A method for the experimental measurement of bulk and shear loss angles in amorphous thin films

Brownian thermal noise is a limiting factor for the sensitivity of many high precision metrology applications, among other gravitational-wave detectors. The origin of Brownian noise can be traced down to internal friction in the amorphous materials that are used for the high reflection coatings. To properly characterize the internal friction in an amorphous material, one needs to consider separately the bulk and shear losses. In most of previous works the two loss angles were considered equal, although without any first principle motivation. In this work we present a method that can be used to extract the material bulk and shear loss angles, based on current state-of-the-art coating ring-down measurement systems. We also show that for titania-doped tantala, a material commonly used in gravitational-wave detector coatings, the experimental data strongly favor a model with two different and distinct loss angles, over the simpler case of one single loss angle

Structure and morphology of low mechanical loss TiO₂-doped Ta₂O₅

The exceptional stability required from high finesse optical cavities and high precision interferometers is fundamentally limited by Brownian motion noise in the interference coatings of the cavity mirrors. In amorphous oxide coatings these thermally driven fluctuations are dominant in the high index layer compared to those in the low index SiO₂ layer in the stack. We present a systematic study of the evolution of the structural and optical properties of ion beam sputtered TiO₂-doped Ta₂O₅ films with annealing temperature. We show that low mechanical loss in TiO₂-doped Ta₂O₅ with a Ti cation ratio = 0.27 is associated with a material that consists of a homogeneous titanium-tantalum-oxygen mixture containing a low density of nanometer sized Ar-filled voids. When the Ti cation ratio is 0.53, phase separation occurs leading to increased mechanical loss. These results suggest that amorphous mixed oxides with low mechanical loss could be identified by considering the thermodynamics of ternary phase formation

Structure and morphology of low mechanical loss TiO₂-doped Ta₂O₅

The exceptional stability required from high finesse optical cavities and high precision interferometers is fundamentally limited by Brownian motion noise in the interference coatings of the cavity mirrors. In amorphous oxide coatings these thermally driven fluctuations are dominant in the high index layer compared to those in the low index SiO₂ layer in the stack. We present a systematic study of the evolution of the structural and optical properties of ion beam sputtered TiO₂-doped Ta₂O₅ films with annealing temperature. We show that low mechanical loss in TiO₂-doped Ta₂O₅ with a Ti cation ratio = 0.27 is associated with a material that consists of a homogeneous titanium-tantalum-oxygen mixture containing a low density of nanometer sized Ar-filled voids. When the Ti cation ratio is 0.53, phase separation occurs leading to increased mechanical loss. These results suggest that amorphous mixed oxides with low mechanical loss could be identified by considering the thermodynamics of ternary phase formation

Investigation of effects of assisted ion bombardment on mechanical loss of sputtered tantala thin films for gravitational wave interferometers

Reduction of Brownian thermal noise due to mechanical loss in high-reflectivity mirror coatings is critical for improving the sensitivity of future gravitational wave detectors. In these mirrors, the mechanical loss at room temperature is dominated by the high refractive index component, amorphous tantala (Ta₂O₅) or tantala doped with titania (Ti∶Ta₂O₅). Toward the goal of identifying mechanisms that could alter mechanical loss, this work investigates the use of assist ion bombardment in the reactive ion beam sputtering deposition of tantala single layers. Low-energy assist ion bombardment can enhance adatom diffusion. Low-energy assist Ar⁺ and Xe⁺ ion bombardment at different conditions was implemented during deposition to identify trends in the mechanical loss with ion mass, ion energy, and ion dose. It is shown that the atomic structure and bonding states of the tantala thin films are not significantly modified by low-energy assist ion bombardment. The coatings mechanical loss remains unaltered by ion bombardment within errors. Based on an analysis of surface diffusivity, it is shown that the dominant deposition of tantala clusters and limited surface diffusion length of oxygen atoms constrain structural changes in the tantala films. A slower deposition rate coupled with a significant increase in the dose of the low-energy assist ions may provide more favorable conditions to improve adatom diffusivity

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

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

A method for the experimental measurement of bulk and shear loss angles in amorphous thin films

Brownian thermal noise is a limiting factor for the sensitivity of many high precision metrology applications, among other gravitational-wave detectors. The origin of Brownian noise can be traced down to internal friction in the amorphous materials that are used for the high reflection coatings. To properly characterize the internal friction in an amorphous material, one needs to consider separately the bulk and shear losses. In most of previous works the two loss angles were considered equal, although without any first principle motivation. In this work we present a method that can be used to extract the material bulk and shear loss angles, based on current state-of-the-art coating ring-down measurement systems. We also show that for titania-doped tantala, a material commonly used in gravitational-wave detector coatings, the experimental data strongly favor a model with two different and distinct loss angles, over the simpler case of one single loss angle