Thermally modulated porous silica multispectral filters and their application in remote imaging

We report a thermally tunable multispectral imaging filter based on reversible condensation of volatile organic fluids within a nanoporous one-dimensional photonic crystal. The photonic crystal (optical rugate filter) comprises oxidized porous silicon, prepared by electrochemical etch of silicon and subsequent air oxidation (porous silica rugate filter, pSiF). The reflectance spectrum of the pSiF is designed and constructed to match two of the red emission bands of the luminescent complex europium(III) tris-dipicolinate, [Eu(dpa)3](3-), which has been used as an indicator for anthrax spores. When the pSiF is fitted with a thermoelectric Peltier cooler/heater and sealed in a container with 2-propanol vapor, microcapillary effects drive the temperature-dependent condensation/evaporation of 2-propanol into/out of the porous nanostructure. Thermal cycling experiments show that the wavelengths of the spectral bands of the pSiF are reversibly tuned by ±35 nm for a temperature change of ±40 °C. Difference images of a UV-illuminated scene containing the [Eu(dpa)3](3-) target, obtained by reflection from pSiF that is continuously thermally cycled through the emission bands of the dye, show that the target can be discriminated from the background or from control targets with overlapping but dissimilar luminescence spectra.