ABSTRACT
To unveil presently inscrutable details of the origins of our universe imprinted in the cosmic microwave background, future experiments in the millimeter and submillimeter range are focusing on the detection of fine features, which necessitate large and sensitive detector arrays to enable multichroic mapping of the sky. Currently, various approaches for coupling light to such detectors are under investigation, namely, coherently summed hierarchical arrays, platelet horns, and antenna-coupled planar lenslets. The last option offers increased bandwidth and a simpler fabrication while maintaining the desired optical performance. In this work, the design, fabrication, and experimental characterization of a prototype planar metamaterial phase-engineered lenslet operating in W-band [75 GHz; 110 GHz] is presented. Its radiated field, initially modeled and measured on a systematics-limited optical bench, is compared against a simulated hyperhemispherical lenslet, a more established technology. It is reported here that our device reaches the cosmic microwave background (CMB) specification for the next stages of experiments, demonstrating power coupling above 95% and beam Gaussicity above 97% while maintaining ellipticity below 10% and a cross-polarization level below -21d B through its operating bandwidth. Such results underline the potential advantages our lenslet can offer as focal optics for future CMB experiments.
ABSTRACT
The high sensitivity requirements set by future cosmic microwave background instruments are pushing the current technologies to produce highly performant focal plane arrays with thousands of detectors. The coupling of the detectors to the telescope optics is a challenging task. Current implemented solutions include phased-array antenna-coupled detectors, platelet horn arrays, and lenslet-coupled planar antennas. There are also recent developments of flat graded-index lenses based on etched silicon. However, there are strong requirements in terms of electromagnetic performance, such as coupling efficiency and bandwidth, as well as requirements in terms of easy manufacturing and scalability, and it is very challenging to meet all these requirements with one of the above solutions. Here, we present a novel approach for producing flat metal-mesh lenslet arrays based on devices previously realized using the mesh-filter technology. We have now adapted the polypropylene-based mesh lens design to silicon substrates, thus providing a good mechanical match to the silicon-based detector arrays. The measured performance of prototype pixels operating at millimeter wavelengths is presented.
