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23 July 2014 Antireflection coatings for submillimeter silicon lenses
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Low-loss lenses are required for submillimeter astronomical applications, such as instrumentation for CCAT, a 25 m diameter telescope to be built at an elevation of 18,400 ft in Chile. Silicon is a leading candidate for dielectric lenses due to its low transmission loss and high index of refraction; however, the latter can lead to large reflection losses. Additionally, large diameter lenses (up to 40 cm), with substantial curvature present a challenge for fabrication of antireflection coatings. Three anti-reflection coatings are considered: a deposited dielectric coating of Parylene C, fine mesh structures cut with a dicing saw, and thin etched silicon layers (fabricated with deep reactive ion etching) for bonding to lenses. Modeling, laboratory measurements, and practicalities of fabrication for the three coatings are presented and compared. Measurements of the Parylene C anti-reflection coating were found to be consistent with previous studies and can be expected to result in a 6% transmission loss for each interface from 0.787 to 0.908 THz. The thin etched silicon layers and fine mesh structure anti-reflection coatings were designed and fabricated on test silicon wafers and found to have reflection losses less than 1% at each interface from 0.787 to 0.908 THz. The thin etched silicon layers are our preferred method because of high transmission efficiency while having an intrinsically faster fabrication time than fine structures cut with dicing saws, though much work remains to adapt the etched approach to curved surfaces and optics < 4" in diameter unlike the diced coatings.
© (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Jordan D. Wheeler, Brian Koopman, Patricio Gallardo, Philip R. Maloney, Spencer Brugger, German Cortes-Medellin, Rahul Datta, C. Darren Dowell, Jason Glenn, Sunil Golwala, Chris McKenney, Jeffery J. McMahon, Charles D. Munson, Mike Niemack, Stephen Parshley, and Gordon Stacey "Antireflection coatings for submillimeter silicon lenses", Proc. SPIE 9153, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VII, 91532Z (23 July 2014);

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