  We present high-reflectivity substrate-transferred single-crystal GaAs/AlGaAs
interference coatings at a center wavelength of 4.54 um with record-low excess
optical loss below 10 parts per million. These high-performance mirrors are
realized via a novel microfabrication process that differs significantly from
the production of amorphous multilayers generated via physical vapor deposition
processes. This new process enables reduced scatter loss due to the low surface
and interfacial roughness, while low background doping in epitaxial growth
ensures strongly reduced absorption. We report on a suite of optical
measurements, including cavity ring-down, transmittance spectroscopy, and
direct absorption tests to reveal the optical losses for a set of prototype
mirrors. In the course of these measurements, we observe a unique
polarization-orientation-dependent loss mechanism which we attribute to elastic
anisotropy of these strained epitaxial multilayers. A future increase in layer
count and a corresponding reduction of transmittance will enable optical
resonators with a finesse in excess of 100 000 in the mid-infrared spectral
region, allowing for advances in high resolution spectroscopy, narrow-linewidth
laser stabilization, and ultrasensitive measurements of various light-matter
interactions.
