Measurement of ultra-low optical absorption in mirror substrates for the next-generation gravitational wave detectors

A key element for the next-generation gravitational wave interferometers are optical substrates characterized by a very low absorption level in the infrared at cryogenic temperatures (a few part per million per cm). In this respect the precise characterization of optical losses is mandatory. However the target absorption levels are so small that a direct measurement is challenging. Moreover there are currently no developed protocols for such characterization in cryogenic conditions. In this thesis we propose to build a setup for this purpose based on the measurement of the heating of a test substrate due to the absorbed power of an IR laser. The final absorption measurement will be performed with the use of an innovative method, compared to the ones that are commonly used in literature, the Modulation Calorimetry technique. The silicon absorption coefficient estimate that is obtained in the experiment that is presented in this thesis will be compared to the only existing measurement of the coefficient in analogous environmental conditions that is currently present in literature and the consequences of this findings on the Einstein Telescope design will be analyzed.