Materials with adjustable refractive index for precision optical filter fabrication

In this work it is shown how by adjusting the porosity of several materials a predetermined effective refractive index can be obtained within a wide range of values. In particular electrochemical etching of silicon wafers results in porous silicon films with a refractive index that can range from 1.4 to 2.1, while grazing angle evaporation can produce porous germanium films with indexes between 1.4 to 4. Moreover, both techniques allow the fabrication of layered structures where the refractive index changes following a predesigned profile, resulting in a monolithic structure with a tailored interference effect. These porous materials and fabrication processes are very promising for the fabrication of precise monolithic optical filters for appplication in the VIS, NIR and SWIR wavelength ranges.