Crystalline coatings (or crystalline mirrors[1]) are a type of thin-film optical interference coating that is made by merging monocrystalline multilayers deposited via processes such as molecular-beam epitaxy (MBE) and metalorganic vapour-phase epitaxy (MOVPE) with microfabrication techniques including direct bonding and selective etching.
In this technique heterostructures such as gallium arsenide / aluminum gallium arsenide (GaAs/AlGaAs) distributed Bragg reflectors (DBRs) are grown and then transferred to polished optical surfaces, resulting in high-performance single-crystal optical coatings on arbitrary, including curved, substrates.
As of 2024[update] the maximum diameter achievable is 20 cm, limited by commercially-available GaAs wafers.
[3] Additional advantages of these coatings include:[4] Owing to the low Brownian noise of crystalline coatings there have been a number of advancements in quantum-limited interferometry, with these mirrors being instrumental in efforts relevant to macroscopic quantum phenomena and enabling the demonstration of ponderomotive squeezing at room temperature,[8] the broadband reduction of quantum radiation pressure noise via squeezed light injection,[9] and the room temperature measurement of quantum back action in the audio band.
[10] Garrett Cole and Markus Aspelmeyer founded Crystalline Mirror Solutions in 2013 to commercialize the technology.