Porous Bragg reflector based sensors: Ways to increase sensitivity

Abstract

In this work, we successfully evaluated the kinetics of adsorption/desorption processes of ethanol and ethanol-water solution in a porous silicon based distributed Bragg reflector (DBR). A highly sensitive structure was designed and then fabricated by electrochemically etching of a silicon wafer. Sensing experiments were realized by monitoring in real-time the reflectance spectra in the visible before, during, and after exposure to different diluted solutions of pure ethanol in water. The transfer matrix method was used to calculate reflectance spectra for a wide range of bilayer combinations [nL, nH] with low nL and high nH refractive indices. It was theoretically and experimentally evidenced that the sensitivity of the DBR sensor is higher for structures with porosity values in the upper range (i.e. refractive indices in the lower range). The sensitivity is also improved for structures with a maximum reflectance peak (λ0) position tuned toward longer wavelengths. The experimental results are in good agreement with the simulated data. We believe that our results can help in developing rapid, sensitive, and reversible sensing devices based on the DBR porous silicon (PS) layer.