The study of the cold Universe allows us to go back to the origins of our solar system by observing the embryos of stars, or to the origins of the Universe by measuring the polarisation of the Cosmic Microwave Background (CMB). These sub-millimetre and millimetre instruments require the use of superconducting detectors cooled down to 100mK. Kinetic Inductance Detectors (KIDs) are based on superconducting resonators and are about to replace bolometers thanks to their natural frequency multiplexing and ease of production (in their basic form). 

  After Planck, the European community, and France in particular, is currently involved in the small Japanese LiteBird space mission, which is dedicated to observing the polarisation of the CMB on large angular scales. In the same field, studies are also underway to develop the 4th generation of ground-based instruments (CMB-S4 project in the USA), with a French contribution based on KIDs currently under construction. It is interesting to note that there is a strong complementarity between space-based and ground-based instruments for CMB observation. In fact, an agreement has been signed between the LiteBird and CMB-S4 collaborations for joint data analysis.

In this type of experiment, optimising the optical performance of the instrument would be greatly simplified if we could reduce the physical dimensions of the focal plane. One way of achieving this would be to build cryogenic multi-frequency detectors, i.e. incorporating new functions that are closer to spectroscopy. In practice, such a detection architecture is based on the use of a broadband antenna connected to a planar circuit and then to KID-type detectors, all at cryogenic temperatures (T<0.3K). Several options are envisaged for the planar circuit, the simplest being a set of filters feeding several detectors. Following an initial dual-frequency design studied as part of an ESA contract (collaboration between the universities of Manchester, Maynooth, Chalmers, Rome and the APC), a prototype has been optimised and is currently being produced as part of a thesis.

We propose to build on these initial results and develop a functional prototype with a few pixels and low spectral resolution (R~10) operating at 150GHz and 220GHz. To achieve this, we will be using KID-type detectors coupled to a broadband antenna and a bank of millimetre filters produced using planar technology. The study will focus on optimising the broadband antenna, choosing the type of filter and improving the coupling with the detector. These studies will be carried out using the electromagnetic modelling and characterisation resources available at the APC (CST MWS, Sonnet, dilution cryostat, adiabatic demagnetisation cryostat, millimetre vector analyser). Samples will be produced at the Paris Observatory and at the C2N technology platform (Orsay). This work will be carried out in collaboration with the Paris Observatory, the Néel Institute and the LPSC in Grenoble.

According to the exact competences and preferences of the student, we could focus her/his activity on one or two of the key aspects: a) modeling and design; b) characterizations at cryogenic temperatures; c) micro-fabrication. The candidate should have competences in at least one of the following domains: RF (radio-frequency) design, instrumentation for astronomy, microfabrication technologies.