Suppression of ageing effects in porous silicon interference filters

Interference filters made from porous silicon can be a useful alternative to conventional dielectric filters because of the fast and cheap fabrication and the compatibility to conventional silicon technology. However, so far the main disadvantages of these structures were ageing effects due to the oxidation of porous silicon in ambient air. In this paper we demonstrate that the problem can be solved by a thermal pre-oxidation of porous silicon. This treatment allows the use of these filters not only at room temperature, but even at high temperatures up to 600°C. Moreover, the pre-oxidation reduces the absorption in the blue and UV which is necessary for future applications in this spectral range. The complex refractive index of the pre-oxidised porous silicon is determined by numerical simulations of reflectance measurements.