Fabry–Perot interferometers (FPIs), comprising foundry-compatible dielectric thin films on sapphire wafer substrates, were investigated for possible use in chemical sensing. Specifically, structures comprising two vertically stacked distributed Bragg reflectors (DBRs), with the lower DBR between a sapphire substrate and a silicon-oxide (SiO2) resonator layer and the other DBR on top of this resonator layer, were investigated for operation in the near-ultraviolet (near-UV) range. The DBRs are composed of a stack of nitride-rich silicon-nitride (SiNx) layers for the higher index and SiO2 layers for the lower index. An exemplary application would be formaldehyde detection at sub-ppm concentrations in air, using UV absorption spectroscopy in the 300–360 nm band, while providing spectral selectivity against the main interfering gases, notably NO2 and O3. Although SiNx thin films are conventionally used only for visible and near-infrared optical wavelengths (above 450 nm) because of high absorbance at lower wavelengths, this work shows that nitride-rich SiNx is suitable for near-UV wavelengths. The interplay between spectral absorbance, transmittance and reflectance in a FPI is presented in a comparative study between one FPI design using stoichiometric material (Si3N4) and two designs based on N-rich compositions, SiN1.39 and SiN1.49. Spectral measurements confirm that if the design accounts for phase penetration depth, sufficient performance can be achieved with the SiN1.49-based FPI design for gas absorption spectroscopy in near-UV, with peak transmission at 330 nm of 64%, a free spectral range (FSR) of 20 nm and a full-width half-magnitude spectral resolution (FWHM) of 2 nm.@en

Exhaust gas measurement in the harsh environment of the tailpipe by optical techniques is a highly robust technique, provided that optical access is maintained in the presence of soot. The design, fabrication, and testing of membranes in SiC-on-Si with integrated heaters to serve as a regenerable MEMS optical window into the tailpipe are presented. Membranes at slightly elevated temperatures are demonstrated to keep the surface transparent by thermophoresis, while surface regeneration is achieved at pulsed high temperatures, which allows long-term optical measurement in the exhaust.

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Ensuring optical transparency over a wide spectral range of a window with a view into the tailpipe of the combustion engine, while it is exposed to the harsh environment of sootcontaining exhaust gas, is an essential pre-requisite for introducing optical techniques for long-term monitoring of automotive emissions. Therefore, a regenerable window composed of an optically transparent polysilicon-carbide membrane with a diameter ranging from 100 µm up to 2000 µm has been fabricated in microelectromechanical systems (MEMS) technology. In the first operating mode, window transparency is periodically restored by pulsed heating of the membrane using an integrated resistor for heating to temperatures that result in oxidation of deposited soot (600–700 °C). In the second mode, the membrane is kept transparent by repelling soot particles using thermophoresis. The same integrated resistor is used to yield a temperature gradient by continuous moderate-temperature heating. Realized devices have been subjected to laboratory soot exposure experiments. Membrane temperatures exceeding 500 °C have been achieved without damage to the membrane. Moreover, heating of membranes to ΔT = 40 °C above gas temperature provides sufficient thermophoretic repulsion to prevent particle deposition and maintain transparency at high soot exposure, while non-heated identical membranes on the same die and at the same exposure are heavily contaminated.

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Continuous monitoring of the composition of E85 biofuel is essential for a quick start and clean and efficient operation of Flex-Fuel Vehicles. The actual ethanol concentration in E85 fuel is in the range 50%–85% and fuel-line sensors are used for ethanol-gasoline composition measurement. However, also a small amount of water is typically present, which cannot be reliably detected using state-of-the-art capacitive fuel-line sensors. Thermal impedance spectroscopy has been investigated as a non-destructive technique to determine the composition of ternary mixtures of biofuels. The principle of the thermal conductivity detector has been extended for measuring both the thermal conductivity and the thermal capacity of biofuel in the range up to 10 kHz using an AC-operated polysilicon heater for injecting a sinusoidal heat flux, and another polysilicon strip at a well-defined spacing or thermopile sensors for measuring the in-phase and quadrature components of the resulting AC temperature difference. Measurements on the components are in reasonable agreement with simulations, with a −3 dB cut-off frequency at 422.5 Hz and 340.8 Hz for ethanol and gasoline, respectively. However, the cut-off frequency of water was found to be significantly lower than simulations due to its high surface tension, thus limiting access to the detector.@en

Silicon waveguide structures with SiO<inf>2</inf> cladding and tapered on-chip couplers are fabricated on Si wafers for use in water-containing biofuel composition measurement in the 2.4- 2.6 μm respectively 3.5-3.8 μm water and ethanol dominated absorption bands.@en

A capacitive probe is generally used in a flex-fuel engine for measuring the ethanol content in biofuel. However, the water content in biofuel of high ethanol content cannot be disregarded or considered constant and the full composition measurement of ethanol, gasoline and water in biofuel is required. Electrical impedance spectroscopy with a customized capacitive probe operating in the 10 kHz to 1 MHz frequency range is combined with optical absorption spectroscopy in the UV spectral range between 230 and 300 nm for a full composition measurement. This approach is experimentally validated using actual fuels and the results demonstrate that electrical impedance spectroscopy when supplemented with optical impedance spectroscopy can be used to fully determine the composition of the biofuel and applied for a more effective engine management. A concept for a low-cost combined measurement system in the fuel line is presented.

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Thermal impedance spectroscopy has been investigated as a non-destructive technique to determine the composition of ternary mixtures of biofuels. The principle of the thermal conductivity detector has been extended for measuring both the thermal conductivity and the thermal capacity of biofuel in the range between 1 to 100 Hz, using an AC-operated polysilicon heater for injecting a sinusoidal heat flux, and another polysilicon strip at a well-defined spacing or thermopile sensors for measuring the in-phase and quadrature components of the resulting AC temperature difference.@en

Measuring the ethanol/water ratio in biofuel of high ethanol content, such as E85, is important when used in a flex-fuel engine. A capacitive probe is generally used for measuring the ethanol/gasoline ratio. However, the water content in E85 biofuel cannot be disregarded or considered constant and full composition measurement of biofuel is required. Electric impedance spectroscopy with a customized coaxial probe operating in the 10 kHz to 1 MHz frequency range was investigated. An in-depth investigation of the electrical impedance domain has led to the conclusion that additional information is required to unambiguously determine the composition of the ternary biofuel mixture. Among the different options of measurement domains and techniques, optical absorption spectroscopy in the UV spectral range between 230 and 300 nm was found to be the most appropriate. The typical absorbance in the UV range is highly dominated by gasoline, while ethanol and water are almost transparent. This approach is experimentally validated using actual fuels.@en