POLARBEAR-2

pb2receiver — Yasuto Hori (top) and Yuki Inoue (bottom) standing beside the POLARBEAR-2 receiver cryostat at KEK in Japan in 2013. Photo credit: Center for Human Imagination at UCSD

POLARBEAR-2 (PB2) is a Stage-3 CMB experiment that was deployed at the Chajnantor Observatory in late 2018. PB2 contains more than 7,000 polarization-sensitive detectors that observe the sky at 90 GHz and 150 GHz simultaneously on a PB1-style telescope. PB2 is led by Adrian Lee at UC Berkeley, Brian Keating at UC San Diego, and Masashi Hazumi at the High Energy Accelerator Research Organization (KEK) in Japan.

pb2figure — A rapid-fire overview of the POLARBEAR-2 hardware. **Top right**: PB2 receiver cryostat, which is responsible for cooling the experiment and guiding the CMB light to the detectors. Note that the PB2 receiver is significantly larger than the PB1 receiver. **Top left**: 3D rendering of the PB2 detector array, which contains more than 7,000 detectors. **Bottom left**: Microscope photograph of the PB2 detector pixel, which contains a broadband, polarization-sensitive Sinuous antenna coupled to four transition-edge sensor (TES) bolometer detectors. **Bottom right**: PB2 telescope under construction in 2016 at the Chajnantor Plateau site. Photo credit: Tomotake Matsumura, Aritoki Suzuki, and Nate Stebor

PB2 is projected to be five times more sensitive than PB1, making it well suited to measure B-modes due to both gravitational distortions by large-scale structure and B-modes due to gravitational waves generated during inflation.

pb2awhwp — A quick snapshot of the PB2 ambient-temperature half-wave plate hardware. The polarization modulator spins at 2 Hz directly in front of the window of the receiver cryostat. Photo credit: Shawn Beckman

My contribution to POLARBEAR-2 has primarily been designing, building, and testing its ambient-temperature continuously-rotating half-wave plate polarization modulator. Specifically, I developed the optical design which utilizes a stack of 20″-diameter sapphire wafers and a two-layer plastic anti-reflection coating to achieve outstanding efficiency across both PB2 frequency channels. In addition to half-wave plates, I have performed instrument modeling, detector evaluation, and optical characterization for PB2.

For more information on PB2, visit the POLARBEAR website!