Tunable Porous Silicon Photonic Band Gap Structures
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Tunable Porous Silicon Photonic Band Gap Structures J. Eduardo Lugo1, Herman A. Lopez, Selena Chan, and Philippe M. Fauchet Department of Electrical and Computer Engineering, University of Rochester, Rochester, New York 14627, U.S.A. 1 Centro de Investigación en Energía, Universidad Nacional Autónoma de México, A.P. 34, 62580 Temixco, Mor. México. ABSTRACT The tuning of one-dimensional photonic band gap structures based on porous silicon will be presented. The photonic structures are prepared by applying a periodic pulse of current density to form alternating high and low porosity layers. The width and position of the photonic bandgap are determined by the dielectric function of each layer, which depends on porosity, and their thickness. In this work we show that by controlling the oxidation of the porous silicon structures, it is possible to tune the photonic bandgap towards shorter wavelengths. The formation of silicon dioxide during oxidation causes a reduction of the refractive index, which induces the blue shift. The photonic band gap is determined experimentally by taking the total reflection of the structures. In order to understand the tuning of the photonic band gap, we developed a geometrical model using the effective medium approximation to calculate the dielectric function of each of the oxidized porous silicon layers. The two key parameters are the porosity and the parameter β, defined as the ratio between the silicon dioxide thickness and the pore radius before oxidation. Choosing the parameter β, to fit the experimental photonic band gap of the oxidized structures, we extract the fraction of oxide that is present. For example, the measured 240 nm blue shift of a photonic bandgap that was centered at 1.7 microns corresponds to the transformation of 30% of the structure into silicon dioxide. A similar approach can be used for oxidized two-dimensional porous silicon photonic structures. INTRODUCTION Porous silicon (PSi) is an interesting optical material that has been researched intensively for a decade. The most attractive property of PSi is its room-temperature luminescence and the possibility to build LED devices [1]. PSi has also been used to fabricate one-, and two-dimensional photonic structures [2][13]. Many other authors are working as well two-, and three-dimensional photonic structures using other materials [14]-[17]. Most of the applications of these PSi based structures are in the field of sensing and communications. For instance, it is well known that the transmission at 1.5 microns is very important in the telecommunications industry. Grüning et al. [9]-[10] have developed two-dimensional PSi based waveguides with the photonic bandgap wavelength range of 3.3-40 microns and Rowson et al. have made macroporous silicon crystals with the bandgap at 1.5 microns [11]. Recently, we have fabricated PSi one-dimensional photonic structures with the bandgap centered on the 1.5 microns wavelength [13]. At this particular wavelength the reflectivity is approximately 100%. The PSi structures are initiall
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