An optical supermirror is a Bragg mirror (typically a dielectric mirror) that is optimized for an extremely high reflectivity – in extreme cases, larger than 99.9999%. This means that the reflection losses are below 1 ppm. Two such ultra-high reflectivity mirrors form a Fabry–Pérot interferometer with a finesse larger than 3 millions and a strong field enhancement within the cavity. The Q factor of a supermirror cavity can be above 1011.
Although most supermirrors are dielectric mirrors (often with Ta2O5/SiO2 layers made by ion beam sputtering), there are also crystalline mirrors  with very high peak reflectivities of e.g. 99.9997% .
The term supermirror is also common for X-ray and neutron reflectors. In that field, it was originally very difficult to achieve high reflectance values. Multilayer mirrors have then been developed, which offer much better performance. Still, the achieved peak reflectivities are far lower in this regime, comparing with optical supermirrors.
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The RP Photonics Buyer's Guide contains 5 suppliers for supermirrors. Among them:
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