Fabrication and Performance of Silicon Immersion Gratings for Infrared Spectroscopy

Silicon immersion gratings open up the possibility of compact infrared spectrometers with high throughput, high spectral resolution, and extensive instantaneous coverage. The performance of the diffraction gratings that we have been developing over the past 15 years has reached the level where it can exceed that of commercially available diffraction gratings. We have produced science-grade immersion grating echelles with coarsely spaced grooves on silicon substrates appropriate for applications in the near-infrared (1.1-5 μm). Devices in the current generation have excellent throughput (60%-80%) and display diffraction-limited performance over apertures of 20 mm or more. Tests of the gratings done in reflection are in good agreement with tests done in immersion. We assess the current state of the silicon grating technology as well as discuss further developments necessary for making gratings on larger silicon substrates.