UC Berkeley

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.