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1. Frequency stability of cryogenic silicon cavities with semiconductor crystalline coatings

   https://doi.org/10.1364/OPTICA.479462  

   Kedar, Dhruv; Yu, Jialiang; Oelker, Eric; Staron, Alexander; Milner, William_R; Robinson, John_M; Legero, Thomas; Riehle, Fritz; Sterr, Uwe; Ye, Jun (April 2023, Optica)

   State-of-the-art optical oscillators employing cryogenic reference cavities are limited in performance by the Brownian thermal noise associated with the mechanical dissipation of the mirror coatings. Recently, crystalline Al_1−xGa_xAs/GaAs coatings have emerged as a promising candidate for improved coating thermal noise. We present measurements of the frequency noise of two fully crystalline cryogenic reference cavities with Al_0.92Ga_0.08As/GaAs optical coatings. We report on birefringent noise associated with anticorrelated frequency fluctuations between the polarization modes of the crystalline coatings and identify variables that affect its magnitude. Comparing the birefringent noise between the two cryogenic reference cavities reveals a phenomenological set of scalings with intracavity power and mode area. We implement an interrogation scheme that cancels this noise by simultaneous probing of both polarization modes. The residual noise remaining after this cancellation is larger than both cavities’ thermal noise limits but still lower than the instabilities previously measured on equivalent resonators with dielectric coatings. Though the source of these noise mechanisms is unclear, we demonstrate that crystalline coatings can provide stability and sensitivity competitive with resonators employing dielectric coatings. 

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2. Crystallization in Zirconia Film Nano-Layered with Silica

   https://doi.org/10.3390/nano11123444  

   Larsen, Brecken; Ausbeck, Christopher; Bennet, Timothy F.; DeSalvo, Gilberto; DeSalvo, Riccardo; LeBohec, Tugdual; Linker, Seth; Mondin, Marina; Neilson, Joshua (December 2021, Nanomaterials)

   Gravitational waves are detected using resonant optical cavity interferometers. The mirror coatings’ inherent thermal noise and photon scattering limit sensitivity. Crystals within the reflective coating may be responsible for either or both noise sources. In this study, we explored crystallization reduction in zirconia through nano-layering with silica. We used X-ray diffraction (XRD) to monitor crystal growth between successive annealing cycles. We observed crystal formation at higher temperatures in thinner zirconia layers, indicating that silica is a successful inhibitor of crystal growth. However, the thinnest barriers break down at high temperatures, thus allowing crystal growth beyond each nano-layer. In addition, in samples with thicker zirconia layers, we observe that crystallization saturates with a significant portion of amorphous material remaining. 

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3. Strategies to reduce the thermoelastic loss of multimaterial coated finite substrates

   https://doi.org/10.1088/1361-6382/acdd49  

   Zhou, R; Molina-Ruiz, M; Hellman, F (June 2023, Classical and Quantum Gravity)

   Abstract Thermoelastic loss is an important energy dissipation mechanisms in resonant systems. A careful analysis of the thermoelastic loss is critical to the design of low-noise devices for high-precision applications, such as the mirrors used for gravitational-wave (GW) detectors. In this paper, we present analytical solutions to the thermoelastic loss due to thermoelasticity between different materials that are in contact. We find expressions for the thermoelastic loss of multimaterial coatings of finite substrates, and analyze its dependencies on material properties, mirror design and operating experimental conditions. Our results show that lower operating mirror temperature, thinner layers and higher number of interfaces in the coating, and the choice of the first layer of the coating that minimizes the thermal expansion mismatch with the substrate are strategies that reduce the thermoelastic loss and, therefore, diminish the thermal noise that limits the resolution in sensing applications. The results presented in this paper are relevant for the development of low-noise GW detectors and for other experiments sensitive to energy dissipation mechanisms when different materials are in contact. 

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4. Prediction of crystallized phases of amorphous Ta2O5-based mixed oxide thin films using a density functional theory database

   https://doi.org/10.1063/5.0035573  

   Fazio, Mariana_A; Yang, Le; Menoni, Carmen_S (March 2021, APL Materials)

   The genomics approach to materials, heralded by increasingly accurate density functional theory (DFT) calculations conducted on thousands of crystalline compounds, has led to accelerated material discovery and property predictions. However, so far, amorphous materials have been largely excluded from this as these systems are notoriously difficult to simulate. Here, we study amorphous Ta2O5 thin films mixed with Al2O3, SiO2, Sc2O3, TiO2, ZnO, ZrO2, Nb2O5, and HfO2 to identify their crystalline structure upon post-deposition annealing in air both experimentally and with simulations. Using the Materials Project open database, phase diagrams based on DFT calculations are constructed for the mixed oxide systems and the annealing process is evaluated via grand potential diagrams with varying oxygen chemical potential. Despite employing calculations based on crystalline bulk materials, the predictions agree well with the experimentally observed crystallized phases of the amorphous thin films. In the absence of ternary phases, the dopant acts as an amorphizer agent increasing the thermal stability of Ta2O5. The least efficient amorphizer agent is found to be Nb2O5, for which the cation has similar chemical properties to those of Ta in Ta2O5. These results show that DFT calculations can be applied for the prediction of crystallized structures of annealed amorphous materials. This could pave the way for accelerated in silico material discovery and property predictions using the powerful genomic approach for amorphous oxide coatings employed in a wide range of applications such as optical coatings, energy storage, and electronic devices. 

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5. Mechanical loss reduction at low temperature after crystallization in a titania-tantala film

   https://doi.org/10.1103/PhysRevD.111.042005  

   Murray, Peter G; Steinlechner, Jessica; Isa, Hafizah Noor; Cummings, Rebecca B; Robie, Raymond; Craig, Kieran; Abernathy, Matthew R; Bassiri, Riccardo; Fejer, Martin M; Hough, James; et al (February 2025, Physical Review D)

   Due to its excellent optical properties, such as low absorption and scattering, amorphous ${\text{Ta}}_2{\mathrm{O}}_5$is commonly used as an optical coating material, often in combination with ${\mathrm{SiO}}_2$layers to produce a highly reflective stack. However, the high mechanical loss of ${\text{Ta}}_2{\mathrm{O}}_5$limits the thermal-noise performance of such coatings when used in precision measurement applications. Doping with ${\mathrm{TiO}}_2$has previously been shown to slightly reduce the mechanical loss, but it is still very high compared to many other materials, particularly at low temperatures. In this paper, we present a detailed study of different heat treatment temperatures and of Ti concentrations of up to nominally 75%. We show a significant mechanical-loss reduction for the mixture with the highest Ti cation content, which crystallized after heat treatment at 500°C. The resulting loss is much lower than that of pure ${\mathrm{TiO}}_2$or that of ${\text{Ta}}_2{\mathrm{O}}_5$after crystallization, making further studies highly interesting, in particular investigations of scattering which may pose a major drawback for optical applications. 

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