Crystallization kinetic of Ta2o5 thin films: engineering amorphous materials for gravitational waves interferometers

Gravitational wave interferometers, the only instruments capable of directly detecting gravitational waves, are crucial for testing General Relativity and enabling new cosmological discoveries. These detectors rely on mirrors coated with multilayer Bragg stacks of amorphous SiO2 and Ti:Ta2O5, which reflect the interferometer's laser beam. A major sensitivity limitation is coating thermal noise (CTN), arising from thermally induced atomic fluctuations in the coatings that displace the mirror surface. Although crystalline materials could theoretically reduce CTN due to their stable atomic structures, current technologies cannot produce large-area crystalline multilayers. As an alternative, we are investigating partially crystallized glassy materials achievable through annealing. We conducted experiments on Ion Beam Sputtered Ta2O5, applying varied annealing conditions to study crystallization. Classical crystallization theory allowed us to extract activation energies for nucleation and growth. Techniques including optical microscopy, Raman spectroscopy, in-situ XRD, and AFM revealed the formation of single-crystal grains with random orientations after annealing. We further explored Ti-doped Ta2O5 to assess how doping affects crystallization behavior. The influence of crystallization on CTN was indirectly evaluated using a Gentle Nodal Suspension system, revealing a link between crystallization degree and thermal noise levels in initial samples. However, increased crystallinity also led to higher optical scattering losses. Despite this trade-off, optimizing grain size distribution via tailored annealing could reduce scattering while preserving thermal noise benefits, suggesting a promising direction for future research into advanced coatings for gravitational wave detectors.