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Fabrication and characterization of large-area crystalline coatings for next-generation gravitational wave detectors

Don't miss our talk at SPIE Optical Systems Design in Frankfurt by our Head of R&D, Christoph Deutsch

Paper 10692-28

Christoph Deutsch, Garrett D. Cole, David Follman, Paula Heu, Dominic Bachmann, Crystalline Mirror Solutions; Philip Koch, Harald Lück, Max-Planck-Institut für Gravitationsphysik; Alexander von Finck, Sven Schröder, Fraunhofer-Institut für Angewandte Optik und Feinmechanik; Ashish Rai, Tobias Zederbauer, Crystalline Mirror Solutions

Crystalline Mirror Solutions has pioneered a novel optical coating technology that enables the transfer of low-loss single-crystal GaAs/AlGaAs heterostructures grown by molecular beam epitaxy (MBE) onto arbitrary optical substrates. One key advantage of these newly-developed semiconductor-based crystalline coatings over state-of-the-art IBS coatings is a tenfold reduction of Brownian noise. Thus, crystalline coatings are currently redefining the performance metrics of a wide range of applications including ultrastable laser systems for optical clocks, stabilized combs for microwave generation, and low-noise test masses for gravitational wave detectors. Over the last few years we have realized significant improvements in the near-IR optical performance of our reflectors, with recently achieved cavity finesse values exceeding 600,000 near 1550 nm. With the continued success in the development of 0.5-inch and 1-inch diameter optics, we have now put a strong focus on the development of large area (>1-inch/25-mm diameter) crystalline coatings, targeting applications in next generation interferometric gravitational wave detectors. Ultimately, we are aiming for >300-mm diameter coatings as installed in Advanced LIGO. To date, our crystalline coating process can immediately be extended to a coating diameter of 200 mm, which is only limited by the maximum commercially available GaAs substrate wafer size. Given the present capabilities of leading GaAs wafer manufacturers, no physical limitations should impede further size-scaling of bulk GaAs boules. To demonstrate the high quality of the epitaxial growth we have successfully verified that the uniformity of production-grade MBE systems is sufficient to meet the stringent specifications for these large optics. We have performed uniformity measurements of the material layer thickness (and thus optical transmission), absorption loss, and scattering loss over areas with a diameter up to 500 mm. These experiments show a clear and promising path towards LIGO-ready “plug-and-play” crystalline coatings and gravitational wave observatories with drastically reduced coating noise.
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