Enhancing Titania-Tantala Amorphous Materials as High-Index Layers in Bragg Reflectors of Gravitational-Wave Detectors
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High Resolution Image Download MS PowerPoint Slide Amorphous mixed titania-tantala coatings are key components of Bragg reflectors in gravitational-wave detectors (GWDs). Attaining the lowest possible values of optical absorption and mechanical losses in coatings is of paramount importance for GWDs, and this requires a complex optimization of the coating deposition and postdeposition annealing. We present here a systematic investigation of the optical properties and internal friction of amorphous mixed titania-tantala coatings grown by ion beam sputtering. We consider coatings with six different cation mixing ratios─defined as Ti/(Ti + Ta)─and we study them both in the as-deposited and annealed states. All coatings have been subject to the same annealing of 500 °C for 10 h in air, which is the postdeposition treatment adopted so far for Bragg reflectors in GWD applications. By exploiting spectroscopic ellipsometry data and modeling, along with ancillary techniques, we retrieved the dielectric function of the coatings in a wide spectral range. When varying the mixing ratio and performing the annealing, we find monotonic─in some cases, almost linear─trends for most of the aforementioned properties. Remakably, the postannealing Urbach energy displays a definite minimum for a mixing ratio around 20%, very close to the composition of the coatings showing the lowest optical absorption for GWD applications. We suggest that the observed minimum in the Urbach energy depends not only on the mixing ratio but also on the annealing parameters. On the other hand, the minimum coating loss angle was found to be weakly dependent on the considered measurement frequency and to lie within a rather broad range of Ti content (cation ratios of 21 and 44%), suggesting that the search for an absolute minimum following postdeposition annealing should be rather sought in the study of the best annealing parameters for each specific cation ratio considered. This work constitutes a reference for the optical properties of the amorphous mixed titania-tantala coatings and highlights the relevance of the Urbach energy as an additional parameter to guide the optimization process of materials for high-performing Bragg reflectors.
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The optimal cation ratio Ti/(Ta + Ti) for the performance of an IBS-deposited Bragg reflector is approximately 21%, as determined by ellipsometry on amorphous oxidic thin films.
“The ellipsometry analysis conducted on the amorphous oxidic thin films concluded that the optimal cation ratio is approximately 21%.”
From Metal Oxide Thin Films for Advanced Photonic Applications (2026) · cites this paper as [51]
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Mirror Coating Research and Developments for Current and Future Gravitational-Wave Detectors
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- Authors
- Amato, A., Magnozzi, M., Shcheblanov, N., Lemaître, A., Cagnoli, G., Granata, M., Michel, C., Gemme, G., Pinard, L., & Canepa, M.
- Affiliations
- Lyon 1 Université (FR)
- Istituto Nazionale di Fisica Nucleare, Sezione di Genova (IT)
- Centre National de la Recherche Scientifique (FR)
- Navier, Ecole des Ponts, Univ Gustave Eiffel, CNRS, F-77420Marne-la-Vallée, France
- Université de Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumiére Matière, F-69622Villeurbanne, France
- University of Genoa (IT)
- Corresponding author
- Amato, Alex
- Magnozzi, Michele
- ORCID
- Venue
- ACS Applied Optical Materials
- Publisher
- American Chemical Society (ACS)
- Published
- 2023-01-11
- Volume
- 1
- Issue
- 1
- Pages
- 395-402
- ISSN
- 2771-9855 (ISSN-L)
- OpenAlex
- W4315645330
- Cited by
- 11
- References
- 57
- Citations by year
- 2025: 6 · 2024: 3 · 2023: 2
- FWCI
- 0.76
- Type
- journal article
- Language
- en
- Open access
- hybrid
- doi.org
- Licence
- Locations
- ACS Applied Optical Materials (publishedVersion, cc-by)
- HAL (submittedVersion)
- Research Publications (Maastricht University) (submittedVersion, cc-by)
- CINECA IRIS Institutial Research Information System (University of Genoa) (submittedVersion)
- Padua Research Archive (University of Padova) (submittedVersion)
- Funders
- Agence Nationale de la Recherche (ViSIONs ANR-18-CE08-0023-03)
- Ministero dell'Universit? e della Ricerca (Dipartimenti di Eccellenza 2017-2022 - DIFILAB)
- Virgo Coating R&D Collaboration
- Topics
- Physical Sciences › Materials Science › Surfaces, Coatings and Films › Optical Coatings and Gratings
- Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Pulsars and Gravitational Waves Research
- Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Mechanical and Optical Resonators
- Keywords
- distributed Bragg reflector, gravitational wave detectors, ion beam sputtering, post-deposition annealing, Urbach energy, optical absorption, mechanical losses, dielectric function, loss angle
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