Characterization of Porous Silicon Fabry-Pérot Optical Sensors for Reflectivity and Transmittivity Measurements

We investigate the effect of fabrication parameters (nonequal surface current densities, impurities inside the structure, etc.) and loss factor on reflectivity and transmittivity measurements from porous silicon Fabry-Pérot cavities with finite-size substrate thicknesses. We apply the formalism based on wave cascade matrix method for obtaining dependencies of reflectivity and transmittivity. From our analysis, we note the following results. First, resonance behavior of reflectivity and transmittivity changes only when optical/physical properties of middle layers of the cavity alter. Second, for lossless cavities, while reflectivity and transmittivity considerably change with surface characteristics (optical/physical properties of first layers), transmittivity is significantly modified by a change of optical/physical properties of middle layers (microcavity region). Third, loss inside a FP cavity makes the transmittivity more immune to variations in optical/physical properties of middle layers. Finally and most importantly, transmittivity values at resonance wavelength as well as the resonance wavelength shift can be utilized for the identification of unknown chemical/biological molecules by lossless FP cavities. For validation of these results, we carried out reflectivity and transmittivity measurements from some arbitrarily chosen positions but around the center of two fabricated FP cavities resonating at 1456 nm and at 542 nm.