Stability of samples in coating research: from edge effect to ageing

Mechanical and optical thermal noises play an important role in many precise opto-mechanical experiments, in which positions of test bodies are monitored by laser beams. Much of the research in this area was driven by the physics of gravitational wave interferometers, to counteract the mirror's multi-layered dielectric coating thermal noise. Coating thermal noise is directly related to structural dissipation inside the material through the loss angle. In view of future upgrades of gravitational wave detectors, increasing the coatings mechanical performances, by lowering the loss angle and retaining their outstanding optical and morphological properties, is fundamental. The measurement of the coating loss angle requires substrates to be stable with respect to their dissipative behavior. It has been seen that fused silica disc losses are resonant-mode shape dependent and are subject to ageing effects, compromising the accuracy of mechanical characterizations of the substrates and, consequently, of the coatings. In commercial samples, the source of this deteriorations can be related to the ground, unpolished lateral surface. In this work we show that the polishing of the sample's edge reduces the amount of spurious losses and ageing effects. A new procedure through CO2 laser polishing of the edge surface is proposed, explained and put in place. The results of these procedures, in terms of roughness and loss behavior is shown. The loss angle measurements are compared with an edge loss model and other existing models.