F. Chaput

High remanent optical anisotropy in photochromic sol–gel films is shown to be photoinduced by discoloration with linearly polarized visible light. This anisotropy results in a linear dichroism in the visible absorption band and in an important birefringence in the near-infrared transparency region. The kinetics of the whole process are quantitatively well described by a simple analytical model. Intrinsic characteristics of the individual molecule, such as the photochemical quantum yield and the anisotropy of their optical properties, are determined. The potential application of this effect to optical data storage is discussed.

International audienceWe report on the synthesis and the optical characterization of a photorefractive sol-gel material which contains nonlinear azo chromophore and carbazole charge transporting molecules. Both of these functional groups are covalently attached to the silica based backbone. Thin sol-gel films exhibit stable optically nonlinear properties: the electro-optic coefficient r13 measured at 633 nm is found to be 17 pm/V one day after corona poling and 15 pm/V after three weeks and over a period of several months. Photorefractivity is demonstrated by two beam coupling experiments without external applied electric field. Sol-gel films present a stable photorefractive memory effect with a net internal gain of 200 cm-

Alumino-silicate aerogels have been prepared from the (BuO)2Al-O-Si (OEt)3 double precursor with different densities (from 50 to 250 kg/m3). SAXS study indicates that both alumino-silicate alcogel and aerogel structures can be represented by primary units which stick together into mass-fractal clusters (D = 1.8-1.9) by a mechanism of cluster-cluster aggregation. Self similarity is displayed in the alumino-silicate aerogels in a large range of densities