Collateral densification associated with the photoresponse of hybrid sol-gel glasses for depositing Bragg gratings on ridge waveguides

Integrated optical devices can be deposited by photolithography in hybrid organic-inorganic glass films prepared by solution sol-gel processing. The latter are made from a generic matrix of methacryloxypropyltrimethoxysilane, methacrylic acid and zirconium oxide. An optical phase mask and pulsed 6.4 eV light were used to deposit 1.5 mm Bragg gratings in ridge waveguide. The gratings are polarisation insensitive and show very high reflectivity at 1.55 mu m(1). The photoresponses that give rise to the grating in the nanocomposite are complex. They express themselves simply as an increase in the material refractive index n. To understand the way in which the underlying photochemistry and physical changes contribute to the refractive index change, we chose to follow structural changes in the polymer and silica framework by combinations of electronic and vibrational spectroscopy(2). Photoinduced polymerization of the organic methacrylate monomer upon UV irradiation triggers a cascade of structural changes within the inorganic matrix. The process is akin to photo-induced sintering. Reduction in non-bonded interactions due to bond formation following polymerization precipitates a localized structural relaxation in the silica network. Collateral densification of the silicate matrix is evident by the shifts to lower energy in the transverse optical phonon mode associated with antisymmetric stretching and decreased Si-O-Si bond angles in the matrix.