Theoretical calculations predict that high-resolution spectroscopy of H2O gas lines in the mid-infrared region is the most promising method to observationally identify the snow-line, which has been proposed as the critical factor separating gas giants from solid planets in the planetary formation process. This requires the spectroscopic observations from space with R = λ/Δλ ≥ 30, 000. For this purpose, we propose a mid-infrared (10-18 μm) high-resolution spectrometer to be onboard the GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) mission. We are developing "immersion grating” spectroscopy technology for high-resolution spectroscopy in space. We have chosen CdZnTe as a candidate for the optical material. We report the current status of the development of the CdZnTe immersion grating, including evaluation of its optical properties (absorption coefficient and refractive index) at cryogenic temperatures, development of an anti-reflection coating with a moth-eye structure for wide-wavelength coverage, and verification of machinability for grating production. We plan to make a prototype spectrometer to demonstrate the capability of the immersion grating with ground-based observations in the N-band (λ = 8–13 μm) and beyond.
We have developed a robust bandpass filter that is suitable for use in mid- and far-infrared cryogenic space telescopes using simple photolithography. The filter consists of Babinet complementary metamaterial mirrors, which are sub-wavelength holes on a silicon wafer with metalized rim and bottom, and non-metalized side wall. The mirrors work as a “Fabry-Perot interference” type bandpass filter and have a spectral resolving power of R~10. The central wavelength is tunable by adjusting the optical distance between the surface and the bottom. It has higher mechanical toughness compared to conventional metal mesh-filters because of the support of the silicon wafer. It also has higher resistance to thermal cycles compared to conventional multi-layer filters because of its simple structure. The central wavelength can be configured only by the sub-wavelength structure without changing the depth of the holes, so that monolithic filters with different central wavelengths at each position can be fabricated.
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