Advanced silicon diffractive optics for cross-dispersed spectroscopy

Our group manufactures diffractive optics directly in silicon. These include transmissive gratings (grisms) and reflective gratings (immersion gratings). These gratings make possible compact medium and high-resolution infrared spectrometers with broad wavelength coverage for airborne (SOFIA), space-based (JWST), and ground-based (IGRINS, iSHELL) infrared spectrometers. We propose here to extend the capabilities of these optics in two important ways that will significantly broaden the usefulness of silicon dispersers for future space infrared instruments: we will create integrated cross-dispersed grisms for medium resolution transmission spectroscopy and we will extend the capabilities of diffraction-limited immersion gratings to the shortest possible wavelengths. Silicon grisms are not only compact at a given dispersive power but also can send dispersed light along an unbent path. This capability makes it possible to contemplate instruments with a single “filter” wheel that permits imaging, and low and medium resolution spectroscopy for planetary transit studies and other observations. In collaboration with JPL, we will develop a medium resolution, monolithic, double-sided silicon grism. This optic combines methods we developed at UT (UV lithographic and wet etching techniques to produce coarse blaze gratings) and at JPL (gray-scale electron beam lithography and plasma transfer etching to produce fine blazed gratings). These devices will enable cross-dispersed spectrographs continuously covering up to an octave in wavelength at R=5000—20,000, and add significantly to the design toolbox for transmission instruments. Silicon immersion gratings reduce instrument volumes by an order of magnitude compared to conventional grating instruments with the same resolving power. We will improve our immersion grating manufacturing precision in several ways to permit good performance down to the newly identified short wavelength cutoff of cryogenic silicon optics (down to 1.065 μm). These improvements will also allow very high resolving power (R>100,000—200,000) throughout the near-IR. With these improved high-R devices, we will open new methods for characterization of exoplanet atmospheres and a different approach to future detection of biosignatures.

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