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

Abstract

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.

Reduction of mechanical losses in ion-beam sputtered tantalum oxide thin films via partial crystallization

“…In recent times, crystalline coatings are emerging as a promising alternative for use in GW detectors [13]. These coatings are characterized by a mechanical loss angle, which is one order of magnitude smaller than that of amorphous HI materials.…”

Clipping losses from a piecewise coated mirror

“…To date, AlGaAs/GaAs, a crystalline coating, has the best CTN characteristics [4], well under the required noise levels, and promising optical losses. Crystalline coatings however may need to address other challenges, such as birefringent noise or increased scattering and absorption defect density [21,22]. In addition, the largest crystalline coating that can currently be fabricated is 10 cm in radius.…”

Unexpected Noise in Next-Generation Mirror Material

“…We don't see the global noise issue at room temperature." Indeed, gallium arsenide coatings outperform dielectric coatings in the room-temperature conditions at which LIGO currently operates [3]. For this reason, LIGO is planning for a possible switch from amorphous to crystalline coatings in the near future, says Steven Penn, the chair of the LIGO Scientific Collaboration Council.…”