Diamond-machined ZnSe immersion grating for NIR high-resolution spectroscopy

ZnSe immersion gratings (n ~ 2.45) provide the possibility of high-resolution spectroscopy for the near-infrared (NIR) region. Since ZnSe has a lower internal attenuation than other NIR materials, it is most suitable for immersion grating, particularly in short NIR region (0.8 - 1.4 μm). We are developing an extremely high-resolution spectrograph with λ/Δλ = 100, 000, WINERED, customized for the short NIR region, using ZnSe (or ZnS) immersion grating. However, it had been very difficult to make fine grooves on ZnSe substrate with a small pitch of less than 50 μm because ZnSe is a soft/brittle material. We have overcome this problem and successfully machined sharp grooves with fine pitch on ZnSe substrates by nano precision fly-cutting technique at LLNL. The optical testing of the sample grating with HeNe laser shows an excellent performance: the relative efficiency more than 87.4 % at 0.633 μm for a classical grating configuration. The diffraction efficiency when used as an immersion grating is estimated to be more than 65 % at 1μm. Following this progress, we are about to start machining a grating on a large ZnSe prism with an entrance aperture of 23mm × 50mm and the blaze angle of 70°.