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1. Measured limits on amplitude dependence of mechanical loss in substrate-transferred GaAs/Al0.92Ga0.08As coatings

   https://doi.org/10.1103/PhysRevD.106.042001  

   Gretarsson, Elizabeth M. ; Gretarsson, Andri M. ; Cole, Garrett D. ; Harry, Gregory M. ; Kinley-Hanlon, Maya M. ; Jones, R. Jason ; Penn, Steven D. ( August 2022 , Physical Review D)

   AIP (Ed.)

   In this Perspective, we summarize the status of technological development for large-area and low-noise substrate-transferred GaAs/AlGaAs (AlGaAs) crystalline coatings for interferometric gravitational-wave (GW) detectors. These topics were originally presented as part of an AlGaAs Workshop held at American University, Washington, DC, from 15 August to 17 August 2022, bringing together members of the GW community from the laser interferometer gravitational-wave observatory (LIGO), Virgo, and KAGRA collaborations, along with scientists from the precision optical metrology community, and industry partners with extensive expertise in the manufacturing of said coatings. AlGaAs-based crystalline coatings present the possibility of GW observatories having significantly greater range than current systems employing ion-beam sputtered mirrors. Given the low thermal noise of AlGaAs at room temperature, GW detectors could realize these significant sensitivity gains while potentially avoiding cryogenic operation. However, the development of large-area AlGaAs coatings presents unique challenges. Herein, we describe recent research and development efforts relevant to crystalline coatings, covering characterization efforts on novel noise processes as well as optical metrology on large-area (∼10 cm diameter) mirrors. We further explore options to expand the maximum coating diameter to 20 cm and beyond, forging a path to produce low-noise mirrors amenable to future GW detector upgrades, while noting the unique requirements and prospective experimental testbeds for these semiconductor-based coatings. 

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2. 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)

   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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3. Clipping losses from a piecewise coated mirror

   https://doi.org/10.1088/1361-6382/acdc01  

   Kontos, Antonios ; Loglia, Balthazar ( June 2023 , Classical and Quantum Gravity)

   The properties of optical coatings play an important role in precision experiments. Gravitational-wave detectors not only require the highest quality coatings but also ones with relatively large diameters. Coatings made with amorphous material can be scaled up easily but for now exhibit relatively high thermal noise characteristics. Crystalline coatings show great promise in terms of their thermal noise properties but cannot easily scale up to the needed sizes. In this paper we explore the possibility of a piecewise coating that includes both amorphous and crystalline material. Specifically, we estimate the scattering losses of such a piecewise coating as a function of the mismatch in the interface between the two coatings. The calculation should be taken as a lower limit to the total losses, as other surface imperfections will play an important role in the final result, but are not considered here. Finally, we present a measurement of the scattering losses from a piecewise coating, to showcase the challenges of realizing such a design.

    

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4. Hydrogen-Induced Ultralow Optical Absorption and Mechanical Loss in Amorphous Silicon for Gravitational-Wave Detectors

   https://doi.org/10.1103/PhysRevLett.131.256902  

   Molina-Ruiz, M. ; Markosyan, A. ; Bassiri, R. ; Fejer, M. M. ; Abernathy, M. ; Metcalf, T. H. ; Liu, X. ; Vajente, G. ; Ananyeva, A. ; Hellman, F. ( December 2023 , Physical Review Letters)

   The sensitivity of gravitational-wave detectors is limited by the mechanical loss associated with the amorphous coatings of the detectors’ mirrors. Amorphous silicon has higher refraction index and lower mechanical loss than current high-index coatings, but its optical absorption at the wavelength used for the detectors is at present large. The addition of hydrogen to the amorphous silicon network reduces both optical absorption and mechanical loss for films prepared under a range of conditions at all measured wavelengths and temperatures, with a particularly large effect on films grown at room temperature. The uptake of hydrogen is greatest in the films grown at room temperature, but still below 1.5 at.% H, which show an ultralow optical absorption (below 10 ppm) measured at 2000 nm for 500-nm-thick films. These results show that hydrogenation is a promising strategy to reduce both optical absorption and mechanical loss in amorphous silicon, and may enable fabrication of mirror coatings for gravitational-wave detectors with improved sensitivity. 

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5. Exploration of co-sputtered Ta 2 O 5 –ZrO 2 thin films for gravitational-wave detectors

   https://doi.org/10.1088/1361-6382/ac1b06  

   Abernathy, M ; Amato, A ; Ananyeva, A ; Angelova, S ; Baloukas, B ; Bassiri, R ; Billingsley, G ; Birney, R ; Cagnoli, G ; Canepa, M ; et al ( September 2021 , Classical and Quantum Gravity)

   Abstract We report on the development and extensive characterization of co-sputtered tantala–zirconia (Ta 2 O 5 -ZrO 2 ) thin films, with the goal to decrease coating Brownian noise in present and future gravitational-wave detectors. We tested a variety of sputtering processes of different energies and deposition rates, and we considered the effect of different values of cation ratio η = Zr/(Zr + Ta) and of post-deposition heat treatment temperature T a on the optical and mechanical properties of the films. Co-sputtered zirconia proved to be an efficient way to frustrate crystallization in tantala thin films, allowing for a substantial increase of the maximum annealing temperature and hence for a decrease of coating mechanical loss φ c . The lowest average coating loss was observed for an ion-beam sputtered sample with η = 0.485 ± 0.004 annealed at 800 °C, yielding φ ¯ c = 1.8 × 1 0 − 4 rad. All coating samples showed cracks after annealing. Although in principle our measurements are sensitive to such defects, we found no evidence that our results were affected. The issue could be solved, at least for ion-beam sputtered coatings, by decreasing heating and cooling rates down to 7 °C h −1 . While we observed as little optical absorption as in the coatings of current gravitational-wave interferometers (0.5 parts per million), further development will be needed to decrease light scattering and avoid the formation of defects upon annealing. 

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