Role of substrate and TiO2 content in TiO2:Ta2O5 coatings for gravitational wave detectors

Gravitational wave detectors (GWDs) are designed to detect the elusive signals produced by spacetime ripples, the GWs. The key to improving GWD sensitivity relies on the reduction of the thermal noise introduced by the mirrors. The high refractive index component of the high-reflectance mirrors installed in the current generation GWDs, such as Advanced LIGO and Advanced Virgo, is made of a mixture of similar to 27% TiO2 and similar to 73% Ta2O5. Such a coating plays a fundamental role in the GWD performance. The 27:73 TiO2:Ta2O5 ratio ensures high structural, optical, and mechanical performances, which allowed for the first ever detection of GWs, but might not be enough for new generation GWDs. Here, we investigate the potential of TiO2:Ta2O5 coatings, in a wider range of Ti/(Ta+Ti) cation ratio. Our research spans over the morphological and structural coating characteristics, and their correlation with optical and mechanical properties. On one hand, we unveil the profound influence of substrate selection and TiO2 content on the quality of coating morphology. On the other, we pinpoint the effect of TiO2 content on the structural properties of the coating, as increasing TiO2 content leads to lower temperature amorphous-to-crystalline transition, and we show that internal strain may arise due to the coexistence of TiO2 and Ta2O5 crystalline phases. Finally, substrate choice, TiO2 concentration, and crystallization characteristics emerge as pivotal factors in the pursuit of precision optics.