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The nanostructured rugate porous silicons (PSi) containing multiple photonic band gaps have been generated by an electrochemical etching through applying a composite waveform summed three computer-generated pseudo-sinusoidal current waveforms. They exhibit three sharp photonic band gaps in the optical reflectivity spectrum, corresponding to the each of the sine components varied from 0.42, 0.36, and 0.30 Hz, with a spacing of 0.06 Hz between each sine component. The sensing experiments using multi-encoded rugate PSi for the detection of nerve agents such as triethyl phosphate (TEP), diethyl chlorophosphate (DCP), dimethyl methylphosphonate (DMMP), and diethyl ethylphosphonate (DEEP) have been achieved. Capillary condensation in the pores causes the reflectivity of rugate PSi to shift to longer wavelengths due to an increase in refractive indices of the porous medium.

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Novel porous Si exhibiting dual optical properties, both Fabry-Perot fringe (optical reflectivity) and photoluminescence, were developed and used as chemical sensors. Porous Si samples were prepared by an electrochemical etch of p-type silicon under the illumination with a 300 W tungsten filament bulb for the duration of etch. The surface of porous Si was characterized by FT-IR instrument. The porosity of samples was about 80%. Both reflectivity and photoluminescence were simultaneously measured under the exposure of organic vapors. The shift of Fabry-Perot fringe to the longer wavelength under the exposure of chloroform vapors was obtained. The steady-state photoluminescence spectra and quenching photoluminescence under the exposure of various organic vapors were obtained. A set of organic compounds were analyzed by both quenching photoluminescence and change of optical thickness.