For the last 12 years, LIRA (formerly LESIA) with CNES support has developed the first high-resolution spectropolarimeters working on a wide UV wavelength range. This has led to several such spectropolarimeters being proposed to fly for the first time on space missions. In particular, the Pollux instrument is proposed by Europe, led by France (Coralie Neiner at LIRA and Jean-Claude Bouret at LAM), to be onboard the next NASA flagship mission HWO (Habitable Worlds Observatory). 

For these instruments, it is very important to minimize and characterize the polarizing effects occurring in the optics in the instrument to avoid degrading the polarimetric measurement accuracy because of instrumental polarization or polarized spectral fringes. 

In this thesis, we propose to study polarizing effects in various optical elements with the goal to understand and minimize these effects in UV spectropolarimeters. In particular, we will study the way to quantify and minimize polarization in echelle gratings and cross-dispersers necessary in high-resolution spectrographs. We will also investigate these effects in the dichroics that are planned in the Pollux instrument to separate the various wavelength channels. In addition, we will model and analyze the polarized fringes created in the modulator of polarimeters made of thin birefringent plates (which is the baseline design for most UV polarimeters, except in the far UV domain) and determine how to best adjust the thickness of the plates to minimize the fringes or move them out of the wavelength domain of interest. 

This work will be performed first thanks to theoretical calculation and modelling work, which will allow us to optimize the design of the instruments such as Pollux. We will then measure polarization in prototypes thanks to a test bench built at LIRA. Samples of UV modulators and dichroics are already available and thus guarantee that experimental results will be obtained during the thesis. Prototypes of gratings are expected to be built in the coming two years through a R&D program in collaboration with Horiba and LAM. All experimental work will be performed in a vacuum chamber in a clean room, as required for UV instruments. 

This work will greatly contribute to improving the design of Pollux for HWO and other UV high-resolution spectropolarimeters to reach a better polarimetric accuracy and precision. 

The candidate for this thesis is expected to have an excellent background in optics, experience with Zemax, and appetite for both theory/modelling in optics and hands-on experience in a clean room. Engineers from IOGS are particularly encouraged to apply. 

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For more Information about the topics and the co-financial partner (found by the lab!); contact Directeur de thèse - coralie.neiner@obspm.fr

Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online before March 13th, 2026 Midnight Paris time!