Crystalline reflectors enable ultralow-thermal-noise optical cavities

Atomic clocks and gravitational-wave detectors involve some of the highest precision experimental measurements in the world. However, both have become limited by thermally induced noise in high-reflectivity mirrors used in optical interferometric cavities. This noise originates from the mechanical damping characteristics of tantala-based high-reflectivity coatings, and has been difficult to reduce. Now, G.D. Cole, W. Zhang, and colleagues at the University of Vienna; Crystalline Mirror Solutions; and JILA, the joint institute of the National Institute of Standards and Technology and the University of Colorado, Boulder, have demonstrated high-reflectivity compound-semiconductor-based crystalline mirrors with a factor-of-ten reduction in thermally induced noise. They report their findings in the July 21 online edition of Nature Photonics (DOI:http://10.1038/NPHOTON.2013.174).

Current ultrahigh-precision optical interferometers use mirrors based on alternating dielectric layers of silica (SiO₂) and tantala (Ta₂O₅) deposited on transparent substrates using ion-beam sputtering. These exhibit optical absorptions as low as a few parts per million. The noise limit for optical cavities formed from these mirrors is dominated by “coating thermal noise,” which is a consequence of the Brownian motion of the surface. This is driven by inherent thermal fluctuations and is controlled by the excess mechanical damping of the tantala layers. For gravitational wave detectors, this means that multi-kilometer-long optical cavities have noise characteristics dominated by optical coatings only a few microns thick. Previous efforts to minimize this noise have involved adding TiO₂ to the tantala layer to reduce the mechanical damping, but have only improved the noise floor by a factor of two.

The researchers hypothesized that mirrors based on epitaxial AlGaAs heterostructures might provide a path to reduced mechanical damping—and thus reduced thermal noise—while still providing ultrahigh reflectivity. Using molecular beam epitaxy, they deposited alternating layers of monocrystal-line GaAs and Al_0.92Ga_0.08As on a GaAs substrate. Then, using a lithographic process, the researchers formed 8-mm-diameter, high-reflectivity discs, which they removed from the growth wafer and direct-bonded to both planar and curved amorphous silica substrates. Next, using a Sr lattice clock laser with record frequency stability and an Yb fiber frequency comb, they measured the noise properties of an optical cavity formed from these mirrors. In close agreement with theory, they found a reduction of at least a factor of 10 in the coating noise at 1 Hz compared to state-of-the-art SiO₂/ Ta₂O₅-based mirrors.

The results suggest that a new generation of room-temperature, ultrahigh-precision measurements may now be possible in atomic clocks, gravitational-wave detectors, and other systems. The researchers said that the fabrication technique does not appear to have any fundamental limits to achieving larger mirror sizes, and relatively simple techniques can likely be applied to tailor these mirrors to a wide range of wavelengths.