High-Performance Mid-Infrared Crystalline Bragg Mirrors at 4.5 µm

We present state-of-the-art mid-infrared high-reflectivity low-loss mirrors at 4.54 µm based on substrate-transferred crystalline coatings. Transmission losses of ~145 ppm and excess losses of <50 ppm are demonstrated via cavity-ringdown, direct transmission, and photothermal measurements.