Metamaterial absorbers are photonic structures composed of an array of sub-wavelength metallic patterns. Results in literature are usually based on structures of nominal dimensions, despite the significant effect of fabrication tolerances on performance. This research aims to identify the main sources of uncertainty and to investigate their effect, notably that of an irregular surface quality (i.e., roughness) of the thin metallic layer and the lithography related variations in size and shape. The effect of the shape and positioning of the resonance peak was investigated and validated using mid-infrared metamaterial absorbers. This sensitivity analysis is essential to the batch fabrication of metamaterial absorbers for MEMS applications.@en

This paper demonstrates the functional integration of a linear variable optical filter (LVOF) and a gas cell at the wafer level, i.e. a gas-filled LVOF, where the resonator cavity of the filter is also used for storing the gas sample. A mm-level effective optical absorption path length is achieved from a μm-level physical path length, by exploiting multiple reflections from highly reflective Bragg mirrors and the high-order operation of the filter. The wideband spectral response of the device is ensured by using a tapered cavity, where the cavity length changes linearly along the length of the filter. Therefore, combined with a detector array and a light source, the gas-filled LVOF enables wideband operation and long absorption path in a single MEMS device, thus ensuring a highly sensitive on-chip gas absorption microspectrometer.@en

The biggest challenge in the miniaturization of spectrometers is achieving high sensitivity and resolving power at reduced device dimensions. The gas-filled linear variable optical filter (LVOF) enhances the interaction between the light and the sample, thereby improving the sensitivity in a small footprint, by using the resonator cavity of a wideband interference filter also as the gas cell
in a wafer-level implementation. The demanding operating conditions of the gas-filled LVOF renders the conventional approximation of the tapered resonator into an array of Fabry-Perot filters invalid. A novel design method using the Fizeau interferometer approach is
described and demonstrated with measurements.@en

Increasing demand for field instruments designed to measure gas composition has strongly promoted the development of robust, miniaturized and low-cost handheld absorption spectrometers in the mid-infrared. Efforts thus far have focused on miniaturizing individual components. However, the optical absorption path that the light beam travels through the sample defines the length of the gas cell and has so far limited miniaturization. Here, we present a functionally integrated linear variable optical filter and gas cell, where the sample to be measured is fed through the resonator cavity of the filter. By using multiple reflections from the mirrors on each side of the cavity, the optical absorption path is elongated from the physical mm -level to the effective mm -level. The device is batch-fabricated at the wafer level in a CMOS-compatible approach. The optical performance is analyzed using the Fizeau interferometer model and demonstrated with actual gas measurements@en

Natural gas is the primary energy resource in both households and industry. Due to the molecular similarity of the main constituents, i.e., hydrocarbons, composition analysis of natural gas requires a highly selective and sensitive measurement technique. Smart sensing of natural gas in a large energy grid favors the use of methods that provide low unit cost in high-volume production, such as onchip devices that can be micromachined at the wafer level, while maintaining the performance of complex benchtop instruments. Optical absorption spectroscopy with wideband optical filters offers a comparable performance in a smaller footprint. However, the gas cell, where the absorption takes place, challenges the miniaturization of the spectrometer. This paper presents two approaches with analysis and experimental validation to integrate the gas cell and the linear variable optical filter (LVOF) at the wafer level. The first approach uses 45° inclined mirrors to steer the light beam through the sample gas, while the second approach, the gas-filled LVOF, functionally integrates the gas cell into the resonator cavity of the filter. Both devices are selfreferenced and compatible with fabrication in a CMOS process, and therefore highly suitable for smart gas sensing.@en

The depletion of domestic reserves and the growing use of sustainable resources forces a transition from the locally produced natural gas with a well-known composition toward the ‘new’ gas with a more flexible composition in the Netherlands. For safe combustion and proper billing, the natural gas must be monitored at both distribution points and households. This calls for a robust and low-cost gas sensor that does not require sample preparation or a lab technician to analyze the results. Optical absorption spectroscopy is a widely used method for material identification due to its nondestructive and self-referencing properties. However, the gas cell, where the sample to be measured is stored, limits the miniaturization of spectrometers to maintain the sensitivity. The gas-filled linear variable optical filter (LVOF) functionally integrates a wideband optical filter and a gas cell by using the resonator cavity of the LVOF as a chamber. The highly reflective mirrors and high operating order of the LVOF enhances the physical dimensions of the cavity into an effective optical path comparable to an external gas cell. Therefore, when combined with a wideband emitter and a detector array, the gas-filled LVOF enables wafer-level spectroscopy with high sensitivity and selectivity. In this dissertation, the design, fabrication and characterization of a gas-filled LVOF for the composition measurement of natural gas is described. The functionality of the sensor is demonstrated by actual gas measurements using the main constituents of natural gas; methane, ethane and propane. @en

A robust and highly miniaturized optical gas sensor based on optical absorption spectroscopy is presented. By using the resonator cavity of a linear variable optical filter (LVOF) also as a gas chamber, a compact and robust optical sensor is achieved. The device operates at the 15th order in 3.2–3.4 μm wavelength range for distinguishing hydrocarbons. The physical cavity length at the μm-level is translated into an effective optical absorption path length at the mm-level by the use of highly reflective (R > 98%) Bragg mirrors. The optical design using the Fizeau interferometer approach is described. Moreover, the CMOS-compatible fabrication method is explained. In addition to the wideband and single wavelength filter characterization, absorption of methane in the LVOF cavity is demonstrated at 3392 nm and 3416.60 nm wavelengths.@en

The design of a metamaterial-based absorber for use in a MEMS-based mid-IR microspectrometer is reported. The microspectrometer consists of a LVOF that is aligned with an array of thermopile detectors, which is fabricated on a SiN membrane and coated with the absorber. Special emphasis is put on the CMOS compatible fabrication, which results in an absorber design based on Al disc resonators and an Al background plane that are separated by an SiO2 layer. Wideband operation over the 3-4 μm spectral range is achieved by staggered tuning of four Al disk resonators in one 1.5 x 1.5 μm2 unit cell, using four different values of the radius of the Al disk between 0.50 μm and 0.63 μm and an SiO2 layer thickness of 150 nm. Simulations reveal an average absorption of about 95% with a ±4% ripple at normal incidence, which reduces to about 80% absorption at a 20° incidence angle. The influence of material choice and dimensions on a single absorption peak was studied and the magnetic polariton was identified as the underlying mechanism of absorption. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.@en

The design of a metamaterial-based absorber for use in a mid-IR microspectrometer-based gas sensor is reported.The microspectrometer consists of a linear variable optical filter (LVOF) that is aligned with an array of thermopiledetectors, which is fabricated on a SiN membrane and covered with the absorber. Special emphasis was put on theCMOS compatible fabrication, which resulted in an absorber design based on aluminium disk resonators and analuminium background plane that are separated by a SiO2 layer. The fabrication process is described, and thechallenges are discussed. @en

A miniaturized methane (CH4) sensor based on nondispersiveinfrared absorption is realized in MEMS technology. A high level offunctional integration is achieved by using the resonance cavity of a linearvariable optical filter (LVOF) also as a gas absorption cell. For effectivedetection of methane at λ = 3.39 µm, an absorption path length of at least 5mm is required. Miniaturization therefore necessitates the use of highlyreflective mirrors and operation at the 15th-order mode with a resonatorcavity length of 25.4 µm. The conventional description of the LVOF interms of the Fabry-Perot resonator is inadequate for analyzing the opticalperformance at such demanding boundary conditions. We demonstrate thatan approach employing the Fizeau resonator is more appropriate.Furthermore, the design and fabrication in a CMOS-compatiblemicrofabrication technolog@en

The increasing demand for handheld systems for absorption spectroscopy has triggered the development of microspectrometers at various wavelength ranges. Several MEMS implementations of the light source, interferometer/optical filter, and detector have already been reported in the literature. However, the size of microspectrometers is still limited by the required absorption path length in the sample gas cell. This paper presents a compact MEMS linear-variable optical filter (LVOF) where the resonator cavity of the filter is also used as a sample gas cell to measure the absorption of methane at 3392nm wavelength. The physical resonator cavity length is elongated 62.2-fold, using multiple reflections from highly reflective Bragg mirrors to achieve a sufficiently long effective optical absorption path. Although the LVOF would in principle enable operation as a robust portable microspectrometer, here it is used in a miniaturized NDIR methane sensor for wavelength selection and calibration. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.@en

The increasing demand for small, robust and low-cost gas sensors triggers the batch fabrication of highly selective and sensitive miniaturized devices. A linear variable optical filter (LVOF) based microspectrometer enables selectivity in a wide wavelength range, while maintaining the robustness and low cost. To achieve sensitivity in an LVOF based absorption spectrometer, a long gas cell is required. In this paper, we propose an on-chip absorption path that also serves as a gas cell, where the light beam is steered using 45° inclined mirrors. The fabrication of 45° inclined mirrors is demonstrated and optical efficiency of the system is analyzed using ray tracing.@en