Macroporous silicon UV filters for space and terrestrial environments

Currently used optical filters exhibit strong limitations in the deep UV and shorter wavelength ranges. We propose an entirely different type of UV filter to solve many of the problems due to inadequate materials and fabrication techniques. These filters consist of three-dimensionally ordered Macroporous Silicon (MPSi), with the pores used as waveguide cores separated by the reflective silicon host. Ordered pores serve as a two-dimensional array of optical waveguides. Multilayer coating of the pore walls results in the band-pass, short-pass, or band-blocking transmittance spectra of MPSi filters. Such filters have a number of advantages. They do not exhibit spectral shifts of the passed or blocked spectral bands with the angle of incidence, permitting operation in tilted and divergent light beams to simplify optical system design and fabrication. Due to their structures (fewer and thinner layers on the pore walls required to gain the same level of rejection), the filters do not exhibit delamination problems and are well suited for operation at extreme temperatures (for space as well as for terrestrial environments). The fabrication process is different from that used for multilayer interference filters. This process permits the fabrication of filters up to 200mm in diameter that are suitable for wavelengths from longer than 400 nm to shorter than 100 nm. Far-UV filters can be manufactured as simply and economically as the near UV ones. The theory of light propagation through the MPSi layers is developed, the main predictions of the theory are experimentally validated, and the fabrication procedure for MPSi UV filters is reported.