One-dimensional porous silicon photonic crystals for chemosensors: Geometrical factors influencing the sensitivity

Abstract

One-dimensional photonic crystals with microcavity were prepared to be used as optical chemosensors. The experimental reflectance of the as-etched devices immersed in solvent solutions appear greater than the theoretical one. The fitting procedure of the experimental data using the transfer matrix method together with the Looyenga effective medium approach reveals that the deviation of the experimental data occurs because of the expansion-contraction effect of the high and low porosity layers, respectively, caused by the presence of the solvent into the porous matrix and is proportional to the solvent effective refractive index. Due to this phenomenon, the sensitivity of the as-etched photonic crystals increases about two times but is not stable because of the aging effect. To avoid this effect, the photonic crystal structures were passivated by thermally silicon oxide growing, gold thin film and titanium oxide deposition. After thermal treatment in an air environment, the optical response achieves its stabilization even after long storage times, thus showing the importance of the passivation procedure. Among them, the passivation by TiO2 shows better performance and lower loss in sensitivity in comparison to the as-etched devices.