Broadband anti-reflective coatings for cosmic microwave background experiments

Abstract

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 $\sim$700 mm diameter) and lenslets ($\sim$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 polytetrafluoroethylene-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.

arXiv comment13 pages, 5 figures

References in corpus (5)

• A Flat Universe from High-Resolution Maps of the Cosmic Microwave Background Radiation
• ACTPol: A polarization-sensitive receiver for the Atacama Cosmology Telescope
• SPTpol: an instrument for CMB polarization measurements with the South Pole Telescope
• CMB Temperature and Polarization Anisotropy Fundamentals
• Commercialization of micro-fabrication of antenna-coupled Transition Edge Sensor bolometer detectors for studies of the Cosmic Microwave Background

Cited by in corpus (7)

• The Simons Observatory Large Aperture Telescope Receiver
• Design and characterization of the SPT-3G receiver
• Broadband, millimeter-wave antireflection coatings for large-format, cryogenic aluminum oxide optics
• Anti-reflection Coated Vacuum Window for the Primordial Inflation Polarization ExploreR (PIPER) balloon-borne instrument
• Characterization and performance of the second-year SPT-3G focal plane
• Anti-reflection coating with mullite and Duroid for large-diameter cryogenic sapphire and alumina optics
• An Ultra-Wideband Leaky Lens Antenna for Broadband Spectroscopic Imaging Applications