<title>Development of silicon grisms and immersion gratings for high-resolution infrared spectroscopy</title>

Abstract

We report new results on silicon grism and immersion grating development using photolithography and anisotropic chemical etching techniques, which include process recipe finding, prototype grism fabrication, lab performance evaluation and initial scientific observations. The very high refractive index of silicon (n = 3.4) enables much higher dispersion power for silicon-based gratings than conventional gratings, e.g. a silicon immersion grating can offer a factor of 3.4 times the dispersion of a conventional immersion grating. Good transmission in the infrared (IR).allows silicon-based gratings to operate in the broad IR wavelength regions (-.1-10 jim and far-IR), which make them attractive for both ground and space-based spectroscopic observations. Coarser gratings can be fabricated with these new techniques rather than conventional techniques, allowing observations at very high dispersion orders for larger simultaneous wavelength coverage.We have found new etching techniques for fabricating high quality silicon grisms with low wavefront distortion, low scattered light and high efficiency. Particularly, a new etching process using tetramethyl ammonium hydroxide (TMAH) is significantly simplifying the fabrication process on large, thick silicon substrates, while providing comparable grating quality to our traditional potassium hydroxide (KOH) process. This technique is being used for fabricating inch size silicon grisms for several JR instruments and is planned to be used for fabricating 4 inch size silicon immersion gratings later.We have obtained complete K band spectra of a total of 6 T Tauri and AeIBe stars and their close companions at a spectral resolution of R 5000 using a silicon echelle grism with a 5 mm pupil diameter at the Lick 3m telescope. These results represent the first scientific observations conducted by the high-resolution silicon grisms, and demonstrate the extremely high dispersing power of silicon-based gratings.The future of silicon-based grating applications in ground and space-based JR instruments is promising. Silicon immersion gratings will make very high-resolution spectroscopy (R > 100,000) feasible with compact instruments for implementation on large telescopes. Silicon grisms will offer an efficient way to implement low-cost medium to high resolution JR spectroscopy (R 1000 -50000) through the conversion of existing cameras into spectrometers by locating a grism in the instrument's pupil location.

Development of an extremely coherent single-mode fiber bundle array for high-contrast imaging of extrasolar planets with visible Terrestrial Planet Finder

“…We have achieved a positioning accuracy of <0.5 m between grooves etched in the silicon, a local roughness of 1-3nm, a large-scale roughness of ~10 nm and a crystal alignment accuracy of <0.1° (Ge et al 2001(Ge et al ,2003. This has been accomplished by having very uniform etching, precision in our mask-writing, precision in photolithography and crystal alignment.…”

Machined immersion grating with theoretically predicted diffraction efficiency

“…They fabricated two Si immersion gratings with this method and demonstrated their viability as diffraction gratings in the infrared region. After a few researchers put a significant effort into realizing a Si immersion grating with the same method [15][16][17], a group at the University of Texas finally succeeded in fabricating such gratings with practical efficiencies [18,19]. The group's immersion gratings, designed for the near-infrared region from 1.2 to 5.5 μm, have relative diffraction efficiencies (called groove efficiencies in the paper, although their definition differs slightly from the exact definition above [20]) of 56%-74%, although prominent ghosts of ∼1% appeared in the spectra due to the groove periodic error of 12-17 nm (rms).…”

Design inputs for a high-performance high-resolution near-infrared spectrograph

“…A very significant risk is that SIGs are not available from any commercial vendor. Prototype SIGs have been manufactured by several research groups 15,16 and recently SIGs have been produced for instruments that are currently under construction 17,18 . There are several possible manufacturing techniques for the SIGs and while SIGs with excellent characteristics have been produced, the optimum manufacturing technique has yet to be identified.…”