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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 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.

click to enlarge

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.

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.		click to enlarge
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.

The silicon diffractive optics we are producing will some day find homes in ground-based, airborne, and space-based infrared spectrometers. We are currently working to improve the already very good quality of the devices and to increase the maximum size we are capable of producing.

click to enlarge		Silicon grating etched into the surface of a thick disk. This photo was made by imaging the grating in "Littrow", that is, taking a picture in the reflection off of the groove surfaces which form a 54.7 degree angle with top of the disk.
		Diffraction spectrum of a silicon grating in green light. The sharp spikes show the response of the grating to green light in 424th-429th order. The smooth curve shows the predicted relative response for perfect, diffraction-limited grooves.
		Completed silicon immersion grating. The light enters by the polished face on the left, strikes the grooved surface (along the hypotenuse of the prism) from the inside, and then re-emerges from the flat front surface.

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

ADS

Micromachined silicon grisms for infrared optics
Mar, D. J.; Marsh, J. P.; Jaffe, D. T., Applied Optics, submitted (2007). [ pdf ]

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, Jasmina Marsh, Doug Mar

24 September 2007
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