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Investigation on the structural, mechanical and optical properties of amorphous oxide thin films for gravitational wave detectors

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dc.contributor.authorCastro Lucas, Samuel, author
dc.contributor.authorMenoni, Carmen, advisor
dc.contributor.authorRocca, Jorge, committee member
dc.contributor.authorSambur, Justin, committee member
dc.date.accessioned2024-05-27T10:31:55Z
dc.date.available2024-05-27T10:31:55Z
dc.date.issued2024
dc.description.abstractAmorphous oxide thin films grown through physical vapor deposition methods like ion beam sputtering, play a crucial role in optical interference coatings for high finesse optical cavities, such as those used in gravitational wave detectors. The stability of these atomically disordered solids is significantly influenced by both deposition conditions and composition. Consequently, these enable the tuning of structural, mechanical, or optical properties. The sensitivity of current gravitational wave interferometric detectors at the frequency range of around 100 Hz is currently limited by a combination of quantum and coating thermal noise (CTN). CTN is associated with thermally driven random displacement fluctuations in the high reflectance amorphous oxide coatings of the end-test masses in the interferometer. These fluctuations cause internal friction, acting as an anelastic relaxation mechanism by dissipating elastic energy. The dissipated internal elastic energy can be quantified through the mechanical loss angle (Q-1). These unwanted fluctuations associated with mechanical loss can be reduced through modifications of the atomic network in the amorphous oxides. Specifically, the combination of two or more metal cations in a mixed amorphous thin film and post-deposition annealing are known to favorably impact the network organization and hence reduce internal friction. The first study of this thesis reports on the structural modifications between amorphous TiO2 with GeO2 and with SiO2. High-index materials for gravitational wave detectors such as amorphous TiO2:GeO2 (44% Ti), have been found to exhibit low mechanical loss post-annealing at 600°C. Reaffirming annealing to be a major contributor to reducing mechanical loss this thesis examines: a) cation interdiffusion between amorphous oxides of TiO2 with GeO2 and with SiO2 and b) the modifications to the structural properties, both after annealing. The annealing temperature, at which this interdiffusion mechanism occurs, is key for pinpointing structural rearrangements that are favorable for reducing internal friction. Furthermore, to determine whether diffusion occurs into SiO2 after annealing is also important, given that the multi-layer mirrors of gravitational wave detectors utilize SiO2 as a low-index layer. The study of cation interdiffusion used nanolaminates of TiO2, SiO2 and GeO2 to identify cation diffusion across the interface. The results show Ge and Ti cation interfacial diffusion, at temperatures above 500°C. Instead, Si cations diffuse into TiO2 at a temperature around 850°C and Ti into SiO2 at around 950°C. These temperatures correspond to an average of 0.8 of the glass transition temperature (Tg), with Tg=606°C for GeO2 and Tg=1187°C for SiO2. These findings support previous research by our group in amorphous GeO2, which showed that elevated temperature deposition and annealing at 0.8 Tg, leads to favorable organization of the atomic network which is associated with low mechanical loss. The second study of this thesis investigates the structural, mechanical, and optical properties of amorphous ternary oxide mixtures following post-annealing. These mixtures consist of TiO2:GeO2 combined with SiO2 and ZrO2, as well as TiO2:SiO2 combined with ZrO2. Candidate high index layers, such as amorphous TiO2:GeO2 (44% Ti), and TiO2:SiO2 (69.5% Ti) exhibit low mechanical loss after post-annealing at 600°C, and 850°C, respectively. The inclusion of a third metal cation is shown to delay the onset of crystallization to temperatures around 800°C. The addition of a third metal cation also modifies the residual stress of the ternary compared to the binary materials. There is an indication of densification when annealing past 600°C. The reduction in residual tensile stress, combined with the higher crystallization temperature of the ternary mixtures, present attractive properties. These properties will expand the parameter space for post-deposition processing, mainly of the TiO2:GeO2 -based mixtures, to further reduce mechanical loss. This advancement paves the way for amorphous oxide coatings for gravitational wave detectors with lower mechanical loss, aligning with plans for future detectors.
dc.format.mediumborn digital
dc.format.mediummasters theses
dc.identifierCastroLucas_colostate_0053N_18255.pdf
dc.identifier.urihttps://hdl.handle.net/10217/238388
dc.languageEnglish
dc.language.isoeng
dc.publisherColorado State University. Libraries
dc.relation.ispartof2020-
dc.rightsCopyright and other restrictions may apply. User is responsible for compliance with all applicable laws. For information about copyright law, please see https://libguides.colostate.edu/copyright.
dc.subjectcation interdiffusion
dc.subjectgravitational waves
dc.subjectternary mixture
dc.subjectcoating thermal noise
dc.subjectamorphous oxides
dc.subjectstructural modifications
dc.titleInvestigation on the structural, mechanical and optical properties of amorphous oxide thin films for gravitational wave detectors
dc.typeText
dcterms.rights.dplaThis Item is protected by copyright and/or related rights (https://rightsstatements.org/vocab/InC/1.0/). You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).
thesis.degree.disciplineElectrical and Computer Engineering
thesis.degree.grantorColorado State University
thesis.degree.levelMasters
thesis.degree.nameMaster of Science (M.S.)

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