Development of CdZnTe immersion grating for spaceborne application

We have been developing an immersion grating for high-resolution spectroscopy in the mid-infrared (MIR) wavelength region. A MIR (12-18 µm) high-resolution (R = 20,000-30,000) spectrograph with the immersion grating is proposed for SPICA, Japanese next-generation space telescope. The instrument will be the world's first high-resolution spectrograph in space, and it would make great impacts on infrared astronomy. To realize a high-efficiency immersion grating, optical properties and machinability of bulk materials are the critical issues. There are three candidate materials with good MIR transmittance; CdTe (n = 2.65), CdZnTe (n = 2.65), and KRS5 (n = 2.30). From measurements of transmittance with FTIR and of homogeneity with phase-shifting interferometry at 1.55 μm, we confirmed that CdZnTe is the best material that satisfies all the optical requirements. As for machinability, by applying Canon's diamond cutting (planing) technique, fine grooves that meet our requirement were successfully cut on flats for all the materials. We also managed to fabricate a small CdZnTe immersion grating, which shows a high grating efficiency from the air. For the reflective metal coating, we tried Au (with thin underlying layer of Cr) and Al on CdZnTe flats both by sputter deposition and vapor deposition. All samples are found to be robust under 77 K and some of them achieve required reflectivity. Despite several remaining technical issues, the fabrication of CdZnTe immersion grating appears to be sound.