Advancements in Microwave Optics for Measuring Polarization Anisotropies in the Cosmic Microwave Background

Abstract

The temperature and polarization anisotropies in the Cosmic Microwave Background (CMB) are direct probes into the physics of the early universe. Increasingly sensitive experiments aim to determine the tensor-to-scalar ratio r through measurement of an impossibly faint B-mode polarized signal shrouded by galactic foregrounds. A direct measurement of primordial B-mode polarization will be a measurement of the energy scale of inflation, unlocking an essential piece of the cosmological puzzle. Next-generation CMB experiments employ a large number of highly sensitive detectors in an attempt to find r and further constrain the cosmological parameters. Such a measurement requires not just high sensitivity to the CMB polarized signal, but large experimental bandwidth to characterize the polarized galactic dust and synchrotron radiation foreground signals.

For experiments using lenslet-coupled planar antenna detector array designs, reflection off the surface of the lenslet must be minimized over a given bandwidth to maximize the measured CMB signal. To this end, antireflection (AR) coatings for lenslets were developed for 30/40 GHz Simons Observatory low-frequency detectors, along with next-generation prototype coatings for 90/150 and 220/270 GHz arrays. The JAXA-led space-based mission LiteBIRD will utilize lenslet-coupled sinuous antenna arrays and TES bolometers for frequencies ranging from 40-195 GHz, necessitating broadband lenslet AR coatings that are robust to launch vibrations and differential thermal contraction. To meet these requirements, a metamaterial AR surface has been proposed. A metamaterial coating designed for the LiteBIRD LF-3 band has been laser etched onto a flat surface, achieving 98% in-band transmission. A six-axis positioning system is used to etch the metamaterial pattern onto a sphere, and a completed prototype LF-3 lenslet is expected to be etched in late 2023. Details of the metamaterial design and the etch process are discussed. Cosmic rays at the Lagrange point L2 pose a threat to LiteBIRD’s sensitivity, as they produce a white noise component that cannot be fully deconstructed in analysis. To mitigate this cosmic ray white noise component, on-chip mitigations have been developed for the purpose of minimizing thermal diffusion from the silicon detector wafer to the TES bolometer detectors. Lastly, the mechanical design and fabrication of a continuously rotating warm half-wave plate for the POLARBEAR-2a experiment, used to minimize noise in large-angular-scale measurements from atmospheric fluctuations, are discussed.