Porous silicon multilayer stack for sensitive refractive index determination of pure solvents

The use of a device consisting of alternating layers of high and low refractive index porous silicon, arranged as a Fabry–Perot cavity, as an inexpensive and miniature refractometer is investigated. The position of the reflectance minimum depends on the pore structure as well as on the material filling the pores. We show that the presence of an interfacial layer at the internal silicon surfaces plays a crucial role in determining the quantitative relationship between the position of the reflectance minimum (i.e. the optical output of the device) and the refractive index of the medium filling the pores (i.e. the material whose presence is being sensed). The intended application of the device is in chemical and possibly biological sensing rather than in the accurate absolute determination of refractive indices.