A Polarization Sensitive Bolometric Detector for Observations of the Cosmic Microwave Background

TL;DR

This paper introduces a polarization-sensitive bolometric detector optimized for cosmic microwave background measurements, featuring a novel design that minimizes systematic errors and achieves background-limited sensitivity across a broad frequency range.

Contribution

The paper presents a new bolometric detector design with intrinsic polarization sensitivity, optimized for CMB polarization measurements, and demonstrates its performance and integration into major experiments.

Findings

Achieved NEPs of 2 x 10^{-17} W/√Hz with low 1/f noise

Designed a detector with minimized systematic effects for polarization measurements

Validated numerical models with experimental data

Abstract

We have developed a bolometric detector that is intrinsically sensitive to linear polarization which is optimized for making measurements of the polarization of the cosmic microwave background radiation. The receiver consists of a pair of co-located silicon nitride micromesh absorbers which couple anisotropically to linearly polarized radiation through a corrugated waveguide structure. This system allows simultaneous background limited measurements of the Stokes I and Q parameters over ~ 30% bandwidths at frequencies from ~ 60 to 600 GHz. Since both linear polarizations traverse identical optical paths from the sky to the point of detection, the susceptibility to systematic effects is minimized. The amount of uncorrelated noise between the two polarization senses is limited to the quantum limit of thermal and photon shot noise, while drifts in the relative responsivity to orthogonal polarizations are limited to the effect of non-uniformity in the thin film deposition of the leads and the intrinsic thermistor properties. Devices using NTD Ge thermistors have achieved NEPs of 2 x 10^{-17} W/sqrt{Hz} with a 1/f knee below 100 mHz at a base temperature of 270 mK. Numerical modelling of the structures has been used to optimize the bolometer geometry and coupling to optics. Comparisons of numerical results and experimental data are made. A description of how the quantities measured by the device can be interpreted in terms of the Stokes parameters is presented. The receiver developed for the Boomerang and Planck HFI focal planes is presented in detail.