The Simons Observatory: Metamaterial Microwave Absorber (MMA) and its Cryogenic Applications

TL;DR

This paper introduces metamaterial microwave absorber tiles designed for cryogenic environments, effectively reducing stray light at millimeter wavelengths with low reflectance and scattering, suitable for applications like the Simons Observatory.

Contribution

The paper presents a novel metamaterial absorber tile fabricated via injection molding with low-cost, scalable production, and demonstrates its effectiveness and thermal stability down to 1 K.

Findings

Reflectance controlled to less than 1% up to 65° incidence

Wide-angle scattering below 0.01%

Dielectric properties stable down to 3 K

Abstract

Controlling stray light at millimeter wavelengths requires special optical design and selection of absorptive materials that should be compatible with cryogenic operating environments. While a wide selection of absorptive materials exists, these typically exhibit high indices of refraction and reflect/scatter a significant fraction of light before absorption. For many lower index materials such as commercial microwave absorbers, their applications in cryogenic environments are challenging. In this paper, we present a new tool to control stray light: metamaterial microwave absorber tiles. These tiles comprise an outer metamaterial layer that approximates a lossy gradient index anti-reflection coating. They are fabricated via injection molding commercially available carbon-loaded polyurethane (25\% by mass). The injection molding technology enables mass production at low cost. The design of these tiles is presented, along with thermal tests to 1 K. Room temperature optical measurements verify their control of reflectance to less than 1\% up to 65$\circ$ angles of incidence, and control of wide angle scattering below 0.01\%. The dielectric properties of the bulk carbon-loaded material used in the tiles is also measured at different temperatures, confirming that the material maintains similar dielectric properties down to 3 K.