  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.
