Experimental characterisation of a planar phase-engineered metamaterial lenslet for millimetre astronomy

To unveil presently inscrutable details of the origins of our Universe imprinted in the Cosmic Microwave Background, future experiments in the millimetre and sub-millimetre 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 whilst maintaining the desired optical performance. In this work, the design, fabrication and experimental characterisation of a prototype planar metamaterial phase-engineered lenslet operating in W-band [75GHz;110GHz] is presented. Its radiated field, initially modelled and measured on a systematics limited optical bench, is compared against a simulated hyper-hemispherical lenslet, a more established technology. It is reported here that our device reaches the CMB specification for the next stages of experiments, demonstrating power coupling above 95%, beam gaussicity above 97% whilst maintaining ellipticity below 10% and cross-polarisation level below -21dB through its operating bandwidth. Such results underlines the potential advantages our lenslet can offer as focal optics for future CMB experiments.