Mechanical characterization of ‘uncoated’ and ‘Ta2O5-single-layer-coated’ SiO2 substrates: results from GeNS suspension, and the CoaCh project

Abstract

Thermal noise of the mirrors limits the sensitivity of interferometric gravitational-wave detectors in the frequency range between 50 and 300 Hz. According to the fluctuation–dissipation theorem, the thermal noise amplitude is proportional to the mechanical loss of the mirrors. Fused silica substrates loss angle widely ranges from 10−9 to 10−6, while for the coating it is around 10−4. Loss angle measurements in ultra low mechanical loss materials are normally affected by a large systematic error due to the excess losses introduced by the suspension system used to hold the samples. An innovative measurement system Gentle Nodal Suspension, where the disk-shaped sample is suspended in equilibrium on the top of a sphere, touching one of the nodal points of vibration, has been developed in INFN Florence Virgo laboratory. The advantages of this system are as follows: (i) the good reproducibility of loss angle measurements; (ii) one surface only (in any case uncoated) of the sample is touched; (iii) the contact surface is minimized because of the absence of applied forces. This suspension has been used to characterize annealing and coating deposition effects on the mechanical quality factor Q. An interesting comparison/analysis of these effects on mechanical, optical, chemical and surface properties using spectroscopic ellipsometry, x-ray photoelectron spectroscopy and atomic force microscopy has been carried out.