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

ZnSe immersion gratings (n similar to 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 mu m). We are deveoping an extremely high-resolution ion spectrograph with lambda/Delta lambda = 100.000. WINERED, customized for the short NIB region, using ZnSe (or ZnS) immersion gratinh.(1) However, it had been very difficult to make fine grooves on ZnSe substrate with a small pitch of less than 50 mu m 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 mu m for a classical grating configuration. The diffraction efficiency when used as an immersion grating is estimated to be more than 65% at 1 mu m. Following this progress, we are about to start machining a grating on a large ZnSe prism with an entrance aperture of 23 mm x 50 mm and the blaze angle of 70 degrees.