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 South Pole. Over next few years, "Stage-3" BICEP Array (BA) telescope will improve program's frequency coverage and sensitivity to primordial B-mode polarization by an order magnitude. first receiver in array, BA1, began observing 30/40 GHz early 2020. two receivers, BA2 BA3, are currently being assembled map southern sky frequencies ranging 95 150 GHz. Common all BA receivers is a refractive, on-axis, cryogenic optical design that focuses microwave radiation onto focal plane populated with antenna-coupled bolometers. High-performance antireflective coatings up 760 mm aperture needed for each element chain, must withstand repeated thermal cycles down 4 K. Here we present fabrication anti-reflection recently deployed BA1 receiver, then discuss laboratory measurements their reflectance. We review lamination method these single- dual-layer plastic indices matched various polyethylene, nylon alumina optics. also describe ongoing efforts optimize cryostats, which may inform technological choices future Small-Aperture Telescopes CMB "Stage 4" experiment.