Developing Polarization Sensitive Detectors for Cosmic Microwave Background Research

The polarization anisotropies in the Cosmic Microwave Background (CMB) are predicted to contain remnant signals of inflationary gravitational waves that hold valuable information about inflation and the early universe. Because these signals are extremely faint, a large developmental effort to fabricate large, uniform, polarization sensitive arrays is necessary. Additionally, detectors and instruments as well as their systematic errors must be well understood and characterized to reach the level of sensitivity necessary to measure the faint CMB polarization signals. The Atacama B-Mode Search (ABS) and the upgrade to the Atacama Cosmology Telescope Polarization receiver (Advanced ACTPol) are currently developing and fielding arrays with hundreds to thousands of state of the art CMB detectors. Significant effort in developing, testing, fielding, and characterizing these detectors is necessary to achieve the scientific goal of precisely measuring the B-mode signal. ABS observed for three seasons from February 2012 to October 2014 with 480 polarization-coupled bolometers designed for observation at 145 GHz. Advanced ACTPol (AdvACT) will upgrade ACTPol with new multichroic arrays to cover five frequency bands ranging from 28-230 GHz, and fielded its first array in the Summer of 2016. My research focused on (1) thoroughly characterizing the ABS instrument and the installed polarization-coupled detectors (which are the predecessors to the AdvACT detectors) through analysis of three seasons of data, (2) analyzing the performance and uniformity of existing detectors to give feedback to the fabrication process, (3) developing cutting-edge instrumentation for AdvACT, and (4) testing state of the art prototypes of future detectors, including multichroic detectors for targeted use in AdvACT. ABS pioneered the use of a continuously-rotating half-wave plate (HWP) for ground-based CMB experiments. The ABS HWP is a birefringent sapphire plate that flips incoming polarization about the fast axis. When the HWP is rotated at frequency f, this results in polarization modulation at 2f, which is detected by the bolometers at 4f. The HWP modulation scheme suppresses 1/f noise, which arises primarily from the atmosphere but can also arise from focal plane temperature and readout electronics. Through analyzing ABS data, I developed several novel techniques for instrument characterization and data selection using the HWP, which will be of great importance for the next generation of instruments like AdvACT and the Simons Array since they now plan to use HWPs after ABS’s demonstration of their effectiveness. The characterization of the ABS instrument and detectors also provided vital input for future detector development. I also designed, modeled, and tested state of the art wideband spline-profiled feedhorns for AdvACT. The first array of these feedhorns was successfully deployed with the array this summer (2016), and the next two arrays are in fabrication. Additionally, I tested the optical properties of high-frequency (220/350 GHz) prototype detectors at NIST. While AdvACT changed the design bands after these tests, they provided valuable input for the design of the final AdvACT pixels.