Thèses

Vendredi 7 Juin 2024 à 10h00.

Development of very low absorption sapphire substrates for the cryogenic gravitational wave detector mirrors

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Membres du jury / jury members :

LEMAITRE Anaël Rapporteur, Professeur, École des Ponts ParisTech
SEGONDS Patricia Rapporteure, Professeure des Universités, Institut Néel
DUJARDIN Christophe Examinateur, Professeur des Universités, Institut Lumière Matière
STEINLECHNER Jessica Examinatrice, Professeure associée, Maastrich University
LEBBOU Kheirreddine Directeur de thèse, Directeur de Recherche CNRS, Institut Lumière Matière
DEGALLAIX Jérôme Co-Directeur de thèse,Chargé de Recherche CNRS, Laboratoire des Matériaux Avancés

Résumé / Abstract :

Sapphire is a unique material combining excellent mechanical properties with exceptional optical performance. It is the second hardest material after diamond and it has a very wide optical transmission band ranging from UV (λ ~ 200 nm) to mid-infrared (λ ~ 5μm). It is therefore naturally used in a wide range of cutting-edge applications ranging from the military field to the extreme physics experiments such detectors for gravitational waves.

The Gravitational wave detectors have opened a new window through which the universe can be observed and a lot of work in the past decade has been dedicated to upgrading the sensitivity of these detectors. One of the pathways to higher sensitivity for future and current detectors is to cool down the mirrors of the Fabry-Perot cavities of the detectors down to cryogenic temperatures to reduce thermal noise.

This temperature operation requires new substrates for these mirrors and two main candidates exist: sapphire and silicon. Sapphire is already in use today in the KAGRA cryogenic gravitational wave detectors. While promising, some problems with the use of sapphire still exists, most notably its optical absorption at 1064 nm, which must be below 50 ppm/cm (or even less for future detectors). Unexpected birefringence, despite using c-axis sapphires has also been measured. The goal of this thesis is to show the feasibility of growing sapphires with sub-50 ppm/cm absorption in a controlled and repeated manner and to investigate the origin of this absorption as well as study their birefringence.

This thesis is to carry out studies of these two parameters and correlate them with:
*crystal growth conditions,
*the presence of impurities in the starting raw material

We therefore propose to understand the origin of optical absorption in sapphire and to develop crystallization process to grow a high-performance sapphire with a reproducible optical absorption below 50 ppm/cm

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