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1. Measured limits on amplitude dependence of mechanical loss in substrate-transferred $\mathrm{Ga}\mathrm{As}/{\mathrm{Al}}_{0.92}{\mathrm{Ga}}_{0.08}\mathrm{As}$ 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. Study on electro-optic noise in crystalline coatings toward future gravitational wave detectors

   https://doi.org/10.1103/PhysRevD.107.022003  

   Tanioka, Satoshi; Vander-Hyde, Daniel; Cole, Garrett D.; Penn, Steven D.; Ballmer, Stefan W. (January 2023, Physical Review D)

   Thermal noise in high-reflectivity mirror coatings is a limiting factor in ground-based gravita-tional wave detectors. Reducing this coating thermal noise improves the sensitivity of detectorsand enriches the scientific outcome of observing runs. Crystalline gallium arsenide and aluminum-alloyed gallium arsenide (referred to as AlGaAs) coatings are promising coating candidates forfuture upgrades of gravitational wave detectors because of their low coating thermal noise. How-ever, AlGaAs-based crystalline coatings may be susceptible to an electro-optic noise induced byfluctuations in an electric field. We investigated the electro-optic effect in an AlGaAs coating byusing a Fabry-Perot cavity, and concluded that the noise level is well below the sensitivity of currentand planned gravitational-wave detectors. 

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3. Experimental demonstration of frequency- downconverted arm-length stabilization for a future upgraded gravitational wave detector

   https://doi.org/10.1364/OL.534141  

   Tanioka, Satoshi; Wu, Bin; Ballmer, Stefan W (October 2024, Optics Letters)

   Ground-based laser interferometric gravitational wave detectors (GWDs) consist of multiple optical cavity systems whose lengths need to be interferometrically controlled. An arm-length stabilization (ALS) system has played an important role in bringing these interferometers into an operational state and enhancing their duty cycle. The sensitivity of these detectors can be improved if the thermal noise of their test mass mirror coatings is reduced. Crystalline AlGaAs coatings are a promising candidate for this. However, the current ALS system with a frequency-doubled 532 nm light is no longer an option with AlGaAs coatings because the 532 nm light is absorbed by AlGaAs coatings due to the narrow bandgap of GaAs. Therefore, alternative locking schemes must be developed. In this Letter, we describe an experimental demonstration of a novel ALS scheme, to the best of our knowledge, which is compatible with AlGaAs coatings. This ALS scheme will enable the use of AlGaAs coatings in current and future terrestrial gravitational wave detectors. 

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4. Effects of mirror birefringence and its fluctuations to laser interferometric gravitational wave detectors

   https://doi.org/10.1103/PhysRevD.109.022009  

   Michimura, Yuta; Wang, Haoyu; Salces-Carcoba, Francisco; Wipf, Christopher; Brooks, Aidan; Arai, Koji; Adhikari, Rana X. (January 2024, Physical Review D)

   Crystalline materials are promising candidates as substrates or high-reflective coatings of mirrors to reduce thermal noises in future laser interferometric gravitational wave detectors. However, birefringence of such materials could degrade the sensitivity of gravitational wave detectors, not only because it can introduce optical losses, but also because its fluctuations create extra phase noise in the arm cavity reflected beam. In this paper, we analytically estimate the effects of birefringence and its fluctuations in the mirror substrate and coating for gravitational wave detectors. 

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5. 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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