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1. Thermal noise and mechanical loss of SiO ₂ /Ta ₂ O ₅ optical coatings at cryogenic temperatures

   https://doi.org/10.1364/OL.413758  

   Robinson, John_M; Oelker, Eric; Milner, William_R; Kedar, Dhruv; Zhang, Wei; Legero, Thomas; Matei, Dan_G; Häfner, Sebastian; Riehle, Fritz; Sterr, Uwe; et al (January 2021, Optics Letters)

   Mechanical loss of dielectric mirror coatings sets fundamental limits for both gravitational wave detectors and cavity-stabilized optical local oscillators for atomic clocks. Two approaches are used to determine the mechanical loss: ringdown measurements of the coating quality factor and direct measurement of the coating thermal noise. Here we report a systematic study of the mirror thermal noise at 4, 16, 124, and 300 K by operating reference cavities at these temperatures. The directly measured thermal noise is used to extract the mechanical loss for ${\mathrm{S}\mathrm{i}\mathrm{O}}_2/{\mathrm{T}\mathrm{a}}_2{\mathrm{O}}_5$coatings, which are compared with previously reported values. 

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2. High-accuracy numerical models of Brownian thermal noise in thin mirror coatings

   https://doi.org/10.1088/1361-6382/acad62  

   Vu, Nils L.; Rodriguez, Samuel; Włodarczyk, Tom; Lovelace, Geoffrey; P Pfeiffer, Harald; S Bonilla, Gabriel; Deppe, Nils; Hébert, François; E Kidder, Lawrence; Moxon, Jordan; et al (January 2023, Classical and Quantum Gravity)

   Abstract Brownian coating thermal noise in detector test masses is limiting the sensitivity of current gravitational-wave detectors on Earth. Therefore, accurate numerical models can inform the ongoing effort to minimize Brownian coating thermal noise in current and future gravitational-wave detectors. Such numerical models typically require significant computational resources and time, and often involve closed-source commercial codes. In contrast, open-source codes give complete visibility and control of the simulated physics, enable direct assessment of the numerical accuracy, and support the reproducibility of results. In this article, we use the open-sourceSpECTREnumerical relativity code and adopt a novel discontinuous Galerkin numerical method to model Brownian coating thermal noise. We demonstrate thatSpECTREachieves significantly higher accuracy than a previous approach at a fraction of the computational cost. Furthermore, we numerically model Brownian coating thermal noise in multiple sub-wavelength crystalline coating layers for the first time. Our new numerical method has the potential to enable fast exploration of realistic mirror configurations, and hence to guide the search for optimal mirror geometries, beam shapes and coating materials for gravitational-wave detectors. 

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3. 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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4. Substrate-transferred GaAs/AlGaAs crystalline coatings for gravitational-wave detectors

   https://doi.org/10.1063/5.0140663  

   Cole, G_D; Ballmer, S_W; Billingsley, G.; Cataño-Lopez, S_B; Fejer, M.; Fritschel, P.; Gretarsson, A_M; Harry, G_M; Kedar, D.; Legero, T.; et al (March 2023, Applied Physics Letters)

   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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5. AlScN-on-SiC microelectromechanical Lamb wave resonators operating at high temperature up to 800 °C

   https://doi.org/10.1063/5.0185606  

   Sui, Wen; Feng, Philip X-L (July 2024, Applied Physics Letters)

   We report on the experimental demonstration of aluminum scandium nitride (AlScN)-on-cubic silicon carbide (3C-SiC) Lamb wave resonators (LWRs) realized via microelectromechanical systems (MEMS) technology, operating at high temperature (T) up to T = 800 °C, while retaining robust electromechanical resonances at ∼27 MHz and good quality factor of Q ≈ 900 even at 800 °C. Measured resonances exhibit clear consistency and stability during heating and cooling processes, validating the AlScN-on-SiC LWRs can operate at high T up to 800 °C without noticeable degradation in moderate vacuum (∼20 mTorr). Even after undergoing four complete thermal cycles (heating from 23 to 800 °C and then cooling down to 23 °C), the devices exhibit robust resonance behavior, suggesting excellent stability and suitability for high-temperature applications. Q starts to decline as the temperature exceeds 400 °C, which can be attributed to energy dissipation mechanisms stemming from thermoelastic damping and intrinsic material loss originating from phonon–phonon interactions. 

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