The infrared high-resolution and highly-sensitive spectroscopy can provide new and deep insights in many fields of astronomy. The 2.0-5.5 μm region is a very unique and important wavelength region for astrochemistry and astrobiology, because the vibrational transitions of C-H, N-H, O-H, C-O, and C-N bonds in many molecules, which are of astrophysical interest, concentrate in this wavelength range. To advance the study in this wavelength range, we are developing a new near-infrared spectrograph: VINROUGE (= Very-compact INfrared high-ResOlUtion Ge-immersion Echelle spectrograph). The instrumental concepts of VINROUGE are “high-resolution”, “highly-sensitive”, and “very-compact instrumentation”. With (i) Germanium immersion grating, (ii) white pupil spectrograph design, (iii) reflective optics using the integrated off-axis mirrors and the optical bench by ceramic (cordierite CO-220), and (iv) highly-sensitive array (HAWAII-2RG 5.3μm cutoff array), we could obtain a solution of optical design with a spectral resolution of 80,000, total throughput of > 0.28, and a compact volume that is smaller than 600 mm×600 mm×600 mm even for 10-m class telescope. We have already completed the development of Germanium immersion grating. In this year, we plan to fabricate a set of integrated off-axis ceramic mirrors together with the ceramic optical bench to demonstrate that the reflective optics was an athermal performance. The first light of VINROUGE is expected in 2019.
Immersion gratings will play important roles for infrared astronomy in the next generation. We have been developing immersion gratings with a variety of kinds of materials and have succeeded in fabricating a high-efficiency germanium (Ge) immersion grating with both a reflection coating on the grating surface and an AR coating on the entrance surface. The grating will be installed in a K-, L-, and M-bands (2-5μm) high-resolution (R=80,000) spectrograph, VINROUGE, which is a prototype for the TMT MIR instrument. In this paper, we report the preliminary results on the evaluation of the Ge immersion grating. We confirmed that the peak absolute diffraction efficiency was in the range of 70-80%, which was as expected from the design, at both room and cryogenic temperatures.
We have been working on a long-term project for developing a variety of infrared immersion gratings for near- to mid-infrared wavelengths. The transmittance of material is essential to realize high-efficiency immersion gratings for astronomical applications. For a typical grating, the attenuation coefficient αatt must be <0.01 cm−1 for the absolute diffraction efficiency of >70%. However, as there are few reports of αatt < 0.01 cm−1 for infrared optical materials in the literatures, we performed high-accuracy measurements of αatt for a variety of infrared materials applicable to immersion gratings. We have already reported αatt at room temperature for single-crystal Si, single-crystal Ge, CVD-ZnS, CVDZnSe, and high-resistivity single-crystal CdZnTe (Ikeda et al. 2009, Kaji et al. 2014, and Sarugaku et al. 2016). Next, we proceeded with the measurements of αatt at cryogenic temperatures of 20–80 K range, which is the typical operational temperatures of infrared instruments, and for which the shifts of the band gap and/or the sharpness of the lattice absorption lines from the corresponding room temperature values are expected. Thus, we developed a new cryogenic FTIR system that enables high-accuracy measurements at cryogenic temperatures. The system has a mechanism with which two sample cells and a reference cell can be easily and quickly switched without any vacuum leak or temperature change. Our preliminary measurement of Ge using this cryogenic FTIR system found that both the cut-on and cut-off wavelengths shift to the shorter (from 2.0 to 1.7 μm) and longer (from 10.6 to 10.9 μm) wavelengths, respectively, when the temperature is decreased from room temperature to the cryogenic temperature (<28 K). We plan to complete cryogenic measurements for a variety of infrared materials by the end of 2016.
ZnSe has a high refractive index (n~ 2.45) and low optical loss (< 0.1/cm) from 0.8 to 12 um. Therefore ZnSe immersion
gratings can enable high-resolution spectroscopy over a wide wavelength range. We are developing ZnSe immersion
gratings for a ground-based NIR high-resolution spectrograph WINERED. We previously produced a large prism-shaped
ZnSe immersion grating with a grooved area 50 mm x 58 mm (Ikeda et al. 2010). However, we find two problems as
NIR immersion grating: (i) serious chipping of the grooves, and (ii) inter-order ghosts in the diffraction pattern. We
believed the chipping to be due to micro cracks just beneath surface present prior to diamond machining. Therefore we
removed this damaged region, a few tens of microns thick, by etching the ZnSe grating blank with a mixture of HCl and
HNO3. Ghosts appearing halfway between main diffraction orders originate from small differences in spacing between
odd and even grooves. Apparently the blank shifts repeatably by about 120 nm in the direction orthogonal to the grooves
depending on whether the translation stage holding the blank is moving right to left or left to right. Therefore we remachined
the grating only cutting grooves with the stage moving from right to left. After re-cutting, we also deposit the
Cu coating with an enhanced interface layer of SiO2 on the groove, which is developed in our previous study. We
evaluated the optical performances of this immersion grating. It shows light scattering of 3.8 % at 1μm, no prominent
ghosts, and a spectral resolution of 91,200 at 1 μm. However we measured an absolute diffraction efficiency of only
27.3% for TE and 25.9 % for TM waves at 1.55 μm. A non-immersed measurement of the diffraction efficiency of the
facet blazed near 20º exceeded 60%, much closer to theoretical predictions. We plan to carry out more tests to resolve
Immersion grating is a next-generation diffraction grating which has the immersed the diffraction surface in an optical
material with high refractive index of n > 2, and can provide higher spectral resolution than a classical reflective grating.
Our group is developing various immersion gratings from the near- to mid-infrared region (Ikeda et al.1, 2, 3, 4, Sarugaku et
al.5, and Sukegawa et al.6). The internal attenuation αatt of the candidate materials is especially very important to achieve
the high efficiency immersion gratings used for astronomical applications. Nevertheless, because there are few available
data as αatt < 0.01cm-1 in the infrared region, except for measurements of CVD-ZnSe, CVD-ZnS, and single-crystal Si in
the short near-infrared region reported by Ikeda et al.7, we cannot select suitable materials as an immersion grating in an
aimed wavelength range. Therefore, we measure the attenuation coefficients of CdTe, CdZnTe, Ge, Si, ZnSe, and ZnS
that could be applicable to immersion gratings. We used an originally developed optical unit attached to a commercial
FTIR which covers the wide wavelength range from 1.3μm to 28μm. This measurement system achieves the high
accuracy of (triangle)αatt ~ 0.01cm-1. As a result, high-resistivity single-crystal CdZnTe, single-crystal Ge, single-crystal Si,
CVD-ZnSe, and CVD-ZnS show αatt < 0.01cm-1 at the wavelength range of 5.5 - 19.0μm, 2.0 - 10.5μm, 1.3 - 5.4μm, 1.7 - 13.2μm, and 1.9 - 9.2μm, respectively. This indicates that these materials are good candidates for high efficiency
immersion grating covering those wavelength ranges. We plan to make similar measurement under the cryogenic
condition as T ≤ 10K for the infrared, especially mid-infrared applications.