The Simons Observatory: metamaterial microwave absorber and its cryogenic applications

Area of Science:

• Optical Engineering
• Materials Science
• Cryogenics

Background:

• Controlling stray light at millimeter wavelengths is critical for sensitive instruments operating in cryogenic environments.
• Existing absorptive materials often have high refractive indices, leading to significant light reflection and scattering before absorption.
• Commercial microwave absorbers are generally unsuitable for cryogenic applications due to material limitations.

Purpose of the Study:

• To introduce a novel solution for stray light control using metamaterial microwave absorber tiles.
• To demonstrate the effectiveness of these tiles in cryogenic conditions and assess their optical performance.

Main Methods:

• Development of metamaterial tiles featuring a gradient index anti-reflection coating.
• Fabrication using injection molding of carbon-loaded polyurethane (25% by mass).
• Thermal testing down to 1 Kelvin and optical measurements at room temperature.

Main Results:

• Metamaterial tiles achieved reflectance below 1% up to 65° angles of incidence.
• Wide-angle scattering was controlled to below 0.01%.
• The carbon-loaded material exhibited stable dielectric properties down to 3 Kelvin.

Conclusions:

• Metamaterial microwave absorber tiles are a viable and cost-effective solution for stray light control in millimeter-wave optics.
• The injection molding process allows for scalable, low-cost mass production.
• The demonstrated performance and cryogenic compatibility make these tiles suitable for demanding applications.