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Published June 5, 2023 | public
Journal Article

Plastic Laminate Antireflective Coatings for Millimeter-Wave Optics in BICEP Array


The BICEP/Keck series of experiments target the cosmic microwave background at degree-scale resolution from the South Pole. Over the next few years, the "Stage-3" BICEP Array (BA) telescope will improve the program's frequency coverage and sensitivity to primordial B-mode polarization by an order of magnitude. The first receiver in the array, BA1, began observing at 30/40 GHz in early 2020. The next two receivers, BA2 and BA3, are currently being assembled and will map the southern sky at frequencies ranging from 95 to 150 GHz. Common to all BA receivers is a refractive, on-axis, cryogenic optical design that focuses microwave radiation onto a focal plane populated with antenna-coupled bolometers. High-performance antireflective coatings up to 760 mm in aperture are needed for each element in the optical chain, and must withstand repeated thermal cycles down to 4 K. Here, we present the design and fabrication of the 30/40 GHz anti-reflection coatings for the recently deployed BA1 receiver, with indices matched to its various polyethylene, nylon and alumina optical components. We describe an epoxy coating technique designed for alumina optics, which achieves better than 80% transmission at room temperature. For polyethylene optical elements, we present a new heat-compression approach that allows low-density polytetrafluoroethylene AR layers to reach sub-percent reflected power. We describe the planned use of these methods for the next BA cryostats, which may inform technological choices for future small-aperture telescopes of the CMB-S4 experiment.

Additional Information

© 2023 Springer Nature. The BICEP/Keck projects have been made possible through a series of grants from the National Science Foundation including Grants No. 0742818, No. 0742592, No. 1044978, No. 1110087, No. 1145172, No. 1145143, No. 1145248, No. 1639040, No. 1638957, No. 1638978 and No. 1638970, and by the Keck Foundation. The development of antenna-coupled detector technology was supported by the JPL Research and Technology Development Fund, and by NASA Grants No. 06-ARPA206-0040, No. 10-SAT10-0017, No. 12- SAT12-0031, No. 14-SAT14-0009 and No. 16-SAT-16- 0002. The development and testing of focal planes were supported by the Gordon and Betty Moore Foundation at Caltech. Readout electronics were supported by a Canada Foundation for Innovation grant to UBC. Support for quasioptical filtering was provided by UK STFC Grant No. ST/N000706/1. The computations in this work were run on the Odyssey/Cannon cluster supported by the FAS Science Division Research Computing Group at Harvard University. The analysis effort at Stanford and S. L. A. C. is partially supported by the U.S. DOE Office of Science. We thank the staff of the U.S. Antarctic Program and in particular the South Pole Station without whose help this research would not have been possible. We thank all those who have contributed past efforts to the BICEP/Keck series of experiments, including the BICEP1 team.

Additional details

August 22, 2023
October 20, 2023