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High finesse semiconductor supermirrors

Substrate-transferred crystalline coatings represent an entirely new concept in high-performance optical interference coatings. This technology was developed as a solution to the long-standing thermal noise limitation found in ultrastable optical interferometers, impacting cavity-stabilized laser systems for precision spectroscopy and optical atomic clocks, as well as interferometric gravitational-wave detectors [1, 2]. In such interferometers, minimization of the limiting thermal noise requires mirrors that simultaneously exhibit minimal optical and mechanical losses. The ultimate stability of these systems is currently dictated by coating Brownian noise, driven by the mechanical losses of the materials that comprise the highly reflective elements of the cavity end mirrors. As a solution to this long-standing problem, in 2013 we demonstrated a microfabrication-based coating technique allowing for the transfer of low-loss GaAs/Al0.92Ga0.08As monocrystalline multilayers onto essentially arbitrary, including curved, optical surfaces. Employed as end mirrors in an optical reference cavity, we observed an exceptionally low mechanical loss angle for these reflectors, with an upper limit of 4×10−5, a tenfold reduction when compared with the best dielectric multilayers at room temperature [3]. Moreover, such coatings were shown to be capable of ppm-levels of optical scatter and absorption losses, showing that crystalline coatings can simultaneously achieve excellent optical and mechanical quality, a key performance metric for demanding applications in precision optical metrology.