Astronomy in the millimetre and sub-millimetre spectrum aims at unveiling the processes behind the origin and evolution of our Universe at various scales, from protostars to the Cosmic Microwave Background. To carry out such observations, novel imaging and spectroscopy instruments covering the Tera-Hertz range are needed. At present, high-resolution spectroscopy is carried out with heterodyne detectors, using either Superconductor-Insulator-Superconductor-mixers or Hot-Electron Bolometer mixers, inherently limited in bandwidth and difficult to multiplex. Imaging is accomplished with detectors working near photo-noise level, primarily Transition Edge Sensors or Microwave Kinetic Inductance Detectors arrays, for which the focal optics needs scaling to reach the sensitivity needed for future cosmology experiments. In this work, the simulated design of a single pixel constituted of a set of antenna-coupled detectors receiving the light focused by a metamaterial-based phase-engineered lenslet is presented. This technology relies on standard lithography fabrication techniques and enables compact and broadband imaging and spectroscopy on-chip.
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 40 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 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 will 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.
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