Researchers in the precision-measurement community have been searching for a solution that allows for the development of high-reflectivity mirrors with simultaneously high mechanical quality for the past decade. Now, an international collaboration of scientists from Vienna, Austria and Boulder, Colorado (US) has demonstrated a novel technology for producing mirrors with a 10x reduction in mechanical loss. The work, reported in the article “Tenfold reduction of Brownian noise in high-reflectivity optical coatings” by Garrett D. Cole, Wei Zhang, Michael J. Martin, Jun Ye and Markus Aspelmeyer in Nature Photonics, represents a new approach for generating high-quality optical coatings, key components of state-of-the art laser systems for precision measurement.
Combining aspects of semiconductor mirrors borrowed from surface-emitting diode lasers, an epitaxial-layer transfer technique gleaned from advanced nanofabrication processes, and an in-depth knowledge of mechanical losses gained from the field of cavity opto-mechanics, the researchers in Vienna realised a novel “crystalline coating” technology. The unprecedented improvement in mechanical quality, verified by researchers in precision measurement in Boulder, arises from the intrinsic order of the high-quality semiconductor materials used to fabricate the mirrors. Previously, the major impediment to utilising such materials in general optics applications was twofold: First, optical surfaces are in many cases curved, which presents a problem for direct crystal growth techniques; second, typical optical substrates are made of glass with an amorphous structure that lacks the order required for seeded crystal growth. Circumventing these limitations, the researchers came up with a micro-fabrication process to separate and then bond high-quality single-crystal films onto curved glass substrates.
The mirror technology described in the article promises to accelerate progress in the development of narrow line-width laser sources for use in precision-measurement systems, spanning time-keeping with optical atomic clocks and fundamental physics research involving precision tests of relativity, cavity quantum electrodynamics, and quantum opto-mechanics. Moreover, leveraging advanced semiconductor production techniques, there is a clear path to implementing large area crystalline coatings in astronomical endeavours, such as gravitational wave detectors.
Following the initial demonstration, the scientists are already hard at work to further improve the technology. Going forward, they plan to combine their novel coatings with the previously demonstrated single-crystal silicon cavity developed by researchers at JILA and Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany. In combination, an all-crystalline cavity (comprised of crystalline coatings, substrates and spacer) would enable outstanding stability and a new milestone in laser technology.