Two-layer anti-reflection coating with mullite and polyimide foam for large-diameter cryogenic infrared filters

TL;DR

This paper introduces a novel two-layer anti-reflection coating for large-diameter alumina IR filters, using thermally-sprayed mullite and tunable polyimide foam, suitable for cryogenic millimeter wave measurements in CMB experiments.

Contribution

The study presents a new AR coating method with large-diameter alumina filters using mullite and Skybond Foam, optimized for cryogenic millimeter wave applications.

Findings

Successful fabrication of large-diameter alumina IR filters with AR coating.

Demonstrated effective IR cutoff and cryogenic performance.

Potential application in next-generation CMB polarization experiments.

Abstract

We have developed a novel two-layer anti-reflection (AR) coating method for large-diameter infrared (IR) filters made of alumina, for the use at cryogenic temperatures in millimeter wave measurements. Thermally- sprayed mullite and polyimide foam (Skybond Foam) are used as the AR material. An advantage of the Skybond Foam is that the index of refraction is chosen between 1.1 and 1.7 by changing the filling factor. Combination with mullite is suitable for wide-band millimeter wave measurements with sufficient IR cutoff capability. We present the material properties, fabrication of a large-diameter IR filter made of alumina with this AR coating method, and characterizations at cryogenic temperatures. This technology can be applied to a low-temperature receiver system with a large-diameter focal plane for next-generation cosmic microwave background (CMB) polarization measurements, such as POLARBEAR-2 (PB-2).