The next generations of ground-based cosmic microwave background experiments will require polarisation sensitive, multichroic pixels of large focal planes comprising several thousand detectors operating at the photon noise limit. One approach to achieve this goal is to couple light from the telescope to a polarisation sensitive antenna structure connected to a superconducting diplexer network where the desired frequency bands are filtered before being fed to individual ultra-sensitive detectors such as Transition Edge Sensors. Traditionally, arrays constituted of horn antennas, planar phased antennas or anti-reflection coated micro-lenses have been placed in front of planar antenna structures to achieve the gain required to couple efficiently to the telescope optics. In this paper are presented the design concept and a preliminary analysis of the measured performances of a phase-engineered metamaterial flat-lenslet. The flat lens design is inherently matched to free space, avoiding the necessity of an anti-reflection coating layer. It can be fabricated lithographically, making scaling to large format arrays relatively simple. Furthermore, this technology is compatible with the fabrication process required for the production of large-format lumped element kinetic inductance detector arrays which have already demonstrated the required sensitivity along with multiplexing ratios of order 1000 detectors/channel.

The Mexico-UK Sub-millimetre Camera for AsTronomy (MUSCAT) is a large-format, millimetre-wave camera consisting of 1,500 background-limited lumped-element kinetic inductance detectors (LEKIDs) scheduled for deployment on the Large Millimeter Telescope (Volcán Sierra Negra, Mexico) in 2018. MUSCAT is designed for observing at 1.1 mm and will utilise the full 4 field of view of the LMTs upgraded 50-m primary mirror. In its primary role, MUSCAT is designed for high-resolution follow-up surveys of both galactic and extra-galactic sub-mm sources identified by Herschel. MUSCAT is also designed to be a technology demonstrator that will provide the first on-sky demonstrations of novel design concepts such as horn-coupled LEKID arrays and closed continuous cycle miniature dilution refrigeration.Here we describe some of the key design elements of the MUSCAT instrument such as the novel use of continuous sorption refrigerators and a miniature dilutor for continuous 100-mK cooling of the focal plane, broadband optical coupling to Aluminium LEKID arrays using waveguide chokes and anti-reflection coating materials as well as with the general mechanical and optical design of MUSCAT. We explain how MUSCAT is designed to be simple to upgrade and the possibilities for changing the focal plane unit that allows MUSCAT to act as a demonstrator for other novel technologies such as multi-chroic polarisation sensitive pixels and on-chip spectrometry in the future. Finally, we will report on the current status of MUSCAT's commissioning.He sorption pump each, this is referred to as a He8 cooler (2 × 4 He). The second stage cooled by a CS cooler uses a system where each precooler consists of a 4 He pump and a 3 He pump, referred to as a He14 cooler (2 × [ 4 He+ 3 He]). The operation of these units including how they are cycled in MUSCAT to accommodate large thermal loads, has been described in a previous paper. 4

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.

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.

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