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Broadband anti-reflective coatings for cosmic microwave background experiments

Conference · · Proc.SPIE Int.Soc.Opt.Eng.
DOI:https://doi.org/10.1117/12.2315674· OSTI ID:1478058

The desire for higher sensitivity has driven ground-based cosmic microwave background (CMB) experiments to employ ever larger focal planes, which in turn require larger reimaging optics. Practical limits to the maximum size of these optics motivates the development of quasi-optically-coupled (lenslet-coupled), multi-chroic detectors. These detectors can be sensitive across a broader bandwidth compared to waveguide-coupled detectors. However, the increase in bandwidth comes at a cost: the lenses (up to ~700 mm diameter) and lenslets (~5 mm diameter, hemispherical lenses on the focal plane) used in these systems are made from high-refractive-index materials (such as silicon or amorphous aluminum oxide) that reflect nearly a third of the incident radiation. In order to maximize the faint CMB signal that reaches the detectors, the lenses and lenslets must be coated with an anti-reflective (AR) material. The AR coating must maximize radiation transmission in scientifically interesting bands and be cryogenically stable. Such a coating was developed for the third generation camera, SPT-3G, of the South Pole Telescope (SPT) experiment, but the materials and techniques used in the development are general to AR coatings for mm-wave optics. The three-layer polytetra uoroethylene-based AR coating is broadband, inexpensive, and can be manufactured with simple tools. The coating is field tested; AR coated focal plane elements were deployed in the 2016-2017 austral summer and AR coated reimaging optics were deployed in 2017-2018.

Research Organization:
Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States); SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States); Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States)
Sponsoring Organization:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
Contributing Organization:
SPT
DOE Contract Number:
AC02-07CH11359
OSTI ID:
1478058
Report Number(s):
arXiv:1809.00030; FERMILAB-CONF-18-447-AE; 1682457
Journal Information:
Proc.SPIE Int.Soc.Opt.Eng., Journal Name: Proc.SPIE Int.Soc.Opt.Eng. Vol. 10708
Country of Publication:
United States
Language:
English

References (10)

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Fabrication of large dual-polarized multichroic TES bolometer arrays for CMB measurements with the SPT-3G camera journal August 2015
BICEP: a large angular scale CMB polarimeter conference February 2003
SPTpol: an instrument for CMB polarization measurements with the South Pole Telescope conference September 2012
CMB-S4 Science Book, First Edition report October 2016
CMB temperature and polarization anisotropy fundamentals journal January 2003
Commercialization of Micro-fabrication of Antenna-Coupled Transition Edge Sensor Bolometer Detectors for Studies of the Cosmic Microwave Background journal April 2018
ACTPol: a polarization-sensitive receiver for the Atacama Cosmology Telescope conference July 2010
Method for Optimized Design of Dielectric Multilayer Filters journal January 1974
Primordial density fluctuations and the microwave background spectrum journal January 1991

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