Atomic Structure of Amorphous Optical Coatings of TiO$_2$-doped GeO$_2$ by Grazing-Incidence Total X-ray Scattering Measurements

K. Prasai; J. Jiang; K. Lee; L. Yang; M. Chicoine; S. Khadka; A. Markosyan; A. Mehta; C. S. Menoni; S. Patel; F. Schiettekatte; B. Shyam; G. Vajente; H-P Cheng; M. M. Fejer; R. Bassiri

arXiv · 2605.27462

Atomic Structure of Amorphous Optical Coatings of TiO$_2$-doped GeO$_2$ by Grazing-Incidence Total X-ray Scattering Measurements

K. Prasai·J. Jiang·K. Lee·L. Yang·M. Chicoine·S. Khadka·A. Markosyan·A. Mehta·C. S. Menoni·S. Patel·F. Schiettekatte·B. Shyam·G. Vajente·H-P Cheng·M. M. Fejer·R. Bassiri

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Abstract

Reducing coating thermal noise in future gravitational-wave detectors requires identifying the atomic motifs that control mechanical loss in amorphous optical coatings. We combine grazing-incidence X-ray pair distribution function measurements with atomic-structure modeling to study amorphous TiO$_2$-doped GeO$_2$ films over Ti cation concentrations from $\sim$11 % to $\sim$48 %, before and after annealing. The structural analysis reveals systematic composition- and annealing-dependent changes in short- and intermediate-range order. Increasing Ti content raises the average Ti coordination and promotes edge- and face-sharing polyhedral connections, while Ge remains predominantly fourfold coordinated. Annealing reduces these compact shared-polyhedron motifs and sharpens the first sharp diffraction peak, indicating a more relaxed intermediate-range network. Among the structural descriptors examined, the clearest correlation with the annealing-induced reduction in mechanical loss is the decrease in edge- and face-sharing polyhedra. These results connect composition, annealing, atomic structure, and mechanical dissipation in TiO$_2$-doped GeO$_2$, providing microscopic guidance for optimizing low-noise mirror coatings.

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K. Prasai, J. Jiang, K. Lee, L. Yang, M. Chicoine, S. Khadka, A. Markosyan, A. Mehta, C. S. Menoni, S. Patel, F. Schiettekatte, B. Shyam, G. Vajente, H-P Cheng, M. M. Fejer, R. Bassiri. 2026-05-25. Atomic Structure of Amorphous Optical Coatings of TiO$_2$-doped GeO$_2$ by Grazing-Incidence Total X-ray Scattering Measurements. https://arxiv.org/abs/2605.27462

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