Broad Bandwidth Metamaterial Antireflection Coatings for Measurement of the Cosmic Microwave Background

The goal for this project is to develop metamaterial antireflection (AR) coatings with broad bandwidth (>3:1 ratio bandwidth), low reflectance (<1%), and negligible dielectric losses for large diameter (> 30 cm diameter) cryogenic silicon lenses and half-wave plates intended for CMB instruments. The coatings consist of subwavelength pillars directly cut into the silicon lens and wave plate substrates. The figure to the left shows an example lens with a metamaterial coating that approximates two dielectric layers.  The coating was fabricated by using a custom three-axis silicon dicing saw to cut a series of perpendicular  grooves  into  the  lens  substrate to leave these sub-wavelength pillars behind. Adding  additional carefully designed “layers” comprised of square  cross-section  pillars  to  these  structures can realize wider bandwidth performance.   The proposed plan was to fabricate and test 3, 4, and 5 layer coatings in year one and scale these coatings across the CMB bands and create coatings for wave plates in year two. This work addresses TABS element 8.1.3.3 (Optical components: Filters and Coatings).
With funds from this grant we developed and demonstrated two new technologies: (1) wide bandwidth millimeter wavelength metamaterial AR coatings on silicon substrates, and (2) broad bandwidth metamaterial silicon half-wave plate modulators.  Both technologies are new and the half-wave plates were entirely conceived with funding from this grant. The need and impact of these new technologies is readily apparent from the fact that these have now been deployed on multiple experiments (the Atacama Cosmology Telescope, and the PIPER balloon) and are proposed for several future NASA balloons.