Design of 280 GHz feedhorn-coupled TES arrays for the balloon-borne polarimeter SPIDER

TL;DR

This paper details the design, fabrication, and testing of 280 GHz feedhorn-coupled TES detector arrays for the SPIDER balloon-borne polarimeter, aimed at measuring cosmic microwave background polarization.

Contribution

It introduces a novel 280 GHz focal plane array with high efficiency and low noise, optimized for large-scale CMB polarization measurements in a balloon-borne experiment.

Findings

Achieved 14.7° Gaussian beam with <1% ellipticity

Demonstrated 94% coupling efficiency in simulations

Measured bolometers with NEP of 2.6×10⁻¹⁷ W/√Hz

Abstract

We describe 280 GHz bolometric detector arrays that instrument the balloon-borne polarimeter SPIDER. A primary science goal of SPIDER is to measure the large-scale B-mode polarization of the cosmic microwave background in search of the cosmic-inflation, gravitational-wave signature. 280 GHz channels aid this science goal by constraining the level of B-mode contamination from galactic dust emission. We present the focal plane unit design, which consists of a 16$\times$16 array of conical, corrugated feedhorns coupled to a monolithic detector array fabricated on a 150 mm diameter silicon wafer. Detector arrays are capable of polarimetric sensing via waveguide probe-coupling to a multiplexed array of transition-edge-sensor (TES) bolometers. The SPIDER receiver has three focal plane units at 280 GHz, which in total contains 765 spatial pixels and 1,530 polarization sensitive bolometers. By fabrication and measurement of single feedhorns, we demonstrate 14.7$^{\circ}$ FHWM Gaussian-shaped beams with $<$1% ellipticity in a 30% fractional bandwidth centered at 280 GHz. We present electromagnetic simulations of the detection circuit, which show 94% band-averaged, single-polarization coupling efficiency, 3% reflection and 3% radiative loss. Lastly, we demonstrate a low thermal conductance bolometer, which is well-described by a simple TES model and exhibits an electrical noise equivalent power (NEP) = 2.6 $\times$ 10$^{-17}$ W/$\sqrt{\mathrm{Hz}}$, consistent with the phonon noise prediction.