Kristallijne oxide-spiegelcoatings voor zwaartekrachtgolfdetectoren

The extraordinary sensitivity achieved by second-generation gravitational wave detectors (GWDs), such as Advanced LIGO (aLIGO) and Advanced Virgo (AdV), enabled the first experimental detection of gravitational waves in 2015, marking the beginning of the era of multi-messenger astronomy. Future upgrades to AdV and the development of third-generation detectors will require a further reduction of the overall interferometer noise budget. One of the fundamental noise sources limiting detector sensitivity in the intermediate frequency range (50-300 Hz) is the so-called coating thermal noise. This consists of the combined contribution of intrinsic Brownian and thermo-optic noises generated within the multilayer coatings deposited on the surfaces of the mirrors located in the interferometer arm cavities. One strategy to minimize this noise source is to identify combinations of coating materials exhibiting loss levels at cryogenic temperatures below the design specifications of next-generation detectors. In this work, corundum crystalline coatings are investigated as a potential alternative to the amorphous multilayers currently in use. Preliminary results on the epitaxial growth of Cr₂O₃, Fe₂O₃, and their solid solutions on sapphire substrates are presented, together with a characterization of their mechanical and optical properties in light of GWD requirements. Furthermore, the work reports the results of theoretical simulations aimed at minimizing thermo-optic noise in corundum multilayers through the use of tailored buffer layers exhibiting a negative coefficient of thermal expansion.