Porous silicon opto-acoustic detector for ternary gas mixture

The use of porous phoxonic crystals with coupled optical and acoustic response has been proposed as a sensing device. Due to the porous nature of the crystal, each layer of the structure can be connected to the environment. As the optical and acoustic performances of the phoxonic crystal change when a gas permeates the pores due to modifications of the effective refractive index and density of the system, it results that these structures are suitable platforms for the detection of gases. The sensor designed following these premises can detect the composition of ternary gas mixtures through optical measurements, while an acoustic wave induces a structural oscillation. The amplified acoustic wave produces a mechanical deformation of the crystal layers that is maximized in the center a resonant microcavity. Therefore, under such experimental conditions, the sensitivity of the optical response is not only due to the optical property changes caused by the gas mixture in contact with the porous structure but also to changes in the mechanical deformations due to modifications of the acoustic properties. In this work, we discuss the device theoretical behavior as a multiparameter sensor that distinguishes the components and concentrations of a ternary gas mixture through the transfer matrix method. For a prototype combination of CO₂-Air-CH₄ mixture, the estimated resolution of the proposed device fabricated in porous silicon can be has high as 0.05% (500 ppm) in the concentration of each individual species.