Auto-embossed Bragg gratings from self-organizing polymers: chemical tuning of periodicity and photoinduced anisotropy

摘要

Abstract: Self-organizing cellulosic polymers are used to explore implications of intrinsic structural anisotropy on light propagation and on Bragg grating replication in thin film waveguides. Polarized waveguide Raman spectroscopy indicates that cellulose acetate and regenerated cellulose polymers show 2D isotropy in the plane of the film. These measurements require that polarization artifacts be carefully separated to recover faithful spectral representations of massive structural order. Parasitic contributions to polarization dependent Raman data are enumerated. Holographic surface relief gratings can be replicated by partially ordered cellulose acetate. Base hydrolysis of the pyranose ester substituents liberates hydroxycellulose (native cellulose) that is largely amorphous, but retains the bulk structural anisotropy of the esterified parent polymer. The latent image of the grating device is passed from the cellulose acetate to the next generation hydroxycellulose film. This process is called `autoembossing', and self-replicates a functional grating device whose period can be effectively doubled. Anisotropy can also be induced with polarized light in hybrid organic/inorganic glass (organosilicate) waveguides prepared by sol-gel polycondensation. We describe novel photopoling experiments that `write' refractive index variations in thin organosilicate films. These written features can be read by a form of autoscopic polarized Raman scattering, and in some cases `erased' by a guided, polarized laser beam. To our knowledge this is the first example of photo-induced anisotropy in hybrid sol-gel glass waveguides that is unrelated to bond isomerization commonly associated with photo-poling in dye-doped polymers or glasses.!16