At LERMA, Observatory of Paris, we are specialized in heterodyne receivers and have contributed to several space missions, e.g. a mixer to the HIFI instrument on Herschel and are now involved in whole frontend receiver for SWI on JUICE. We are also participating in the planning of future mission, such as FIRSPEX, Millimetron and the Origins Space Telescope. In these new missions, the astronomical community asked for arrays of heterodyne receivers and focal plan arrays of up to 128 pixels are part of the study. In the future even larger arrays will be desired.
However, currently only very small arrays exist, basically made up of individual pixels. Therefore, it is necessary to carry out development work for large arrays, in particular to simplify the design, fabrication and assembly of large arrays. We have already started such R&D work on focal plane arrays. We have compared traditional horns and antennas to an array of patch antennas that we have designed. The patch antenna array is planar, i.e. less spacious, and has a very good beam pattern. However, it requires a ground plane, which is difficult to fabricate. It also use strip lines to carry the signal, which has high losses unless superconducting materials are used. The patch antenna arrays are hence not optimal for heterodyne receivers. As an alternative solution to simplify focal plan arrays, we have started to look at transmit arrays, which are metamaterials designed to add a phase shift to an incoming beam. These transmit arrays can be designed such that the phase shifts causes the beam to focussed on the mixer. Simulations of Duccio Delfini, our 3rd year Ph.D. student funded by CNES, show that the transmit arrays have nearly no losses compared to the patch antenna array. D. Delfini designed an array that allows focussing a beam in one dimension. Colleagues in Rennes have developed planar lenses with a similar approach, albeit at much lower frequencies (17 GHz). It would be very interesting to find a similar design of these planar lenses out of metamaterials for THz frequencies. There are also ideas to use a coupling structure using metamaterials that could couple both LO and sky signal to the mixer. Other possibilities include using such phase shifting arrays as a beam splitter. However, these promissing ideas go well beyond the current R&D work, as well as the thesis of D. Delfini.
The objective of this research thesis is to use planar lenses probably made using metamaterials to shape electromagnetic wave fronts in the optical path of THz heterodyne receivers. This project uses an interdisciplinary approach to apply very innovative aspects in engineering to astronomical instrumentation and the student will be supervised by an astronomical instrumentalist at the Observatory of Pars as well as by researchers of the electrical engineering department (either Rennes or UPMC).
The Ph.D. project requires on one hand, the development of THz frequency planar lenses, (which are typically at much lower frequencies), and on the other hand, the application of these promising techniques to astronomical receivers. The student will first get an introduction to heterodyne receivers by participating in some of the laboratory work at the observatory. In a heterodyne receiver the signal from the sky is overlaid onto an artificial well-known monochromatic wave of a local oscillator (LO). Both signals are sent to a horn or lens which focuses the radiation onto a mixer, where the beat frequency of the sky and the LO is created and - in a further step - spectrally detected. Here we propose to replace the horn or lens by a planar lens ideally made from a transmit array. The student will receive an introduction to different simulation software such as HFSS and FEKO. Soon the student will simulate planar lenses for submillimeter and THz frequencies. We will first try to make the planar lenses out of metamaterials. If this should prove to difficult, we will explore the possibility of Fresnel lenses. The design needs to be carried out in close contact with the instrumentation group to ensure the desired specifications are met and on the other hand in close contact to the clean room engineers that will advice us on the fabrication possibilities. Once a realistic planar lens has been designed it will be fabricated by our engineers in the clean room and mounted on an existing mixer. The Ph.D. student will then test the planar lens in particular its optical beam shape and its efficiency. After careful analysis the student will iterate his/her design to improve the transmit array and a second version will be fabricated and tested. This second iteration is likely to be necessary as the planar lens for astronomical THz receivers is a completely new concept. This innovative concept could be an essential building block for a breakthrough to large heterodyne receiver arrays, which are essential for the next generation of far-infrared satellites such as the Origins Space Telescope.
The Ph.D. candidate should have a master degree in engineering, physics or astronomy and posses good fundamental knowledge of physics and/or engineering. The candidate needs to be able to think critically and analyse simulations and measurements where there are no standard methods, but logical deduction is required. The candidate needs to be at ease with computers and be prepared to learn new simulation software and design the transmit arrays with them. He/ she will also need to carry out laboratory measurements and should have prior experience and should have show that he/she can interpret the results independently. As we are working in a very specialized field with only a few experts worldwide all papers and exchange is in English and a good knowledge of English is required. Some knowledge of French is of advantage to communicate with the clean room staff. Most of all the candidate needs to be very interested in the proposed research and highly motivated.
To apply, we invite you to contact the PhD/research supervisor and fill, with him/her, the co-financing part of the online application form (Reply to the offer) by April 1st, 2019.