We present the design and performance of broadband and tunable infrared-blocking filters for millimeter and sub-millimeter astronomy composed of small scattering particles embedded in an aerogel substrate. The ultra-low-density (typically < 150 mg/cm^3) aerogel substrate provides an index of refraction as low as 1.05, removing the need for anti-reflection coatings and allowing for broadband operation from DC to above 1 THz. The size distribution of the scattering particles can be tuned to provide a variable cutoff frequency. Aerogel filters with embedded high-resistivity silicon powder are being produced at 40-cm diameter to enable large-aperture cryogenic receivers for cosmic microwave background polarimeters, which require large arrays of sub-Kelvin detectors in their search for the signature of an inflationary gravitational-wave background.
@article{arxiv.1807.04194,
title = {Aerogel scattering filters for cosmic microwave background observations},
author = {Thomas Essinger-Hileman and Charles L. Bennett and Lance Corbett and Haiquan Guo and Kyle Helson and Tobias Marriage and Mary Ann B. Meador and Karwan Rostem and Edward J. Wollack},
journal= {arXiv preprint arXiv:1807.04194},
year = {2021}
}