Recent advancements in substrate transferred crystalline coatings

The current bounds of stability and sensitivity in precision optical interferometers are dictated by the mechanical dissipation, and thus the corresponding Brownian noise level, of the high-reflectivity coatings that comprise the cavity end mirrors. A spin-off of fundamental quantum optics research from the University of Vienna, Crystalline Mirror Solutions has developed a proprietary microfabrication technique that enables the transfer of low-loss single-crystal semiconductor heterostructures onto essentially arbitrary optical surfaces. These "crystalline coatings" simultaneously exhibit both high reflectivity and minimal mechanical damping, with room temperature loss angles an order of magnitude below that of state-of-the-art ion-beam sputtered (IBS) dielectric coatings and cryogenic loss angles reduced by a further factor of ten. These excellent optomechanical properties pave the way for the implementation of crystalline coatings in the next generation of cavity-stabilized laser systems and interferometric gravitational-wave detectors. Over the last two years we have undertaken a focused optimization effort, culminating in the realization of parts-per-million levels of optical scatter and sub-ppm absorption for center wavelengths spanning 1000 to 1600 nm, proving that crystalline coatings are capable of optical performance rivaling that of IBS multilayers. Moving to longer wavelengths, first attempts at fabricating reflectors for the 3–4 µm spectral region have already yielded optical losses on par with the best coatings present on the commercial market. Taken together, mirrors fabricated with our crystalline coating technique exhibit the lowest mechanical loss (and thus Brownian noise), the highest thermal conductivity, and the widest spectral coverage of any “supermirror” technology.