  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 in a workshop{\dag} 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 AlGaAs mirrors amenable
to future GW detector upgrades, while noting the unique requirements and
prospective experimental testbeds for these novel materials.
