Simons Observatory: Broadband Metamaterial Anti-Reflection Cuttings for Large Aperture Alumina Optics

TL;DR

This paper details the design, fabrication, and testing of metamaterial anti-reflection coatings for large alumina optics, achieving broadband reflection control suitable for the Simons Observatory's frequency range.

Contribution

It introduces a novel metamaterial ARC fabrication method with micron accuracy, enabling broadband anti-reflection performance on large alumina filters for millimeter-wave astronomy.

Findings

Percent-level reflection control achieved across 75-170 GHz and 200-300 GHz bands.

Demonstrated effective broadband performance on large filters and prototypes.

Identified scattering effects at shorter wavelengths near the metamaterial scale.

Abstract

We present the design, fabrication, and measured performance of metamaterial Anti-Reflection Cuttings (ARCs) for large-format alumina filters operating over more than an octave of bandwidth to be deployed on the Simons Observatory (SO). The ARC consists of sub-wavelength features diced into the optic's surface using a custom dicing saw with near-micron accuracy. The designs achieve percent-level control over reflections at angles of incidence up to 20$^\circ$. The ARCs were demonstrated on four 42 cm diameter filters covering the 75-170 GHz band and a 50 mm diameter prototype covering the 200-300 GHz band. The reflection and transmission of these samples were measured using a broadband coherent source that covers frequencies from 20 GHz to 1.2 THz. These measurements demonstrate percent-level control over reflectance across the targeted pass-bands and a rapid reduction in transmission as the wavelength approaches the length scale of the metamaterial structure where scattering dominates the optical response. The latter behavior enables the use of the metamaterial ARC as a scattering filter in this limit.