by Gloria Nobile
Halfway between material science and gravitational astronomy, research on the optical coatings of the Einstein Telescope (ET) mirrors plays a crucial role in ensuring the high performance of the interferometer. Coatings are thin layers of specific materials applied to mirror surfaces to provide the desired optical properties, such as the required level of reflection or transmission.
Various sources of noise (which we discussed here) can affect the experiment’s measurements. Coatings also have a direct impact on the detector’s sensitivity: any optical or mechanical losses can introduce noise, interfering with the detection of gravitational wave signals.
«Thermal noise is among the most insidious. It’s the noise generated within the interferometer by continuous microscopic rearrangements of the atomic structure of the mirrors», explains Hanna Skliarova, a researcher in the ETIC project at the University of Padua. «One way to reduce this noise is to keep the ET mirrors at low temperatures. This is why part of the interferometer is designed to operate in cryogenic conditions».
Finding the least “noisy” materials and optimizing the coatings’ properties to reduce the disturbances they generate is essential to achieving the expected sensitivity of the Einstein Telescope. This goal requires state-of-the-art equipment and significant research and development efforts. The University of Padua, together with the local sections of INFN and INAF, is heavily involved in both theoretical and experimental fronts of this large-scale project, particularly in the production of experimental optical coatings for material research for ET mirrors. A central role will be played by the COMET (Coating Materials for Einstein Telescope) laboratory within the Department of Physics and Astronomy “Galileo Galilei” at the University of Padua, which will be established in Rovigo as part of the ETIC project.
«This center will be dedicated to the production of high-quality samples and the study of deposition processes during fabrication. The laboratory will focus on research and development of innovative optical films characterized by extreme transparency and a high mechanical quality factor», Skliarova points out. «Among its main objectives is identifying and eliminating the physical causes of optical absorption in coatings and sources of mechanical dissipation. Both parameters are critically influenced by complex factors such as the structural, mechanical, compositional, and surface properties of the materials».
COMET will be able to produce samples on demand for the international scientific community, fostering the development of new materials and treatments that may eventually be scaled up to create the mirrors for ET and other gravitational interferometers.
The laboratories of the University of Bologna are also at the forefront of this research. «After a preliminary evaluation of several materials, we identified a thin film composed of mixed titanium and silicon dioxides as the most promising for our research», says Sara Cepić, PhD student at the University of Bologna and a researcher in the ETIC project. «These thin coatings will be prepared using the radio-frequency magnetron co-sputtering technique. The deposition chamber was specifically set up for this project, allowing us to use two different sources simultaneously».
This setup makes it possible to create composite films with extremely precise ratios between titanium and silicon dioxides, enabling detailed studies of how variations in these ratios influence the material’s properties. The analysis starts with a uniform distribution of the two components, gradually increasing the proportion of one over the other.
Advanced techniques are employed for morphology studies and microanalysis of the materials. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) provide high-resolution images of the sample surfaces at the nanometer level. Integrated with these techniques, energy-dispersive X-ray spectroscopy (EDS) identifies and characterizes the chemical elements present and their concentrations.
«The University of Bologna is actively collaborating with other groups to develop increasingly high-performance materials», concludes Cepić. «The research includes design, characterization, deposition, and optimization of thin films to be applied to the ET mirrors, a truly revolutionary project of global importance».
To learn more, watch the interviews with Hanna Skliarova and Sara Cepić on the ET Italy YouTube channel.
Credits featured image: EGO/Virgo

Sara Cepić in the laboratory