Production and evaluation of silicon diffractive optics for infrared astronomy

The silicon diffractive optics we have been developing over the past 15 years have reached the level where they compete with and, in some cases, exceed the performance of commercially available diffraction gratings. The main goal of our program is to produce high quality immersion gratings with coarsely spaced grooves appropriate for use in the near-infrared (1.1 - 5 µm), as well as a set of grisms for the near-IR and longer wavelength bands (5 - 35 µm). We tested all gratings as front-surface devices as well as immersed gratings or grisms. Results of our testing show that our echelles behave according to the predictions of the scalar efficiency model and that tests done on front surfaces are in good agreement with tests done in immersion. Relative efficiencies of all gratings were better than 60% and as high as 97% at 632.8 nm. All gratings except our older prototype had diffraction limited performance at 632.8 nm. Having produced several diffraction gratings on silicon substrates up to 75 mm in diameter, we evaluate the current state of the silicon grating technology as well as discuss further developments necessary for making gratings on larger silicon substrates.