Photonic bandgap effect in periodic porous silicon planar waveguides
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Photonic bandgap effect in periodic porous silicon planar waveguides P. Ferrand and R. Romestain Laboratoire de Spectrométrie Physique, Université Joseph Fourier Grenoble 1, CNRS UMR 5588, BP 87, F-38402 Saint Martin d'Hères, France ABSTRACT 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 photoinduced index contrast (∆n = 0.5) at a submicronic scale. INTRODUCTION The optical properties of porous silicon (PS) have been studied for many years: as a light emitter, since the discovery of photoluminescence at room temperature [1], but also as a very promising optical material, using the structuration of the optical index. Using periodic modulations of the optical index in depth, distributed Bragg reflectors that forbid the propagation of light in a given spectral and angular range, have been demonstrated [2]. Microcavities have also been reported as an efficient way to control both the spectral and angular range of the emission of a PS layer [3] or an impregnated laser dye [4]. In addition, a strong effort has been made, in order to increase the optical quality, especially by reducing the scattering losses due to the interface roughness [5]. PS waveguides have also been demonstrated as a promising way to carry the light in integrated optical circuits [6,7]. In the same time, photonic bandgap (PBG) materials have been the subject of intense work, since it has been suggested that they can allow a total control of spontaneous emission [8] and localization of light [9]. In addition, because they can guide the light with a very high efficiency, even across very sharp bends [10], they constitute a promising way to increase the integration density of optical integrated circuits in the future. In this paper, we report the fabrication of a PS planar waveguide with a submicronic periodic modulation of the optical index along one direction of plane. We demonstrate experimentally strong PBG effects for guided light in the near-infrared (NIR) range, and the spectral analysis yields quantitative measurements of the optical index structuration at a microscopic scale. EXPERIMENTAL SETUP Our waveguides were obtained on p-type (100) bulk silicon 100 µm-thick substrates using a two-layer configuration: light was guided by total internal reflection inside the top layer, thanks to the presence of a low index layer (cladding), between the guiding layer and the substrate. The stacking was basically obtained by etching successive PS layers, using successive current densities of 16.6 and 50 mA.cm-2, in a HF:H2O:C2H5OH (35:35:30) solution at room temperature, yielding respective
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