Hybrid metamaterial lenslet arrays for millimeter and submillimeter imaging

We are developing broadband metamaterial planar lenslet arrays for millimeter and submillimeter imaging using stacked silicon wafers patterned with subwavelength copper squares, deep reactive ion etched (DRIE) holes, or a hybrid combination of the two. Beam-forming is accomplished through spatial variation in refractive index within each pixel created by these subwavelength features, a gradient-index (GRIN) design. However, the optical properties of both the metal mesh and DRIE metamaterials exhibit dependence on polarization orientation and wavelength, posing challenges for lens design. We combine metal mesh and DRIE GRIN sections to leverage the contrasting polarization- and frequency-dependent properties of the two material types. Here we present measurements of our most recent prototype metamaterial GRIN lens on sinuous antenna-coupled TES detectors from 88 to 225 GHz. We also present design studies extending to higher frequencies and optimizing for different pixel pitches.