Tunable Porous Silicon Photonic Band Gap Structures

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.