Characterization and Effect of Hydrogen Treatment and UV Irradiation on Photosensitive Sol-Gel Derived Aluminosilicate Planar Waveguides J. M. Nedelec,* ,† J. Grimblot, ‡ S. Turrell, † and M. Bouazaoui § Laboratoire de Spectrochimie Infrarouge et Raman, CNRS UMR 8516, Ba ˆ t. C8, Laboratoire de Catalyse de Lille, UPRESA 8010, Ba ˆ t. C3, and Laboratoire de Physique des Lasers, Atomes et Mole ´ cules, CNRS UMR 8523, Ba ˆ t P5, Centre d’Etudes et de Recherches Laser et Applications, UniVersite ´ des Sciences et Technologies de Lille, 59655 VilleneuVe d’Ascq Cedex, France ReceiVed: August 4, 1999; In Final Form: October 30, 1999 Photosensitive Ce 3+ -doped aluminosilicate planar waveguides have been prepared by a sol-gel process. The waveguides, which have been characterized using different techniques, appear to be totally amorphous and of good optical quality. The m-lines technique and waveguide Raman spectroscopy have shown that the doping of the guides with Ce 3+ ions slows the densification process and that consequently the effect of doping has to be taken into account for the characterization of such materials. X-ray photoelectron spectroscopy (XPS) analysis of the samples has revealed a very high concentration of nonbridging oxygens (NBO) at the surface. To study the mechanisms responsible for the photosensitivity of these waveguides, selected samples have been hydrogenated and irradiated with UV light and then analyzed by XPS. Results have shown a strong decrease of the O/(Al + Si) atomic ratio, especially at the surface of these guides, after H 2 loading and UV irradiation. This decrease is correlated with the high concentration of NBO at the surface of the samples and has been interpreted as resulting from the formation of molecular water accompanied by the creation of defects in the glass. 1. Introduction Over the past few years, there has been great enthusiasm for the study of passive and active planar waveguides because of their potential applications in the realization of integrated all- optical devices. 1-3 Among the materials used in optronics, photorefractive materials are of particular interest. In effect, one can induce a permanent variation of the refractive index in such materials by laser irradiation. If the irradiation is periodic in space, then the resulting modulation of the refractive index is also periodic and the final material acts as a Bragg grating. This effect, initially observed in germanosilicate optical fibers, 4 leads to a photoinduced refractive index change that offers potential applications in optical communications. 5 In recent years, the photorefractive effect has been observed in numerous other materials, including rare-earth-doped aluminosilicate fibers 6-8 and most recently, within our research group, in rare-earth-doped aluminosilicate planar waveguides. 9 In all cases, the preliminary exposition of the material to hydrogen has been shown to enhance the photorefractive effect. 10,11 If the study of the photosensitivity of the different systems is quite well documented in the literature, 12 there is a recognized lack of understanding of the mechanisms responsible for the observed photosensitivity. However, a thorough understanding of these mechanisms is the key to the production of new materials with enhanced properties. Most of the studies devoted to the comprehension of the microscopic mechanisms have concerned germanosilicate glasses 13 because of their extensive use in telecommunications. In the present work, we examine Ce 3+ -doped aluminosilicate planar waveguides. Since the sol-gel method has been proven to be a suitable method for the preparation of homogeneous multicomponent glasses both as bulk materials and as thin films, 14,15 in the present work the waveguides were produced by combining sol-gel and dip-coating processes. The waveguides were first characterized by optical-loss measurements, m-lines techniques, and waveguide Raman spectroscopy (WRS) and X-ray photoelectron spectros- copy (XPS). The materials were then hydrogenated and irradi- ated, and changes at each step were investigated by XPS. 2. Experimental Section 2.1. Waveguide Elaboration. The procedure for the prepara- tion of aluminosilicate planar waveguides has been already detailed elsewhere. 14,16 Briefly, a silicon sol is prepared by hydrolysis of tetraethyl orthosilicate in an acidic alcoholic medium. This sol is mixed with an aluminum sol prepared by dissolving aluminum tri-sec-butoxide in alcohol. The amounts of reagents were chosen to obtain a molar ratio of x ) Al/Si ) 2, which was kept constant throughout the study. The final sol was doped with Ce 3+ ions by adding hydrated cerium nitrate salt (Ce(NO 3 ) 3 ‚6H 2 O). The concentration of cerium (Ce/(Al + Si) molar ratio) varied from 0 to 2%. The resulting solution was then used in a dip-coating process. 17 The substrates were either optical grade 25 mm × 75 mm × 1 mm Suprasil slides or monocrystalline silicon wafers coated with 2 μm of silica deposited by plasma enhanced chemical vapor deposition (PECVD). The substrates were dipped and withdrawn from the coating solution at a constant rate of 40 mm/min. After each dip, the sample was treated at 650 °C, and after every five dips it was annealed at 900 °C for 1 h. This procedure allowed the densification of the films and the complete elimination of organic residues. * To whom correspondence should be addressed. Permanent address: Dr. J. M. Nedelec, Laboratoire des Mate ´riaux Inorganiques ESA 6002, Universite ´ Blaise Pascal, 24 Avenue des Landais, 63 177 Aubiere Cedex, France. E-mail: jnedelec@chimtp.univ-bpclermont.fr. † Laboratoire de Spectrochimie Infrarouge et Raman. ‡ Laboratoire de Catalyse de Lille. § Laboratoire de Physique des Lasers, Atomes et Mole ´cules. 926 J. Phys. Chem. B 2000, 104, 926-930 10.1021/jp992767n CCC: $19.00 © 2000 American Chemical Society Published on Web 01/19/2000