Role of substrate and TiO$_{2}$ content in TiO$_{2}$:Ta$_{2}$O$_{5}$ coatings for gravitational wave detectors
Résumé
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 ∼27% TiO$_{2}$ and ∼73% Ta$_{2}$O$_{5}$. Such a coating plays a fundamental role in the GWD performance. The 27:73 TiO$_{2}$:Ta$_{2}$O$_{5}$ 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 TiO$_{2}$:Ta$_{2}$O$_{5}$ 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 TiO$_{2}$ content on the quality of coating morphology. On the other, we pinpoint the effect of TiO$_{2}$ content on the structural properties of the coating, as increasing TiO$_{2}$ content leads to lower temperature amorphous-to-crystalline transition, and we show that internal strain may arise due to the coexistence of TiO$_{2}$ and Ta$_{2}$O$_{5}$ crystalline phases. Finally, substrate choice, TiO$_{2}$ concentration, and crystallization characteristics emerge as pivotal factors in the pursuit of precision optics.
Mots clés
gravitational wave detectors
coatings
morphological characteristics
structural characteristics
optical properties
crystallization
noise
thermal
crystal
optical
gravitational radiation detector
mirror
performance
space-time
temperature
quality
gravitational radiation
sensitivity
LIGO
refractive index
VIRGO
correlation
optics
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