Research

Instrumentation & Device Development

EXES: mid-IR spectrograph

GMTNIRS

HETDEX: Dark Energy Experiment

IGRINS

Micromachined Silicon Diffractive Optics

Micromachined Silicon Diffractive Optics

We are using silicon micromachining techniques to fabricate immersion gratings and grisms for infrared spectroscopy. These devices offer substantial advantages in compactness, formatting, and efficiency over other dispersive devices. For example, high resolution spectrographs designed around immersion gratings can have volumes an order of magnitude smaller than comparable instruments built around conventional gratings. In addition, the ability to make coarse grooves using micromachining allows us to produce gratings that make it possible for the first time for infrared instruments to have continuous wavelength coverage over large bands at high resolution.

In an immersion grating, the light enters from the left and is incident on the grating surface from the inside, where the wavelength is shortened by a factor equal to the refractive index (3.44 for Si). This allows a grating of a given size to have 3.44 times the resolving power of a conventional front-surface device.

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A grism is a transmission grating mounted (or etched into a prism-shaped dielectric substrate. The light enters on the left and is diffracted by the grating as it exits. The resolving power of devices with the same opening angle depends on the refractive index of the substrate as (n-1). Silicon grisms of a given size have resolving powers 3-4 times greater than those of grisms made from glass or other low index materials.

We use the same silicon fabrication techniques used in nanotechnology to produce the grooves in our diffractive optics. We pattern the grooves onto silicon using photolithography and then etch away part of the crystal to form V-shaped grooves. The feature size of our grooves is large (typically 20-150 mm), but the groove positions must come at regular intervals with a precision of only 20-30 nm over the entire size of the piece.

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Scanning electron micrograph of grooves micromachined into a silicon wafer. The opening angle for such grooves is 70°, rather than the 90° angle produced by conventional ruling. The groove surfaces are much flatter than in ruled gratings.

End view of micromachined grooves, The flat areas at the top are etch-stops used in the patterning process. They are shadowed by the grooves when viewed from inside the silicon.

In grisms and in many immersion gratings, the grooves must be etched at an angle with respect to the silicon crystal planes. Here we show a test which produced grooves at a very steep bias angle.

Recent Grating Production

We have recently produced a grating for IGRINS, an R=40,000, 1.4-2.5 μm spectrograph for the McDonald 2.7m telescope. This instrument uses a 90mm long R3 immersion grating.

Figure 1: Machined grating surface for the IGRINS immersion echelle. (left) The 30mm thick ~100mm diameter substrate. (right) An image seen in Littrow in the grating grooves.


Figure 2: Shaped IGRINS immersion grating. Light enters through the face seen in the foreground. The grating surface is on the top.


Figure 3: Monochromatic spectrum taken on the front surface of the IGRINS grating at 543nm. The intensity scale is logarithmic.


Figure 4: 633nm interferogram of the IGRINS grating surface. The surface accuracy is λ/6 peak to valley over the full 25mm beam..


Figure 5: Efficiency measurements for the IGRINS grating in immersion. The on-blaze efficiency is about 75% across the H and K bands.

Applications

UT silicon diffractive optics have been incorporated into three very different instruments to date. These include the FORCAST camera on NASA’s SOFIA Airborne Observatory and the NIRCAM instrument on JWST.


Figure 6: UT has fabricated grisms for FORCAST that give it a spectroscopic capability in the 5-30 micron range.


Figure 7: Our group has produced a set of 4 grisms incorporated into the long wavelength arm of NIRCAM giving it an R=1800 capability for slitless spectroscopy.

UT has produced an immersion grating for IGRINS that gives it a resolving power of 40,000 across all of the H and K atmospheric windows in a single exposure.

In addition to these three instruments, UT is fabricating an immersion grating for a next-generation spectrograph on the NASA IRTF and for prototype instruments for spectroscopy of trace gases in the Earth’s atmosphere from space.

You can learn more about our work on silicon micromachined gratings by looking at our publications:

ADS

Near-infrared metrology of high-performance silicon immersion gratings

Gully-Santiago, M., Wang, W., Deen, C., Modern Technologies in Space and Ground-Based Telescopes and Instrumentation II, Edited by Ramon Navarro, Colin Cunningham, and Eric Prieto, Proc. SPIE, 8450, E2SG (2012). [ web ]

iSHELL: a 1-5 micron cross-dispersed R=70,000 immersion grating spectrograph for IRTF

Rayner, J., Bond, T., Bonnet, M., Jaffe, D., Muller, G., and Tokunaga, A. , Ground Based and Airborne Instrumentation for Astronomy IV, Edited by Ian McLean, Suzanne Ramsay, and Hideki Takami, Proc. SPIE 8446, E2CR (2012). [ web ]

High-performance silicon grisms for 1.2-8.0 micron: detailed results from the JWST-NIRCam devices

Gully-Santiago, M.; Wang, W.; Deen, C.; Kelly, D.; Greene, T.P.; Bacon, J.; Jaffe, D.T.; Modern Technologies in Space- and Ground-based Telescopes and Instrumentation. Edited by Atad-Ettedgui, Eli; Lemke, Dietrich. Proceedings of the SPIE, Volume 7739, pp. 77393S-7 (2010). [ web ]

Manufacturing of silicon immersion gratings for infrared spectrometers

Wang, Weisong; Gully-Santiago, Michael; Deen, Casey; Mar, Douglas J.; Jaffe, Daniel T. ; Modern Technologies in Space- and Ground-based Telescopes and Instrumentation. Edited by Atad-Ettedgui, Eli; Lemke, Dietrich. Proceedings of the SPIE, Volume 7739, pp. 77394L-9 (2010) [ web ]

GMTNIRS (Giant Magellan Telescope near-infrared spectrograph): design concept

Lee, Sungho; Yuk, In-Soo; Lee, Hanshin; Wang, Weisong; Park, Chan; Park,Kwi-Jong; Chun, Moo-Young; Pak, Soojong; Strubhar, Joseph; Deen, Casey; Gully-Santiago, Michael; Rand, Jared; Seo, Haingja; Kwon, Jungmi; Oh, Heeyoung; Barnes, Stuart; Lacy, John; Goertz, John; Park, Won-Kee; Pyo, Tae-Soo; Jaffe, Daniel T.; Ground-based and Airborne Instrumentation for Astronomy III. Edited by McLean, Ian S.; Ramsay, Suzanne K.; Takami, Hideki. Proceedings of the SPIE, Volume 7735, pp. 77352K-9 (2010). [ web ]

Preliminary design of IGRINS (Immersion GRating INfrared Spectrograph)

Yuk, In-Soo; Jaffe, Daniel T.; Barnes, Stuart; Chun, Moo-Young; Park, Chan; Lee, Sungho; Lee, Hanshin; Wang, Weisong; Park, Kwi-Jong; Pak, Soojong; Strubhar, Joseph; Deen, Casey; Oh, Heeyoung; Seo, Haingja; Pyo, Tae-Soo; Park, Won-Kee; Lacy, John; Goertz, John; Rand, Jared; Gully-Santiago, Michael ; Ground-based and Airborne Instrumentation for Astronomy III. Edited by McLean, Ian S.; Ramsay, Suzanne K.; Takami, Hideki. Proceedings of the SPIE, Volume 7735, pp. 77351M-10 (2010). [ web ]

Science instrument development for the Giant Magellan Telescope

Jaffe, D. T.; Depoy, D. L.; Fabricant, D. G.; Hinz, P. M.; Jacoby, G.; Johns, M.; McCarthy, P.; McGregor, P. J.; Shectman, S.; Szentgyorgyi, A. ; Ground-based and Airborne Instrumentation for Astronomy III. Edited by McLean, Ian S.; Ramsay, Suzanne K.; Takami, Hideki. Proceedings of the SPIE, Volume 7735, pp. 773525-8 (2010) [ web ]

A silicon and KRS-5 grism suite for FORCAST on SOFIA

Deen, Casey P.; Keller, Luke; Ennico, Kimberly A.; Jaffe, Daniel T.; Marsh, Jasmina P.; Adams, Joseph D.; Chitrakar, Nirbhik; Greene, Thomas P.; Mar, Douglas J.; Herter, Terry; Ground-based and Airborne Instrumentation for Astronomy II. Edited by McLean, Ian S.; Casali, Mark M. Proceedings of the SPIE, Volume 7014, pp. 70142C-10 (2008). [ web ]

Silicon immersion grating spectrograph design for the NASA Infrared Telescope Facility

Tokunaga, A. T.; Bond, T.; Jaffe, D. T.; Mumma, M. J.; Rayner, J. T.; Tollestrup, E. V.; Warren, D. W.; Ground-based and Airborne Instrumentation for Astronomy II. Edited by McLean, Ian S.; Casali, Mark M. Proceedings of the SPIE, Volume 7014, pp. 70146A-11 (2008). [ web ]

Fabrication and test of silicon grisms for JWST-NIRCam

Jaffe, D. T.; Wang, W.; Marsh, J. P.; Deen, C. P.; Kelly, D.; Greene, T. P. ; Space Telescopes and Instrumentation 2008: Optical, Infrared, and Millimeter. Edited by Oschmann, Jacobus M., Jr.; de Graauw, Mattheus W. M.; MacEwen, Howard A. Proceedings of the SPIE, Volume 7010, pp. 70103L-6 (2008).[ web ]

Micromachined silicon grisms for infrared optics

Douglas J. Mar, Jasmina P. Marsh, Casey P. Deen, Hao Ling, Hosung Choo, and Daniel T. Jaffe; Appl. Opt. 48, 1016-1029 (2009) [ web ]

Production and Evaluation of Silicon Immersion Gratings for Infrared Astronomy

Marsh, J. P.; Mar, D. J.; Jaffe, D. T., Applied Optics, 46, 3400 (2007). [ pdf ]

Performance of large chemically etched silicon grisms for infrared spectroscopy

Mar, D. J.; Marsh, J. P.; Jaffe, D. T.; Keller, L. D.; Ennico, K. A., 2006, Proceedings of the SPIE, 6269, 62695R (2006). [ pdf ]

Fabrication and Performance of Silicon Immersion Gratings for Infrared Spectroscopy

Marsh, Jasmina P.; Mar, Douglas J.; Jaffe, Daniel T., Proceedings of the SPIE, 6269, 62694J (2006). [ pdf ]

GMTNIRS - The High Resolution Near-IR Spectrograph for the Giant Magellan Telescope

Jaffe, D. T.; Mar, D. J.; Warren, D.; Segura, P. R., Proceedings of the SPIE, 6269, 62694I (2006). [ pdf ]

Infrared grisms using anisotropic etching of silicon to produce a highly asymmetric groove profile

Ershov, Oleg A.; Marsh, Jasmina P.; Allers, K. N.; Jaffe, Daniel T., IR Space Telescopes and Instruments. Edited by John C. Mather. Proceedings of the SPIE, Volume 4850, pp. 805-812 (2003). [ pdf ]

Silicon grisms and immersion gratings produced by anisotropic etching: testing and analysis

Marsh, Jasmina P.; Ershov, Oleg A.; Jaffe, Daniel T., IR Space Telescopes and Instruments. Edited by John C. Mather. Proceedings of the SPIE, Volume 4850, pp. 797-804 (2003). [ pdf ]

Production of high-order micromachined silicon echelles on optically flat substrates

Ershov, Oleg A.; Jaffe, Daniel T.; Marsh, Jasmina P.; Keller, Luke D., Proc. SPIE Vol. 4440, p. 301-308, Lithographic and Micromachining Techniques for Optical Component Fabrication, Ernst-Bernhard Kley; Hans-Peter Herzig; Eds. [ pdf ]

Large-area silicon immersion echelle gratings and grisms for IR spectroscopy

Keller, Luke D.; Jaffe, Daniel T.; Ershov, Oleg A.; Marsh, Jasmina P., Proc. SPIE Vol. 4485, p. 385-392, Optical Spectroscopic Techniques, Remote Sensing, and Instrumentation for Atmospheric and Space Research IV, Allen M. Larar; Martin G. Mlynczak; Eds. [ pdf ]

Dan Jaffe, Cindy Brooks, Michael Gully-Santiago