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’re developing an immersion grating made of CdZnTe designed for a high-dispersion mid-infrared spectrograph (10-18 μm, R = λ/Δλ ∼ 30, 000) to be onboard the next-generation infrared space telescope GREX-PLUS. The adoption of an immersion grating will reduce the spectrometer size to 1/n (1/n3 in volume, n: refractive index) compared to conventional diffraction gratings. To determine this absorption coefficient accurately, we need to take the effect of multiple reflection into account that depend on the refractive index. However, the accurate refractive index of CdZnTe (Δn < 10−4) at 10-18 μm below 20 K has not been measured yet. Therefore, we’re developing a measurement system of the refractive index at cryogenic temperatures in the mid-infrared range. We adopt the minimum deviation method in this system to measure the refractive index, measuring the apex and deviation angle of the prismatic sample of material to be measured. Here we give an overview of the measurement system, as well as preliminary results of the refractive index measurement.
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