est consacré à l’élaboration du matériau de cathode Nd2NiO4+d ainsi que du matériau d'électrolyte Ce1-xAxO2-d. Les méthodes mises en œuvre sont la synthèse de nanopoudres en milieux éthanol/eau supercritiques et par voie pyrosol ainsi que le dépôt de couches minces en milieu CO2 supercritique. Les objets obtenus ont enfin été caractérisés par spectroscopie d'impédance électrochimique afin de quantifier leur performance pour l’application SOFC. / The ongoing development of Intermediate Temperature Solid Oxide Fuel Cells implies the optimization of the synthesis and deposition methods for the new materials developed these past years. Indeed, electrochemical properties of these materials are closely linked to the initial powder characteristics as well as the electrode (or electrolyte) microstructure after deposition. Significant improvement of the aforementioned properties can be obtained via nanostructuration of the materials. Thus, this thesis is dedicated to the synthesis of the cathode material Nd2NiO4+d and the electrolyte material Ce1-xAxO2-d. Methods employed are namely nanopowder synthesis in water/ethanol supercritical mixtures and spray pyrolysis as well as thin film deposition in supercritical fluids. The obtained objects have finally been characterized by electrochemical impedance spectroscopy in order to assess their performance for the SOFC application.