Design and Fabrication of Metamaterial Anti-reflection Coatings for the Simons Observatory

The Simons Observatory (SO) will be a cosmic microwave background (CMB) survey experiment with three
small-aperture telescopes and one large-aperture telescope, which will observe from the Atacama Desert in Chile.
In total, SO will field over 60,000 transition-edge sensor (TES) bolometers in six spectral bands centered between
27 and 280 GHz in order to achieve the sensitivity necessary to measure or constrain numerous cosmological
quantities, as outlined in The Simons Observatory Collaboration et al. (2019). These telescopes require 33 highly
transparent, large aperture, refracting optics. To this end, we developed mechanically robust, highly efficient,
metamaterial anti-reflection (AR) coatings with octave bandwidth coverage for silicon optics up to 46 cm in
diameter for the 22-55, 75-165, and 190-310 GHz bands. We detail the design, the manufacturing approach
to fabricate the SO lenses, their performance, and possible extensions of metamaterial AR coatings to optical
elements made of harder materials such as alumina.