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

Abstract

The noise caused by internal mechanical dissipation in the high refractive index amorphous thin films in dielectric mirrors is an important limitation for gravitational wave detection. The objective of this study is to decrease this noise spectral density, which is linearly dependent on such dissipation and characterized by the loss angle of the Young's modulus, by adding zirconia to titania-doped tantala, from which the current mirrors for gravitational wave detection are made.The purpose of adding zirconia is to raise the crystallization temperature, which allows the material to be more relaxed by raising the practical annealing temperature. The Ta, Ti and Zr oxides are deposited by reactive magnetron sputtering in an Ar:O2 atmosphere using radio-frequency and high power impulse plasma excitation. We show that thanks to zirconia, the crystallization temperature rises by more than 150 • C, which allows one to obtain a loss angle of 2.5 × 10 −4 , that is, a decrease by a factor of 1.5 compared to the current mirror high-index layers. However, due to a difference in the coefficient of thermal expansion between the thin film and the silica substrate, cracks appear at high annealing temperature. In response, a silica capping layer is applied to increase the temperature of crack formation by 100 • C.

Hydrogen Concentration and Mechanical Dissipation upon Annealing in Zirconia-doped Tantala Thin Films for Gravitational Wave Observatory Mirrors

“…The internal dissipation in amorphous materials usually decreases upon thermal annealing as they structurally relax, in particular in the case of tantala [6]. Some doping elements such as Ti are found to decrease the loss angle or to frustrate its crystallization, allowing further annealing and relaxation [7]. However, impurities such as hydrogen are known to desorb from materials upon annealing, including ZrO2 [8].…”

Imaging Scatterometer for Observing Changes to Optical Coatings During Air Annealing

“…The titania dopant is added to further decrease the mechanical loss and the absorption of the Ta 2 O 5 layers [12]. We note that other dopants such as zirconia can be added to frustrate crystallization in tantala [13] and titania-doped-tantala [14], though the samples used here do not include zirconia. There is currently heavy research into identifying coatings for future detectors that will have even lower coating thermal noise [15] and as good optical properties as the current coatings.…”