Speaker
Description
One of the two nested interferometers of the Einstein Telescope (ET-LF) is designed to operate at cryogenic temperatures. At such low temperatures, thermo-elastic interactions between the coating and the substrate become crucial, making the characterization of optical and
mechanical losses in mirror coatings extremely important. However, these same interactions complicate the interpretation of standard characterization methods based on Q-factor measurements of coated substrates.
To overcome these limitations, we are developing a novel experimental setup to probe both optical losses and mechanical dissipation using freestanding coating membranes across a broad temperature range. Suspending the coating as a thin membrane strongly suppresses substrate–coating coupling, enabling direct and precise evaluation of the coating’s properties from room temperature down to a few kelvins.
The apparatus includes a low-vibration cryostat and a high-finesse optical cavity used to measure optical losses. The membrane is positioned inside the resonator, and optical losses are determined by monitoring the cavity finesse as a function of membrane position along the optical axis. Mechanical losses are measured instead using a Michelson interferometer as a displacement readout, where the membrane serves as one of the end mirrors, enabling sensitive measurement of its vibrational modes. Preliminary tests on low-stress silicon nitride (Si3N4) membranes confirm the functionality of the setup.
In parallel, we have started developing an in-house membrane fabrication process and we are working on the realization of Si3N4 membranes using laser cutting and chemical wet etching. Once optimized, the process will then be applied to other materials as well.