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Design and manipulation of 1-D rugate photonic crystals of porous silicon for chemical sensing applications


Porous silicon rugate photonic crystals are an attractive optical sensor material due to their high surface area, naked eye response, and controllable optical, morphological, and chemical characteristics. This thesis presents new ways to improve the selectivity, reversibility, and stability against interferents of remotely interrogated porous silicon sensors. After a brief introduction to rugate porous silicon, Chapters 2-5 present methods of directly sensing the interaction of organic vapors with the porous layer by chemical and physical sensor modulation. A 0.3 mm² fiber optic-coupled porous silicon sensor is constructed in Chapter 2 and implanted in a bed of activated carbon, demonstrating carbon bed end-of-service-life sensing. Chapter 3 furthers this concept by incorporating chemically modified sensor surfaces, with selectivity between water vapor, isopropanol, and heptane vapors demonstrated using acetylated and oxidized sensor chemistries. Chapter 4 introduces physical modulation of the porous silicon sensor, with thermal modulation of the photonic crystal to 160°C employed to rapidly and repeatedly desorb methyl salicylate and octanol vapors that foul the sensor response. Thermal modulation is applied to discrimination of pure heptane, cyclohexane, and isopropanol vapors in Chapter 5 by rapidly cycling a rugate sensor between 25- 80°C while exposed to partial pressures of organic vapors up to 7.5 Torr. Sensor responses to the thermally modulated sorption equilibrium allow discrimination of these pure analyte vapors. The final three chapters describe using porous silicon as tailored interference filters that increase the specificity of standoff optical detection. In Chapter 6, the stop bands of rugate filters are tuned to match mid-infrared molecular absorbance bands, including the 1250 cm⁻¹ P=O bond stretch. Standoff gas sensing is demonstrated with filters matched to the 2350cm⁻¹ stretch of CO₂. In Chapter 7, selective 2-D imaging of target compounds is demonstrated by matching filters to visible emission peaks of photoluminescent dyes that bind to dipicolinic acid, found in anthrax spores. Finally, wavelength separated, ratiometric referencing is demonstrated in Chapter 8, where a pH-responsive NH₃(g) sensor based on a dye-infused rugate filter with two stop bands encoded into the porous layer is shown to compensate for large fluctuations in probe light intensity and increase signal to noise

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