We have obtained a porous silicon optical planar waveguide, with a submicronic periodic modulation of the optical index along one direction of plane, using a holographic process. Near-infrared continuous transmission spectra of guided light across not less than 3000 periods show several strong stopbands, with a decrease of intensity by two orders of magnitude. By means of the coupled-mode theory, we were able to deduce from the spectra a realistic map of the optical index at a microscopic scale, demonstrating a strong photo-induced index contrast (△n = 0.5) at a submicronic scale.
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