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Elastic and Structural Anisotropy in Silica Thin Films for Gravitational-Wave Detectors

Brenda Bracco1,2,*, Michele Magnozzi3,4, Stefano Colace3, Maurizio Canepa3, Giulio Favaro5,6, Marco Bazzan6, Massimo Granata7, David Hofman7, Alessandro Di Michele8,2 et al.

Laura Silenzi9,2, Gianpietro Cagnoli10, Giovanni Carlotti8, Paola Sassi1,2,†, and Silvia Corezzi8,2,*,‡

  • *These authors contributed equally to this work.
  • †Contact author: paola.sassi@unipg.it
  • ‡Contact author: silvia.corezzi@unipg.it
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Phys. Rev. X 16, 021036 – Published 15 May, 2026

DOI: https://doi.org/10.1103/fmrf-dys9

Abstract

The thermal noise of mirror coatings for gravitational-wave detectors critically depends on the elastic properties of the constituent materials. Data analyses and theoretical models typically assume each material is homogeneous and isotropic, but isotropy has never been explicitly verified. Using Brillouin light scattering (BLS), we demonstrate that ion-beam-sputtered SiO2—a material still viable for future mirror coatings—exhibits cylindrical elastic symmetry, with in-plane isotropy but a notable 6% compressive anisotropy along the film normal. This anisotropy remains unchanged after the postdeposition heat treatment currently used in ground-based detectors (500 °C, 10 h) but is nearly eliminated at 900 °C. Infrared reflectivity experiments support these findings by directly revealing heterogeneities in the distribution of bridging and nonbridging oxygen structures along the growth axis. While BLS measures the real part of the elastic constants at gigahertz frequencies, the data reveal negligible contributions from mechanical relaxations in the kilohertz to gigahertz range, making BLS a valid substitute for low-frequency properties obtained from standard anisotropy-insensitive techniques. Our results highlight that restoring isotropy through heat treatment—by softening the material, enabling more than 7% out-of-plane expansion, and smoothing out structural heterogeneities—may play a key role in reducing thermal noise. This proof-of-concept study extends beyond silica, providing critical insights for the design of future coatings.

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Popular Summary

Thermal noise in mirror coatings is a primary factor limiting the sensitivity of interferometric gravitational-wave detectors. Using Brillouin light scattering and infrared reflectivity, we demonstrate that ion-beam sputtered silica thin films exhibit significant elastic and structural anisotropy, characterized by a difference between in-plane and out-of-plane elastic response. Our results show that standard postdeposition heat treatments fail to eliminate this anisotropy—which is suppressed only at much higher temperatures—and establish a direct link between restoring isotropy and thermal noise reduction. These insights challenge the long-standing assumption of material isotropy and provide a critical pathway for designing next-generation, high-performance optical coatings.

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Phys. Rev. X 16, 021036– Published 15 May, 2026
  • Received 25 March 2025
  • Accepted 6 March 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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