Thermo-optic noise for coating application at room and cryogenic temperature

Coating thermo-optic noise arises from thermal fluctuations that modify a mirror’s reflection phase via two mechanisms: the thermoelastic effect (changes in coating thickness due to thermal expansion) and the thermorefractive effect (variations in thickness and refractive index). Their sum defines the total thermo-optic noise. The temperature dependence of the optical path length is given by 1/L dL/dT = α + β/n, where α is the thermal expansion coefficient and β/n accounts for refractive index changes. Measurements follow shifts in interference peaks versus temperature, first during controlled heating in an oven and now also in a cooling setup reaching cryogenic conditions. In this context, I will present the data collected and analysed from measurements on titania-silica, considered one of the leading candidates for coating applications at room temperature. In this case, the measurements are performed by gradually increasing the sample temperature in a controlled manner. Similarly, I will present the results obtained for amorphous silicon, which is regarded as one of the most promising materials for coatings operating at cryogenic temperatures. In this case, the measurements are carried out by progressively decreasing the temperature, allowing us to probe the material’s behaviour under cooling conditions relevant for cryogenic operation.