Speaker
Description
One of the most critical components of gravitational wave interferometers is their mirror test masses, as coating thermal noise is a primary limiting factor in the 20–2000 Hz frequency range. For this reason, one of the two nested interferometers composing Einstein Telescope (ET-LF) is designed to operate under cryogenic conditions. However, characterizing both mechanical and optical losses in mirror coatings under realistic conditions is challenging, especially at cryogenic temperatures, where thermo-elastic interactions between the coating and the substrate makes very hard to use standard characterization methods based on the measurements of mechanical Q factors of substrates coated with the materials to be characterized.
To address this issue, we propose an innovative experimental setup to measure optical losses and mechanical dissipation in freestanding coating membranes over a broad temperature range. By suspending the coating as a thin membrane, the thermo-elastic interactions between the coating and the substrates are minimized, which enables precise measurement of the coating's properties in the whole range between room temperature and few Kelvins.
The experimental apparatus features a low-vibration cryostat housing an optical cavity, along with piezo actuators for precise membrane positioning and alignment. The measurement consists in placing the membrane inside the resonator so that it couples with the stationary electromagnetic field circulating inside the cavity. The optical losses are determined by monitoring the finesse of the Fabry-Perot cavity as a function of membrane position along the optical axis. The mechanical dissipations are instead measured using the cavity as a sensitive transducer of the membrane vibration spectrum, with dissipation values extracted through a dedicated data analysis procedure.
Preliminary tests on low-stress silicon nitride (SiN) membranes confirm the functionality of the setup. In the next phase, we will test membranes made of materials with lower optical absorption than SiN to further refine our measurements. This will help in optimizing the apparatus and expanding instrumentation to enable more comprehensive studies.
