Downhole FT-MIR Spectrometer Using Perfect Metasurface Absorber

Abstract

Realization of “Fourier transform infrared (FT-IR) on a chip” holds the potential of a disruptive technology for downhole chemical analysis within the oil and gas industry. One of the critical obstacles to downhole integration though has been the cooling requirements of conventional technologies. Here, we report the design and numerical analysis of an uncooled miniaturized Fourier transform mid-infrared (FT-MIR) spectrometer compatible with downhole thermal environments, enabled by a broadband mid-infrared metasurface detector/source combination derived from a geometric inversion of a set of conformal mapping contours. The metasurface is numerically found to exhibit a near-zero index metamaterial (NZIM) behavior with absorption characterized by surface plasmon resonances confined to the ultrathin ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula> /300) metasurface plane, making the absorption properties of the microbolometer design much less sensitive to the remaining support structure than in typical designs. This feature allows the metasurface to be integrated on a single VO2 substrate operated at elevated downhole temperatures that coincide with the metal–insulator transition region. Within this transition region, the VO2 material exhibits enhanced thermometric properties, enabling an uncooled microbolometer design with predicted maximum detectivity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${D}^{*} = 1.5\times {10}^{{10}}\,\,\text {cm} \sqrt {\text {Hz}}/\text {W}$ </tex-math></inline-formula> and noise equivalent difference temperature (NEDT) of 1 mK at a modulation frequency of 500 Hz. These parameters approach entry-level lab FT-MIR spectrometers and could represent a significant step in deploying mid-infrared spectroscopy into oilfield downhole logging applications.