Demonstration of 220/280 GHz Multichroic Feedhorn-Coupled TES Polarimeter

TL;DR

This paper presents the design, measurement, and validation of a multichroic feedhorn-coupled TES polarimeter operating at 220 and 280 GHz, extending previous technology to higher frequencies for cosmic microwave background observations.

Contribution

The work demonstrates the operation of feedhorn-coupled TES polarimeters at frequencies up to 315 GHz, including design validation and comparison of power-termination schemes.

Findings

Achieved optical efficiency of approximately 77% at 220 GHz and 75% at 280 GHz.

Measured passbands closely match simulations, confirming design accuracy.

Found equivalent performance between different power-termination schemes.

Abstract

We describe the design and measurement of feedhorn-coupled, transition-edge sensor (TES) polarimeters with two passbands centered at 220 GHz and 280 GHz, intended for observations of the cosmic microwave background. Each pixel couples polarized light in two linear polarizations by use of a planar orthomode transducer and senses power via four TES bolometers, one for each band in each linear polarization. Previous designs of this detector architecture incorporated passbands from 27 GHz to 220 GHz; we now demonstrate this technology at frequencies up to 315 GHz. Observational passbands are defined with an on-chip diplexer, and Fourier-transform-spectrometer measurements are in excellent agreement with simulations. We find coupling from feedhorn to TES bolometer using a cryogenic, temperature-controlled thermal source. We determine the optical efficiency of our device is $\eta$ = 77%$\pm$6% (75%$\pm$5%) for 220 (280) GHz, relative to the designed passband shapes. Lastly, we compare two power-termination schemes commonly used in wide-bandwidth millimeter-wave polarimeters and find equal performance in terms of optical efficiency and passband shape.