Structural Evolution that Affects the Room-Temperature Internal Friction of Binary Oxide Nanolaminates: Implications for Ultrastable Optical Cavities

Abstract

Internal friction in oxide thin films imposes a critical limitation to the sensitivity and stability of the ultrahigh finesse optical cavities for gravitational wave detectors. Strategies like doping or creating nanolaminates (NL) are sought to introduce structural modifications that reduce internal friction. This work describes an investigation of the morphological changes SiO2/Ta2O5 and TiO2/Ta2O5 nanolaminates undergo with annealing and their impact on room-temperature internal friction. It is demonstrated that thermal treatment results in a reduction of internal friction in both nanolaminates but through different pathways. In the SiO2/Ta2O5 nanolaminate, the layers of which remain intact after annealing, the total reduction in internal friction follows the reduction in the composing SiO2 and Ta2O5 layers. In contrast, interdiffusion initiated by annealing at the interface in the TiO2/Ta2O5 nanolaminate leads to the formation of a mixed phase. It is the interfacial reaction upon annealing that dictates the more significant reduction in internal friction to ∼2.6 × 10–4, a value lower than any other Ta2O5 mixture coating with similar cation concentration.

Low Mechanical Loss TiO2:GeO2 Coatings for Reduced Thermal Noise in Gravitational Wave Interferometers

“…The SiO 2 layers can already be produced with low enough mechanical loss angle [22], so the main focus of the current research is on improving the high refractive index material. Several different approaches have been investigated, including deposition at elevated substrate temperatures [28,29] and with assist ion bombardment [30,31], doping and nanolayering of Ta 2 O 5 [32][33][34][35], and the use of nitrides [36,37]. Here we report results on amorphous oxide coatings based on mixtures of GeO 2 and TiO 2 .…”

Zirconia-titania-doped tantala optical coatings for low mechanical loss Bragg mirrors

“…However, the improvement obtained by annealing at temperatures above 750 • in terms of loss angle appears to be marginal in comparison to the loss angle of current a-LIGO mirrors and recent results from the literature for titania-doped tantala [12,13], and involves annealing the eventual mirrors at extremely high temperatures, > 800 • C, with potential problems such as cracking.…”

“…In order to reach this objective, the following paths are being examined: using different layer compositions, deposition tools and methods (including nanolayers), post-treatment strategies, or using mono-crystalline layers instead of amorphous ones [10,11]. Recently, Fazio et al [12] were able to reach a loss angle of 2.8 × 10 −4 in 27% Ti-doped tantala deposited by ion beam sputtering (IBS) by annealing the sample at 600 • C. Yang et al [13] achieved a loss angle of 2.6 × 10 −4 in titania-tantala nanolaminates annealed at 650 • C, the titania nanolayer however becoming discontinuous.…”

Enhanced medium-range order in vapor-deposited germania glasses at elevated temperatures

“…a-GeO 2 thin films were deposited with a 4Wave Laboratory Alloy and Nanolayer System physical vapor deposition system ( 45 , 46 ). A gridless ion source was used to generate low-energy Ar ions that were accelerated toward a high-purity Ge target negatively biased at 800 V. This created a sputter plume that deposited onto the substrate.…”