Critical microwave instruments for applications such as cosmic microwave background studies require large, strong, low-dielectric-loss refractive optics that are compatible with vacuum environments and cryogenic temperatures. In refracting telescope designs, the lens diameter limits the resolution of an instrument, so it is desirable to use as large a lens as possible. The lens diameter is especially important for low-frequency observations. In addition, large- diameter optics allow for a focal plane with a large useable area, which provides more space for detectors. For experiments with detectors that are approximately photon shot-noise-limited, increasing the number of detectors is a means of increasing the sensitivity of the experiment. However, high-density polyethylene or ultra-high-molecular-weight polyethylene have low refractive index. Silicon and alumina have high indices of refraction, but are not generally available in large areas, and require custom machining and polishing. Also, there are no existing anti-reflection approaches that are compatible with the vacuum impedance and cryogenic temperatures. Low-loss millimeter-wave (mm-wave) optics will be made by inkjet-print additive manufacturing using gradient dielectric and magneto-dielectric nanocomposites optimized for mm-wave instruments used on high-energy physics (HEP) applications. The inkjet printing process allows for freeform gradient-index patterns to be formed that eliminate or reduce the need of lenses for surface curvature, and the gradient-index patterns can be used to achromatize the lens. One-meter scale lens printing is available today, limited only by the scalable number of printheads used in the fabrication process. Large-area low-loss dielectric millimeter-wave optics made using tough gradient-index nanocomposites will be designed and characterized for mm-wave instruments. A tradeoff study of gradient-index lens designs will be conducted and, from candidate devices, print-compatible feedstock materials will be identified, synthesized, and characterized over the operating conditions. Using our existing gradient-index ceramic additive-manufacturing process as a baseline, radially, axially, and freeform gradient dielectric mm-wave lenses, sized up to 50 cm, will be fabricated and tested. The electrical, thermal, and mechanical properties of the sample lenses will be tested over the 4K to 350K temperature range and compared to models. The data will be used to refine the design of a mm- wave lens to be delivered in Phase II. The innovation enables a broad range of mm-wave applications. The mm-wave region of the electromagnetic spectrum is increasingly exploited for a wide range of commercial, industrial, and military applications. Millimeter- wave frequencies are being used globally for applications like radio astronomy, remote sensing, automotive radar, imaging, security screening, and telecommunications, to name a few. Millimeter waves are also notably effective for military applications—including for the detection of explosives on personnel—because the waves readily pass through common clothing materials and reflect from the body and any concealed items.
