UV-induced permanent gratings in sol-gel germanosilicate thin films

Germanosilicate thin films have been elaborated by the sol-gel process and the dip-coating technique. Pulsed or continuous wave UV laser (244 nm) was used to write permanent gratings in these films. In the case of exposure to cw laser, the grating diffraction efficiencies were measured using a focus ed beam from a He-Ne laser at 633 nm and photoinduced changes in refractive index as high as 4 x 10(-3) have been obtained. The thermal behaviour of t hese gratings has been investigated showing a good stability up to 400 degr ees C. Exposure to pulsed fringe pattern led to a glass photo-expansion mod ulated by a strong corrugation which can be due mainly to photo-ablation at the places of the bright fringes. The waveguide surface at the grating pla ces was investigated through Atomic Force Microscopy (AFM) and microscopic profilometry techniques. Preliminary results on the kinetics of the grating growths are also reported. (C) 2000 Elsevier Science B.V.. All rights rese rved.