1. Institute for National Measurement Standards, National Research Council, K1A 0R6 Ottawa , Canada
2. Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, USA
3. Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Cantoblanco , 28049 Madrid, Spain
4. Institute for Chemical Process and Environmental Technology, National Research Council, K1A 0R6 Ottawa, Canada

Correspondence to: Pavel Cheben¹ Correspondence and requests for materials should be addressed to F.dM. (e-mail: Email: delmonte@icmm.csic.es) or P.C. (e-mail: Email: pavel.cheben@nrc.ca ).

Photorefractive materials¹ exhibit a spatial modulation of the refractive index due to redistribution of photogenerated charges in an optically nonlinear medium. As such, they have the ability to manipulate light and are potentially important for optical applications¹ including image processing, optical storage, programmable optical interconnects and simulation of neural networks. Photorefractive materials are generally crystals, polymers and glasses with electro-optic or birefringent properties and non-centrosymmetric structure². Here we report the photorefractive effect in both non-centrosymmetric and centrosymmetric azo-dye-doped silica glasses, in which refractive index gratings that are spatially phase-shifted with respect to the incident light intensity pattern are observed. The effect results from a non-local response of the material to optical illumination, and enables the transfer of energy between two interfering light beams (asymmetric two-beam coupling). Although the writing time for the present grating is relatively slow, we have achieved a two-beam coupling optical gain of 188 cm ^-1 in the centrosymmetric glasses, and a gain of 444 cm ^-1 in the non-centrosymmetric structures. The latter are fabricated using a corona discharge process³ to induce a permanent arrangement of azo-dye chromophores.