In this approach, AlGaAs-based epitaxial Bragg reflectors are directly bonded to super-polished bulk optical substrates, producing low-noise near-infrared (NIR) mirrors for cavity-stabilized lasers employed in optical atomic clocks and next-generation gravitational-wave detectors, as well as ultralow-loss mid-infrared (MIR) mirrors for cavity ring-down spectroscopy. These “semiconductor supermirrors,” first demonstrated in Markus Aspelmeyer’s group in Vienna in 2013, uniquely combine ultralow optical and elastic losses. With continuous improvements in fabrication and metrology, crystalline coatings now routinely achieve combined scatter and absorption losses below 5 parts per million (ppm) across the NIR and MIR, spanning 940 nm to 4600 nm. This enables NIR reference cavities with record-setting stability, outperforming sputtered amorphous mirrors and powering the world’s lowest-noise laser systems. In MIR trace-gas sensing, crystalline coatings similarly surpass the detection limits achievable with PVD-deposited amorphous multilayers. Beyond passive mirrors, this bonding-based manufacturing technique supports a range of active and integrated photonic devices. Select examples include optically pumped semiconductor disk lasers for a sodium laser guide star, as well as active electro-optic and nonlinear components and photonic integrated circuits.