Mid-Infrared Crystalline Supermirrors for Optical Cavities with 231 000 Finesse

We fabricate and characterize substrate-transferred single-crystal mirror coatings with 9.33 ± 0.17 ppm of transmittance and 4.27 ± 0.52 ppm of excess optical loss, corresponding to a transmission-loss dominated reflectance of 99.9986% at 4.45 μm.