Photonic bandgap approach in designing microcavity-based optical sensors

Multi-layer structures, such as Bragg reflectors, rugate filters, and optical microcavities are widely used in optical sensing. They are characterised by a periodical modulation of the refractive index so that they can be classified as 1-D photonic crystals. In this communication, the optical features of such a class of sensors are analyzed from the band structure point of view. This general approach is then applied to the case of vapour sensors based on a porous silicon microcavity. A numerical analysis of the photonic bands, when the porous microcavity is exposed at chemical vapours, is presented and discussed for design optimisation purposes. In particular, we investigate how the photonic band gap changes when a volatile substance condensates in the silicon pores inducing a variation of the refractive indices of the layers forming the microcavity. Results are also compared with those obtained by the usual optical transfer matrix method.