François Schiettekatte

Cosmic Explorer is a next-generation ground-based gravitational-wave observatory that is being designed in the 2020s and is envisioned to begin operations in the 2030s together with the Einstein Telescope in Europe. The Cosmic Explorer concept currently consists of two widely separated L-shaped observatories in the United States, one with 40 km-long arms and the other with 20 km-long arms. This order of magnitude increase in scale with respect to the LIGO-Virgo-KAGRA observatories will, together with technological improvements, deliver an order of magnitude greater astronomical reach, allowing access to gravitational waves from remnants of the first stars and opening a wide discovery aperture to the novel and unknown. In addition to pushing the reach of gravitational-wave astronomy, Cosmic Explorer endeavors to approach the lifecycle of large scientific facilities in a way that prioritizes mutually beneficial relationships with local and Indigenous communities. This article describes the (scientific, cost and access, and social) criteria that will be used to identify and evaluate locations that could potentially host the Cosmic Explorer observatories.

Cosmic Explorer (CE) is a next-generation ground-based gravitational-wave observatory that is being designed in the 2020s and is envisioned to begin operations in the 2030s together with the Einstein Telescope in Europe. The CE concept currently consists of two widely separated L-shaped observatories in the United States, one with 40 km-long arms and the other with 20 km-long arms. This order of magnitude increase in scale with respect to the LIGO-Virgo-KAGRA observatories will, together with technological improvements, deliver an order of magnitude greater astronomical reach, allowing access to gravitational waves from remnants of the first stars and opening a wide discovery aperture to the novel and unknown. In addition to pushing the reach of gravitational-wave astronomy, CE endeavors to approach the lifecycle of large scientific facilities in a way that prioritizes mutually beneficial relationships with local and Indigenous communities. This article describes the (scientific, cost and access, and social) criteria that will be used to identify and evaluate locations that could potentially host the CE observatories.

One of the main noise sources in current gravitational-wave detectors is the thermal noise of the high-reflectivity coatings on the main interferometer optics. Coating thermal noise is dominated by the mechanical loss of the high-refractive index material within the coating stacks, Ta 2 O 5 mixed with TiO 2 . For upgrades to room-temperature detectors, a mixture of GeO 2 and TiO 2 is an interesting alternative candidate coating material. While the rather low refractive index of GeO₂ increases with increasing TiO 2 content, a higher TiO 2 content results in a lower threshold temperature before heat treatment leads to crystallisation, and potentially to a degradation of optical properties. For future cryogenic detectors, on the other hand, a higher TiO 2 content is beneficial as the TiO₂ suppresses the low-temperature mechanical loss peak of GeO₂. In this paper, we present the optical properties of coatings—produced by plasma-assisted ion-beam evaporation—with high TiO₂ content at 1550 nm, a laser wavelength considered for cryogenic gravitational-wave detectors, as a function of heat-treatment temperature. For comparison, the absorption of pure GeO₂ was also measured. Furthermore, results at the currently-used wavelength of 1064 nm are presented.

Blistering is a phenomenon sometimes observed in sputtered-deposited thin films but seldom investigated in detail. Here, we consider the case of titania-doped germania (TGO)/silica multilayers deposited by ion beam sputtering. TGO is a candidate as high refractive index material in the Bragg mirrors for the next iteration of gravitational waves detectors. It needs to be annealed at 600 ^∘C for 100 h in order to reach the desired relaxation state. However under some growth conditions, in 52-layer TGO/silica stacks, blistering occurs upon annealing at a temperature near 500 ^∘C, which corresponds to the temperature where Ar desorbs from TGO. In order to better understand the blistering phenomenon, we measure the Ar transport in single layers of TGO and silica. In the case of < 1 µm-thick TGO layers, the Ar desorption is mainly limited by detrapping. The transport model also correctly predicts the evolution of the total amount of Ar in a 8.5 µm stack of TGO and silica layers annealed at 450 ^∘C, but in that case, the process is mainly limited by diffusion. Since Ar diffusion is an order of magnitude slower in TGO compared to silica, we observe a correspondingly strong accumulation of Ar in TGO. The Ar transport model is used to explain some regimes of the blisters growth, and we find indications that Ar accumulation is a driver for their growth in general, but the blisters nucleation remains a complex phenomenon influenced by several other factors including stress, substrate roughness, and impurities.

Amorphous thin films of Ti doped GeO₂ are of interest for coatings of the mirrors in gravitational wave detectors (GWDs) due to their low internal friction (Vajente et al 2021 Phys. Rev. Lett. 127 071101). The addition of Ti to amorphous GeO₂ (a-GeO₂) enables tailoring of the optical and structural properties of the mixtures. However, the specific modifications that occur in the amorphous network with the addition of Ti are not known. In this work, x-ray photoelectron spectroscopy is used to identify modifications to the bonding of Ge and Ti atoms in mixtures of Ti doped a-GeO₂ with different Ti cation content. The formation of (Ti–O–Ge) bonds is evidenced from: (1) the presence of a peak which intensity increases with Ti content and causes a shift to lower binding energy (BE) of the core level O 1s peak; (2) the shift to higher BE of the Ti 2p _3/2 peak and a decrease in the energy split; and (3) the shift to lower BE of the Ge 3d _5/2 peak and increase in the energy split. These changes reflect modifications to the bonding when Ge replaces Ti in Ti–O–Ti bonds and Ti replaces Ge in Ge–O–Ge bonds due to their difference in electronegativity. A decrease in the O–O nearest-neighbour distance due to the incorporation of Ti atom is also observed from the broadening of the valence band spectra. The results show the 0.44 Ti doped a-GeO₂ mixture has a balance between the (Ti–O–Ge) and the (Ge–O–Ge) networks, not observed in Ti poor and Ti rich mixtures. This finding could have important consequences in the optimisation of amorphous Ti doped a-GeO₂ mixtures for low internal friction coatings of GWDs.