Cosmology Large Angular Scale Surveyor (CLASS) Focal Plane Development

TL;DR

The CLASS project aims to measure the polarization of the Cosmic Microwave Background across large angular scales to detect inflationary signatures, utilizing advanced polarization modulation and multi-frequency detectors from the Atacama Desert.

Contribution

This paper details the development of the CLASS focal plane with innovative polarization modulation, multi-frequency detectors, and integrated band definition for precise CMB polarization measurements.

Findings

Successful implementation of VPM for polarization modulation.

Multi-frequency focal planes covering 38-217 GHz.

Enhanced detector uniformity and efficiency using single-crystal silicon.

Abstract

The Cosmology Large Angular Scale Surveyor (CLASS) will measure the polarization of the Cosmic Microwave Background to search for and characterize the polarized signature of inflation. CLASS will operate from the Atacama Desert and observe $\sim$70% of the sky. A variable-delay polarization modulator (VPM) modulates the polarization at $\sim$10 Hz to suppress the 1/f noise of the atmosphere and enable the measurement of the large angular scale polarization modes. The measurement of the inflationary signal across angular scales that span both the recombination and reionization features allows a test of the predicted shape of the polarized angular power spectra in addition to a measurement of the energy scale of inflation. CLASS is an array of telescopes covering frequencies of 38, 93, 148, and 217 GHz. These frequencies straddle the foreground minimum and thus allow the extraction of foregrounds from the primordial signal. Each focal plane contains feedhorn-coupled transition-edge sensors that simultaneously detect two orthogonal linear polarizations. The use of single-crystal silicon as the dielectric for the on-chip transmission lines enables both high efficiency and uniformity in fabrication. Integrated band definition has been implemented that both controls the bandpass of the single mode transmission on the chip and prevents stray light from coupling to the detectors.