Electrically induced concentration profiles of nanoparticles in a MMA-silane matrix: A new method to obtain GRIN-lenses.

A new technique to produce a radial gradient in the refractive index (r-GRIN) in organic-inorganic nanocomposite materials using sol-gel techniques in combination with electrophoretically induced concentration profiles of oxide nanoparticles is presented. The composite material is based on methacryloxylpropyl trimethoxysilane (MPTS), zirconium n-propoxide (ZR), Methacrylic acid (MA) and tetraethyleneglycol dimethacrylate (TEGDMA). Irgacure 184 was used as a photosensitive initiator. The surface of these particles is enriched with MA which is linked by a chelating complex to the initial ZR component. The TEGDMA component is used to introduce more flexibility in the organic inorganic network and to reduce polymerization stresses. Electric charges on the ZrO2 nanoparticle surface force the particles to diffuse in the gel state by electric fields employed by appropriate electrodes in presence of an electric field. The movement and interdiffusion of the Zr-nanoparticles in the matrix were measured by zeta-potential measurements and by photon-correlation spectroscopy (PCS). In the performed experiments, a radial electric field amplitude of 200 V/cm was used and held for 5 hours keeping the material in the gel state. The variation of refractive index in real time was measured by Mach-Zehnder interferometry. After the electrophoretic process, a polymerization step was carried out to immobilize the Delta n gradient. Delta n was measured by ellipsometry and the value of 0.07 was obtained for a sample of 1 cm in diameter. The form of the concentration profile and hence from the index profile was detected by EDX (Energy Dispersive X-ray analysis) measurements.