Fabrication of silicon grisms up to 2 inches in dimension has become a routine process at Penn State thanks to newly developed techniques in chemical etching, lithography and post-processing. The newly etched silicon grisms have typical rms surface roughness of ~ 9 nm with the best reaching 0.9 nm, significantly lower than our previous attempts (~ 20-30 nm). The wavefront quality of the etched gratings is high. Typical wavefront error is ~ 0.035 wave at 0.6328 micron, indicating diffraction-limited performance in the entire infrared wavelengths (1.2-10 microns) where silicon has excellent transmission. These processes have also significantly eliminated visible defects due to grating mask breaks during chemical etching. For the best grisms, we have less than 1 defect per cm 2 . The measured total integrated scatter is less than 1% at 0.6328 micron, indicating similar or lower scatter in the IR when grisms are operated in transmission. These new generation grisms are being evaluated with our Penn State near IR Imager and Spectrograph (PIRIS) in cryogenic temperature. We are applying the new techniques in etching an 80x40 mm 2 grating on 30 mm thick substrate to make an anamorphic silicon immersion grating, which can provide a diffraction-limited spectral resolution of R = 220,000 at 2.2 micron. We plan to put this immersion grating in a modified PIRIS to measure magnetic field strength using the Fe I line at 1.56 micron among hundreds of nearby solar type stars to investigate the probability of the Maunder Minimum using the Mt. Wilson 100inch with adaptive optics in 2003.

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.

Future 30m telescopes provide enormous challenges for IR high resolution spectrograph design. The spectrograph collimated beam size will reach ~ 400 mm in order to reach R ~ 25,000 under 0.4 arcsec seeing-limited images. This beam size will push an IR spectrograph volume larger than that of the giant optical echelle spectrograph at 10m telescopes, e.g. the Keck HIRES is 6 6 4 m 3 (Vogt et al. 1994). The cost would be enormous considering the entire instrument must be cooled to cryogenic temperatures to be feasible. Here we propose a new kind of IR spectrometer using silicon anamorphic immersion gratings as the main disperser. By operating silicon immersion gratings in an anamorphic immersion mode, the increase in spectral resolving power can be up to a factor of n 2 or ~12 times at Brewster's angle (Dekker 1987). Hence, to reach the same spectral resolution, the collimated beam size is reduced to ~33mm in diameter, which makes the design of the instrument relatively easy.The recent breakthrough in silicon immersion grating technology at Penn State has allowed us to routinely fabricate high quality silicon grisms and immersion gratings with sizes of up to 2 inches, <1% integrated scattered light, and diffraction-limited performance thanks to newly developed techniques. Silicon anamorphic immersion gratings with etched dimensions of ~4 inches are being developed at Penn State. The first grating will be available for testing in late 2002. Currently, industry can supply up to 12 inch diameter silicon ingots. We plan to develop a new tool to handle this large grating size in our stateof-the-art nanofabrication facility. A silicon anamorphic grating of this size can provide a seeing-limited (0.4 arcsec) spectral resolution of R ~ 30,000 or diffraction-limited spectral resolution of R ~ 750,000 at 2.2 microns. In this paper, technical issues related to the design of an anamorphic grating spectrograph are discussed.

The breakthrough of silicon immersion grating technology at Penn State has the ability to revolutionize high-resolution infrared spectroscopy when it is coupled with adaptive optics at large ground-based telescopes. Fabrication of high quality silicon grism and immersion gratings up to 2 inches in dimension, less than 1% integrated scattered light, and diffraction-limited performance becomes a routine process thanks to newly developed techniques. Silicon immersion gratings with etched dimensions of ~ 4 inches are being developed at Penn State. These immersion gratings will be able to provide a diffraction-limited spectral resolution of R = 300,000 at 2.2 micron, or 130,000 at 4.6 micron. Prototype silicon grisms have been successfully used in initial scientific observations at the Lick 3m telescope with adaptive optics. Complete K band spectra of a total of 6 T Tauri and Ae/Be stars and their close companions at a spectral resolution of R ~ 3000 were obtained. This resolving power was achieved by using a silicon echelle grism with a 5 mm pupil diameter in an IR camera. 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. New discoveries from this high spatial and spectral resolution IR spectroscopy will be reported. The future of silicon-based grating applications in ground-based AO IR 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 IR spectroscopy (R ~ 1000-50000) through the conversion of existing cameras into spectrometers by locating a grism in the instrument's pupil location.

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