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

Abstract

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