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1. 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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2. Effects of elevated-temperature deposition on the atomic structure of amorphous Ta2O5 films

   https://doi.org/10.1063/5.0170100  

   Prasai, K; Lee, K; Baloukas, B; Cheng, H-P; Fazio, M; Martinu, L; Mehta, A; Menoni, C S; Schiettekatte, F; Shink, R; et al (December 2023, APL Materials)

   Brownian thermal noise as a result of mechanical loss in optical coatings will become the dominant source of noise at the most sensitive frequencies of ground-based gravitational-wave detectors. Experiments found, however, that a candidate material, amorphous Ta2O5, is unable to form an ultrastable glass and, consequently, to yield a film with significantly reduced mechanical loss through elevated-temperature deposition alone. X-ray scattering PDF measurements are carried out on films deposited and subsequently annealed at various temperatures. Inverse atomic modeling is used to analyze the short and medium range features in the atomic structure of these films. Furthermore, in silico deposition simulations of Ta2O5 are carried out at various substrate temperatures and an atomic level analysis of the growth at high temperatures is presented. It is observed that upon elevated-temperature deposition, short range features remain identical, whereas medium range order increases. After annealing, however, both the short and medium range orders of films deposited at different substrate temperatures are nearly identical. A discussion on the surface diffusion and glass transition temperatures indicates that future pursuits of ultrastable low-mechanical-loss films through elevated temperature deposition should focus on materials with a high surface mobility, and/or lower glass transition temperatures in the range of achievable deposition temperatueres. 

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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. Network structure of amorphous ${\mathrm{TiO}}_2$ -doped ${\mathrm{GeO}}_2$ via atomistic model and simulations of the Raman activity

   https://doi.org/10.1103/7sch-kndg  

   Zhang, Rui; Osovsky, Ruth; Jiang, Jun; Fry, James N; Fejer, Martin M; Menoni, Carmen S; Cheng, Hai-Ping (January 2026, Physical Review Materials)

   In the pursuit of identifying improved amorphous oxide coatings for gravitational wave detector test masses, 44% Ti-doped GeO2 has emerged as a potential candidate due to its low mechanical loss [Vajente et al. Phys. Rev. Lett. 127, 071101 (2021)]. It has been proposed and experimentally demonstrated that a reduction in mechanical loss in amorphous oxides is associated with medium-range order (MRO) dominated by cornersharing links in the amorphous network. In this work, we describe a combined experimental and theoretical approach to investigate the MRO of Ti-doped GeO2. Using atomistic modeling and simulations, we calculate the Raman spectrum of 44% Ti-doped GeO2, and we show that it predicts the main features observed in the experiments and further provides insight into the MRO, which would otherwise be hidden. It is shown that the amorphous oxide mixture exhibits a distribution of predominantly large (>4 member) rings formed by corner-shared tetrahedral Ge-O-Ti connections. It is also shown that the addition of Ti to a-GeO2 does not largely compromise corner-sharing connections, as the smaller populations of Ti-O-Ti and Ge-O-Ge links in the network are predominantly corner-shared. The mainly covalently bonded representation of the MRO of Ti-doped GeO2 revealed by this analysis validates the importance of the MRO network organization in influencing mechanical loss in amorphous oxides. 

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5. 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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