Aerogel scattering filters for cosmic microwave background observations

Abstract

We present the design and performance of broadband and tunable infrared-blocking filters for millimeter and submillimeter astronomy composed of small scattering particles embedded in an aerogel substrate. The ultralow-density (typically &lt;<!-- 150 m g / c m 3 ) aerogel substrate provides an index of refraction as low as 1.05, removing the need for antireflection 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.

Characterization of aerogel scattering filters for astronomical telescopes

“…* By varying the scattering particle size and density relative to the base aerogel, we tuned the optical performance of the filters. While our group's previous work had investigated using silicon scattering particles, 12 we chose to use diamond for this work due to greater commercial availability of diamond particles with known size distributions. The optical properties of diamond are also well-known, most notably low loss in the far-infrared and millimeter spectral ranges.…”

Experiment for cryogenic large-aperture intensity mapping: instrument design

“…Two polyimide aerogel filters loaded with diamond scattering particles act as low-pass filters with a cutoff of ∼1 THz. 74 High-and low-pass heat-pressed metal-mesh filters 75 define the band. Each detector in the spectrometer is sensitive to a bandwidth Δν ≈ 0.9 GHz, giving it a coherence length of c∕Δν ¼ 33 cm.…”