Fabrication of multi-optical filters based on encoded rugate porous silicon

Flexible rugate porous silicon (PSi)/polystyrene composite films were prepared for the possible use of a tunable band-rejection filter. Rugate PSi was generated by applying a computer-generated pseudo-sinusoidal current waveform through an electrochemical etching. A composite waveform of current density containing two to four sine components was used for the optical encoding method. The values of used frequency for each of the sine components varied from 0.38 to 0.44 Hz, with a spacing of 0.02 Hz between each sine component. When the composite film was exposed to the air-saturated vapor of media, the transmission resonances were shifted to the longer wavelength, indicating that the transmission resonances were tunable by the control of a medium dose. Two other rugate PSi/polymer composite films were prepared by the modification of surface of rugate PSi through either thermal oxidation or photolytic hydrosilation. The shift of transmission resonances depends on both the surface functionalities of composite films and the properties of dose media. Hydrophobic composite films modified with dodecyl functionality displayed a much greater affinity for hydrophobic media.