Painting a rainbow on silicon - a simple method to generate a porous silicon band filter gradient

A band filter gradient structure was obtained in porous Si by electrochemically etching a rugate structure into a Si wafer using an asymmetrically placed counter electrode. The resulting porous Si film displays a rejection band whose wavelength maximum varies spatially due to the uneven distribution of current across the wafer during the etch. The rainbow-like band filter can be converted to a porous SiO2 graded band filter by thermal oxidization. A robust and flexible composite silicone/porous silicon gradient structure is achieved by impregnation with a silicone polymer. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.