Optical Designs and Antireflection Coatings for CMB Instruments

Abstract

The cosmic microwave background (CMB) is one of the richest observables in cosmology. The B-mode polarization pattern is of particular interest as a direct probe of inflation. CMB polarization measurements provide additional cosmological information, such as the reionization optical depth, as well as particle physics information, such asthe sum of neutrino masses. Measuring the CMB polarization requires sensitive and stable mm-wave polarimeters. I worked to improve the optical design, antireflection coatings, and bolometers used in these polarimeters. I designed and analyzed Gregorian, Crossed, and Open Dragone CMB telescopes as a member of the EBEX, CORE, and PICO projects. The Open Dragone I developed for PICO is the first use of this optical system in a CMB instrument design. It provides a similar field of view to the Crossed Dragone used in CORE, while avoiding the far sidelobe issues. To facilitate the design process, I developed a white noise only, end-to-end sensitivity model for CMB instruments. This model includes correlated Bose photon noise between tightly packed pixels and the effects of multichroic pixels. I also developed prototype antireflection coatings on silicon, sapphire, and alumina using laser machining and dicing saws to produce sub-wavelength structures. These prototype coatings are cryogenically robust, low loss, have broad bandwidth (∆ν/νC = 55% on alumina), and illustrate a path to coatings on large, > 10 cm, lenses with ∆ν/νC ≈ 150%. Finally, as part of the EBEX project, I measured the optical time constants of flight bolometers and the optical efficiency of new, low thermal conductance, G = 9 pW/K, bolometers developed for future balloon-borne or space-based CMB instruments. Overall, my work has contributed to the newest generation of high sensitivity CMB polarimeters which will probe the earliest moments and fundamental properties of the Universe.