Design and characterization of the Cosmology Large Angular Scale Surveyor (CLASS) 93 GHz focal plane

TL;DR

This paper details the design, fabrication, and characterization of the first 93 GHz detector array for the CLASS telescope, demonstrating photon-noise limited performance and high array yield for cosmological polarization measurements.

Contribution

It introduces a novel 93 GHz detector array with optimized silicon wafer fabrication, providing detailed characterization and performance metrics for cosmological observations.

Findings

Detectors are photon-noise limited.

Array yield is 82%.

Total NEP is 2.1 aW/√s.

Abstract

The Cosmology Large Angular Scale Surveyor (CLASS) aims to detect and characterize the primordial B-mode signal and make a sample-variance-limited measurement of the optical depth to reionization. CLASS is a ground-based, multi-frequency microwave polarimeter that surveys 70% of the microwave sky every day from the Atacama Desert. The focal plane detector arrays of all CLASS telescopes contain smooth-walled feedhorns that couple to transition-edge sensor (TES) bolometers through symmetric planar orthomode transducer (OMT) antennas. These low noise polarization-sensitive detector arrays are fabricated on mono-crystalline silicon wafers to maintain TES uniformity and optimize optical efficiency throughout the wafer. In this paper, we discuss the design and characterization of the first CLASS 93 GHz detector array. We measure the dark parameters, bandpass, and noise spectra of the detectors and report that the detectors are photon-noise limited. With current array yield of 82%, we estimate the total array noise-equivalent power (NEP) to be 2.1 aW$\sqrt[]{\mathrm{s}}$.