Broadband, millimeter-wave antireflection coatings for large-format, cryogenic aluminum oxide optics

TL;DR

This paper develops and tests broadband millimeter-wave antireflection coatings for cryogenic aluminum oxide optics, suitable for large lenses and dense arrays, improving transmittance at 150 GHz.

Contribution

It introduces two practical coating prescriptions for large and array optics, including fabrication methods and laboratory performance measurements.

Findings

Large-format coating achieves 97% transmittance at 150 GHz.

Lenslet coating achieves 94% transmittance at 150 GHz.

Coatings are effective for cryogenic millimeter-wave applications.

Abstract

We present two prescriptions for broadband (~77 - 252 GHz), millimeter-wave antireflection coatings for cryogenic, sintered polycrystalline aluminum oxide optics: one for large-format (700 mm diameter) planar and plano-convex elements, the other for densely packed arrays of quasi-optical elements, in our case 5 mm diameter half-spheres (called "lenslets"). The coatings comprise three layers of commercially-available, polytetrafluoroethylene-based, dielectric sheet material. The lenslet coating is molded to fit the 150 mm diameter arrays directly while the large-diameter lenses are coated using a tiled approach. We review the fabrication processes for both prescriptions then discuss laboratory measurements of their transmittance and reflectance. In addition, we present the inferred refractive indices and loss tangents for the coating materials and the aluminum oxide substrate. We find that at 150 GHz and 300 K the large-format coating sample achieves (97 +/- 2)% transmittance and the lenslet coating sample achieves (94 +/- 3)% transmittance.