Structure and morphology of low mechanical loss TiO${}_2$-doped Ta${}_2$O${}_5$

Abstract

Amorphous oxide thin films play a fundamental role in state-of-the art interferometry experiments, such as gravitational wave detectors where these films compose the high reflectance mirrors of end and input masses. The sensitivity of these detectors is affected by thermal noise in the mirrors with its main source being the mechanical loss of the high index layers. These thermally driven fluctuations are a fundamental limit to optical interferometry experiments and there is a pressing need to understand the underlying processes that lead to mechanical dissipation in materials at room temperature. Two strategies are known to lower the mechanical loss: employing a mixture of Ta${}_2$O${}_5$ with $\approx$ 20\% of TiO${}_2$ and post-deposition annealing, but the reasons behind this are not completely understood. In this work, we present a systematic study of the structural and optical properties of ion beam sputtered TiO${}_2$-doped Ta${}_2$O${}_5$ films as a function of the annealing temperature. We show for the first time that low mechanical loss is associated with a material morphology that consists of nanometer sized Ar-rich bubbles embedded into an atomically homogeneous mixed titanium-tantalum oxide. When the Ti cation ratio is high, however, phase separation occurs in the film which leads to increased mechanical loss. These results indicate that for designing low mechanical loss mixed oxide coatings for interferometry applications it would be beneficial to identify materials with the ability to form ternary compounds while the dopant ratio needs to be kept low to avoid phase separation.

References in corpus (13)

• Advanced Virgo: a 2nd generation interferometric gravitational wave detector
• Advanced LIGO
• Sr lattice clock at 1x10^{-16} fractional uncertainty by remote optical evaluation with a Ca clock
• Internal thermal noise in the LIGO test masses : a direct approach
• Titania-doped tantala/silica coatings for gravitational-wave detection
• Mechanical Loss in Tantala/Silica Dielectric Mirror Coatings
• Amorphous optical coatings of present gravitational-wave interferometers
• Mechanical loss in state-of-the-art amorphous optical coatings
• Effect of heat treatment on mechanical dissipation in Ta${}_2$O${}_5$ coatings
• Optical spectrum analyzer with quantum limited noise floor
• Audio-band Coating Thermal Noise Measurement for Advanced LIGO with a Multi-mode Optical Resonator
• Material loss angles from direct measurements of broadband thermal noise
• Investigation of the Young's modulus and thermal expansion of amorphous titania-doped tantala films

Cited by in corpus (8)

• Low mechanical loss TiO${}_2$:GeO${}_2$ coatings for reduced thermal noise in Gravitational Wave Interferometers
• Comprehensive study of amorphous metal oxide and Ta${}_2$O${}_5$-based mixed oxide coatings for gravitational-wave detectors
• Optical and mechanical properties of ion-beam-sputtered Nb${}_2$O${}_5$ and TiO${}_2$-Nb${}_2$O${}_5$ thin films for gravitational-wave interferometers
• Structural evolution of binary oxide nanolaminates with annealing and its impact on room-temperature internal friction
• Effects of mixing and annealing on the optical and mechanical properties of TiO${}_2$:Ta${}_2$O${}_5$ amorphous coatings
• Development of ion-beam sputtered silicon nitride thin films for low-noise mirror coatings of gravitational-wave detectors
• Measurements of Optical Scatter Versus Annealing Temperature for Amorphous Ta2O5 and TiO2:Ta2O5 Thin Films
• Imaging Scatterometer for Observing In-Situ Changes to Optical Coatings During Air Annealing