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Mid-wave and long-wave infrared (IR) are two bands of interest for uncooled infrared imaging cameras. While longwave infrared detectors are sensitive to human body temperature, mid-wave infrared detectors are useful to detect “hot sources”. In addition, various gases have absorption bands in the mid-wave IR range, so that environmental monitoring or gas detection should be mentioned as further applications. To realize multispectral uncooled thermal imaging detectors, Fraunhofer IMS investigated the absorption properties of plasmonic metamaterial absorbers made of metal-insulator-metal (MIM) structures. High and multispectral absorption is particularly desirable for various microengineering applications, including microbolometers. The MIM absorbers are developed to be adaptable to Fraunhofer IMS nanotube microbolometer technology.
We report here the first results of simulation and experimental characterization of MIM test structures for multispectral absorption. The test structures consist of upper periodic metal structures, a middle dielectric layer and a lower metal reflector layer to produce surface plasmon resonance at desired absorption wavelengths. For a CMOS-compatible MIM absorber, various materials and thicknesses are being studied to realize selective absorption. We demonstrate the optical characterization of the test structures by Fourier transform infrared (FTIR) measurements and the influence of size, thickness and materials of MIM structures to achieve high selective absorption in a narrow wavelength range.
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