Multichroic detectors for Cosmic Microwave Background polarisation observation

The study of polarized fluctuations in the 3K Cosmic Microwave Background (CMB) appears today as an essential way to progress in our understanding of the Universe. The European community is now taking part in the LiteBird small satellite and is also studying a large mission in preparation for the ESA M6 call.  
The optimization of the instrument's optical performance would be greatly simplified if the physical dimensions of the focal plane could be reduced without decreasing the number of detectors. An elegant way to reach such architecture would be to realize multi-frequency cryogenic detectors, i.e. integrating new functionalities approaching spectroscopy. In practice, such a detection architecture is based on the use of a broadband antenna connected to a planar circuit and then to detectors (bolometers or Kinetic Inductance Detectors), all at cryogenic temperatures (T<0.3K). Several options are considered for the planar circuit, the simplest being a set of filters feeding several detectors. A first dual-frequency version has been studied in the framework of a contract with ESA between the universities of Manchester, Maynooth, Chalmers, Rome and APC.
We propose to start from the first results to produce a functional prototype with a few pixels and a low spectral resolution (R=10) operating around 150GHz. To achieve this, we will use Kinetic Inductance Detectors coupled with a wideband antenna and a bank of millimetre-wave filters realized in planar technology. The study will initially focus on the optimization of the broadband antenna, then on the choice of the type of filter and the improvement of the coupling with the detector. These studies will be carried out with the electromagnetic softwares and facilities available at APC (CST MWS, Sonnet, dilution cryostat, adiabatic demagnetisation cryostat, 70GHz-250GHz vector network analyser). Samples will be realized in the clean room facility of Observatoire de Paris as well as in C2N-MINERVE (Orsay). This work will be carried out in collaboration with the Paris Observatory, the Néel Institute and the LPSC in Grenoble as well as the Centro Atómico in Bariloche and the CNEA in Buenos Aires (Argentina).
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. 


Michel Piat






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