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1. 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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2. Frequency Stability of $2.5\times {10}^{-17}$ from a Si Cavity with AlGaAs Crystalline Mirrors

   https://doi.org/10.1103/zgrm-cjbb  

   Lee, Dahyeon; Hu, Zoey Z; Lewis, Ben; Aeppli, Alexander; Kim, Kyungtae; Yao, Zhibin; Legero, Thomas; Nicolodi, Daniele; Riehle, Fritz; Sterr, Uwe; et al (January 2026, Physical Review Letters)

   Developments in ultrastable lasers have fueled remarkable advances in optical frequency metrology and quantum science. A key ingredient in further improving laser frequency stability is the use of low-noise mirror materials such as AlGaAs crystalline coatings. However, excess noise observed with these coatings limits the performance of cryogenic silicon cavities with AlGaAs mirrors to similar levels achieved with conventional dielectric coatings. With a new pair of crystalline coated mirrors in a 6-cm-long cryogenic silicon cavity operated at 17 K, we demonstrate a clear advantage of crystalline coatings over dielectric coatings. The achieved fractional frequency stability of 2.5 × 10−17 at 10 s is four times better than expected for dielectric mirrors and corresponds to more than a tenfold reduction in the coating mechanical loss factor. We also combine two silicon cavities to demonstrate optical frequency averaging for enhanced stability. In addition, we present a long-term frequency drift record of four cryogenic silicon cavities measured over several years. These results open up realistic prospects for cavity-stabilized lasers with 10−18 fractional stability, as well as an all-optical timescale with continuously operating optical local oscillators. 

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3. Nonuniform birefringence in highly reflective substrate-transferred ${\mathrm{GaAs}/\mathrm{Al}}_{0.92}{\mathrm{Ga}}_{0.08}\mathrm{As}$ coatings at 1064 nm

   https://doi.org/10.1103/xtsf-t8y4  

   Gretarsson, Andri M; Juston, Ambroise_L M; Nicolai, Benjamin; Borg, Naomi; Meagher, Breck N; Cole, Garrett D; Billingsley, GariLynn; Makarem, Camille N; Gretarsson, Elizabeth M; Harry, Gregory M; et al (January 2026, Physical Review Research)

   Using a custom-built scanning system, we generated maps of birefringence on reflection at $\lambda =1064$ nm from single-crystal GaAs/ ${\mathrm{Al}}_{0.92}{\mathrm{Ga}}_{0.08}\mathrm{As}$ Bragg reflectors (henceforth “AlGaAs coatings”). Ten coatings were bonded to fused silica substrates and one remained on the epitaxial growth wafer. The average phase difference on reflection between beams polarized along the fast and slow axes of the coating was found to be $\psi =1.09\pm 0.18$ mrad, consistent with values observed in high-finesse optical reference cavities using similar AlGaAs coatings. Scans of substrate-transferred coatings with diameters between 18 and 194 mm showed birefringence nonuniformity at a median level of $0.1$ mrad. A similar epitaxial multilayer that was not substrate transferred, but remained on the growth wafer, had by far the least birefringence nonuniformity of all mirrors tested at $0.02$ mrad. On the other hand, the average birefringence of the epi-on-wafer coating and substrate-transferred coatings was indistinguishable. Excluding nonuniformity found at the location of crystal and bonding defects, we conclude that the observed nonuniformity was imparted during the substrate transfer process, likely during bonding. Quantifying the impact on the scatter loss in a LIGO (Laser Interferometer Gravitational wave Observatory) interferometer, we find that birefringence nonuniformity at the levels seen here is unlikely to have a significant impact on performance. Nonetheless, future efforts will focus on improved process control to minimize and ultimately eliminate the observed nonuniformity. 

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4. 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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5. Low mechanical loss and high refractive index in amorphous ${\mathrm{Ta}}_2{\mathrm{O}}_5$ films grown by magnetron sputtering

   https://doi.org/10.1103/PhysRevMaterials.8.035603  

   Molina-Ruiz, M; Shukla, K; Ananyeva, A; Vajente, G; Abernathy, M R; Metcalf, T H; Liu, X; Markosyan, A; Bassiri, R; Fejer, M M; et al (March 2024, Physical Review Materials)

   The ability to observe astronomical events through the detection of gravitational waves relies on the quality of multilayer coatings used on the optical mirrors of interferometers. Amorphous Ta2O5 (including TiO2:Ta2O5) currently limits detector sensitivity due to high mechanical loss. In this paper, mechanical loss measured at both cryogenic and room temperatures of amorphous Ta2O5 films grown by magnetron sputtering and annealed in air at 500 ◦C is shown to decrease for elevated growth temperature. Films grown at 310 ◦C and annealed yield a mechanical loss of 3.1×10−4 at room temperature, the lowest value reported for pure amorphous Ta2O5 grown by magnetron sputtering to date, and comparable to the lowest values obtained for films grown by ion beam sputtering. Additionally, the refractive index n increases 6% for elevated growth temperature, which could lead to improved sensitivity of gravitational-wave detectors by allowing a thickness reduction in the mirrors’ coatings. Structural characterization suggests that the observed mechanical loss reduction in amorphous Ta2O5 films with increasing growth temperature correlates with a reduction in the coordination number between oxygen and tantalum atoms, consistent with TaOx polyhedra with increased corner-sharing and reduced edge- and facesharing structures. 

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