Crystallization of amorphous tantala coatings and its implication on GW interferometry

This thesis is a significant contribution to the ongoing international effort, particularly within the LIGO-Virgo collaboration, aimed at enhancing the sensitivity of gravitational wave interferometers. The primary focus is on mitigating the impact of coating thermal noise (CTN), a major limitation affecting the interferometers' sensitivity, especially in their most critical frequency band. Gravitational astronomy, a noteworthy achievement of the new millennium, has provided a novel perspective on distant and enigmatic cosmic phenomena like black holes and neutron stars. The technological cornerstone of this field lies in gravitational wave interferometers, recognized as among the most sensitive instruments ever developed. These instruments rely on special mirror test masses coated with materials designed to reflect laser signals in the interferometer. However, the sensitivity of these interferometers is hindered by coating thermal noise, prompting a concerted global research initiative to explore new avenues for reducing this noise. This thesis, conducted within the framework of the LIGO-Virgo collaboration, focuses on the potential of controlled partial crystallization to improve the performance of coatings, particularly those made of Ta2O5—a material commonly used in gravitational wave interferometry and prone to thermal noise issues. The research involved exploring a wide range of annealing conditions to induce the formation of crystalline grains within amorphous films of Ta2O5 deposited through Ion Beam Sputtering. In-situ XRD experiments were utilized to characterize the transition kinetics from amorphous to crystalline states. The study employed classical crystallization theory to determine activation energies for the growth process and parameters for describing the characteristic crystallization time in the Avrami model. Various characterization techniques, including optical microscopy, Raman spectroscopy, XRD, and AFM measurements, revealed the presence of single-crystalline grains with random orientation, comparable or larger in size than the film thickness. A dedicated in-situ total scattering experiment at the ESRF synchrotron radiation facility illustrated the nucleation of grains with a considerable size of several tens of nanometers. The crystalline phase was further analyzed through a Rietveld analysis of the diffraction spectra, identifying a predominant hexagonal unit cell. The impact of partial crystallization on the mechanical losses of the samples was investigated using a Gentle Nodal Suspension system, revealing, for the first time, a dependence of mechanical losses on the crystallized fraction. Notably, a 1% crystalline volume led to a 15% reduction in mechanical losses compared to standard coatings. Furthermore, the optical properties of the films were examined using a dedicated setup for angle-resolved optical scattering and total integrated scattering measurements. Preliminary results indicated a moderate increase in optical scattering at the wavelength of 1064 nm at the optimal crystallization level for minimizing mechanical losses. The study concludes by suggesting that further engineering of the crystallized grain size distribution may help optimize optical properties. In summary, this comprehensive project systematically addresses thermal Brownian noise, offering insights into the potential benefits of controlled partial crystallization in improving the performance of gravitational wave interferometers.