Transmittance correlation of porous silicon multilayers used as a chemical sensor platform

This work presents a system of two optical microcavities made of mesoporous silicon that have been analyzed as a platform for either chemical sensing or biosensing. When a porous microcavity is exposed to an analyte, the effective refractive index of its layers change, and its optical transmittance shifts towards lower wavenumbers. We constructed a device that employs two identical porous silicon microcavities, one of them is allowed to be in contact with the analyte, whereas the other remains unexposed. The transmitted intensity of the system results in the integrated product of the transmittances of both multilayers, which can be approximated to the autocorrelation function of the transmittance of the microcavity. Its value depends on the analyte concentration, so it can be used for sensing purposes. This results in a sensor that requires neither a wavelength-sensitive detector nor a monochromatic source of illumination, and is robust to changes in temperature, because it only depends on the relative changes in the microcavities. The sensor's response can be optimized by modifying the angular position of the second microcavity. A sensor based on this principle is demonstrated for isopropyl alcohol detection. The minimum concentration change that can be measured is about 30 ppm, which is equivalent to a minimum measurable change of refractive index of 5 x 10(-5). (C) 2015 Elsevier B.V. All rights reserved.