Innovative research for coating thermal noise reduction in gravitational wave detectors and other high precision experiments

Mechanical and optical thermal noise is a limiting source of noise in many high precision opto-mechanical experiments that involve stabilized optical cavities. Most of the theoretical and experimental research in this field was driven by gravitational wave physics and interferometric detectors. The Virgo-LIGO collaboration was able to detect for the first time gravitational wave on September 14th, 2015 opening the gravitational wave astronomy era. Since then, many other signals followed, during the successive observing runs. Nowadays the possible upgrades of different subsystems is ongoing and will be implemented in order to increase more and more the sensitivity, and then observing volume and detection rates. The mid frequency band, from few to hundreds Hz, has proven to be the most fruitful bandwidth of gravitational signals, until now. In this region the thermal noise of the multi-layered dielectric coating of the mirrors is become the most limiting noise. Being able to reduce it, is an essential step in achieving the desired sensitivity goals. Coating thermal noise arises from fluctuations of the mirror surface under thermally activated transitions between equilibrium configurations of structure in coatings. Its amplitude is linked to the amount of internal friction and stresses inside the multilayer dielectric coatings. In this framework, it is important to accelerate their investigation and optimization. Coating thermal noise is an issue common to all experiments that strive for really small movement measures. The mission is pursued mainly in gravitational wave community by Virgo Coating Research and Development Collaboration and in LIGO Optical Working Group, by exploiting the recent advances in amorphous thin film synthesis, by utilizing high throughput characterization of elastic loss, by drawing on a new theoretical understanding of atomic scale mechanism of elastic loss, and by employing non-destructive methods to determine the short and medium range atomic arrangement of sub-micron amorphous films. The goal is to develop coatings that meet the thermal noise requirements, preserving outstanding optical properties. Present coatings are ion-beam sputtering multi-layers, alternatively made of titanium dioxide-doped tantalum pentoxide (Ti:Ta2O5, high refractive index) and fused silica (SiO2, low refractive index). They are deposited by the Laboratoire de Mat´eriaux Avanc´es, on massive, large-area fused silica substrates. Ti:Ta2O5 layer is the dominant source of thermal noise in coatings. For this reason, there is interest in finding new possible high-index material or new deposition techniques that allows the production of coatings with lower internal frictions. I II Acronyms This theses work is framed inside this perspective, especially involving characterization of substrates’ (both amorphous like fused silica, and crystalline, like silicon) and coatings’ mechanical losses, along with morphological and optical characterizations. Main analysed coatings are examples of possible new coatings, under investigation inside the Virgo Coating Research and Development Collaboration. In the first chapter a general introduction to thermal noise, starting from thermodynamic equilibrium in systems at a temperature different from zero, passing through fluctuation dissipation theorem up to inelastic behaviour and mechanical dissipations in solids, is shown. In the second chapter the more specific topic of mirror’s thermal noise is faced. The main contributions of all the system’s part are analysed, highlighting the more relevant contribution of coating thermal noise. Two sections are dedicated to two high precision experiments that suffer for coating thermal noise: optical cavities’ experiments and gravitational wave detector experiments. In the third chapter the state of the art of nowadays gravitational wave detector coatings is illustrated; after this, the general guidelines of the research plans devoted to the coating thermal noise reduction are presented. The three main sections shown are metrology, deposition techniques and treatments, and new materials. This chapter lay the foundation for the next experimental chapters, introducing the method to measure mechanical losses in coatings. In the fourth and fifth chapters the main issues regarding the substrates upon which coatings can be deposited are shown. A general description of the constituent material comes with a specific treatment of the substrates experimentally measured and characterized. Analytical models of their inelastic behaviour are presented and compared with experimental results. In the sixth chapter the topic of new coatings is faced in a twofold way. Through the study and characterization of a candidate material for coating research, the zinc sulphide (ZnS). Furthermore, the nano-layered coating technique is illustrated, together with initial morphological and mechanical characterization of some samples.