The POLARBEAR Cosmic Microwave Background Polarization Experiment and Anti-Reflection Coatings for Millimeter Wave Observations
New technology has rapidly advanced the field of observational cosmology over the last 30 years. This trend will continue with the development of technologies to measure the Cosmic Microwave Background (CMB) polarization. The B-mode component of the polarization map will place limits on the energy scale of inflation and the sum of the neutrino masses. This thesis describes the \pb instrument which will measure the CMB polarization anisotropy to unprecedented sensitivity. POLARBEAR-I is currently observing, and an upgraded version, POLARBEAR-II, is planned for the future.
The first version of the experiment, POLARBEAR-I, is fielding several new technologies for the first time. POLARBEAR-I has high sensitivity due to its detector count. It employs a 1274 detector Transition-Edge Sensor (TES) bolometer array. The bolometers are coupled to a planar array of polarization sensitive antennas. These antennas are lithographed on the same substrate as the TES detectors, allowing on-chip band defining filters between the antenna and detector. The focal plane is composed of seven hexagonal detector modules. This modular scheme can be extended to create larger focal plane arrays in the future. POLARBEAR-I is observing at a single band near 150 GHz, the peak in the CMB blackbody curve.
The lenslet antenna coupled detector technology, fielding for the first time in POLARBEAR-I, is naturally scalable to larger arrays with multi-chroic pixels. This broadband technology will have higher sensitivity and better capability for astronomical foreground contaminant removal. The antenna geometry can be changed to receive a wider frequency bandwidth. This bandwidth can be broken into multiple frequency bands with the on-chip band defining filters. Each band will be read out by one TES detector. A dual band instrument, \pbtwo, is in development with bands at 90 and 150 GHz.
One challenge for all CMB polarization measurements is minimization of systematic errors. One source of error is polarized reflections off of the refractive optics inside the receiver. Specifically, the antenna-coupled detector scheme relies on a high dielectric lenslet for each pixel on the focal plane. A large portion of this thesis discusses development of anti-reflection (AR) coatings for the high curvature lenslet surface. The AR coating technologies discussed are also applicable to other optical elements, such as reimaging lenses and half-wave plates. A single layer coating is used on the \pbone lenslet array, and a two layer coating is presented for use in \pbtwo. The two layer coating method can be extended to wider bandwidth AR coatings.