Diamond-machined ZnSe immersion grating for NIRhigh-resolution spectroscopy

摘要

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 forimmersion grating, particularly in short NIR region (0.8 — 1.4 pm). 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 witha small pitch of less than 50 pm because ZnSe is a soft/brittle material. We have overcome this problem andsuccessfully machined sharp grooves with fine pitch on ZnSe substrates by nano precision fly-cutting techniqueat LLNL. The optical testing of the sample grating with HeNe laser shows an excellent performance: the relativeefficiency more than 87.4 % at 0.633 pm for a classical grating configuration. The diffraction efficiency whenused as an immersion grating is estimated to be more than 65 % at 1μm. Following this progress, we are aboutto start machining a grating on a large ZnSe prism with an entrance aperture of 23mm x 50mm and the blazeangle of 70°.