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Mirror, mirror on the wall, who has the lowest noise of them all

Researchers in Vienna and Boulder demonstrate a leap forward in optical coating technology.

Although it may not be immediately obvious, the mechanical properties of optical components have a significant impact on the performance of lasers employed in precision sensing applications. Currently, the mechanical damping of such components, and the inherent mechanical fluctuations they generate, present a roadblock to further advancement of ever more precise measurements of time and space.

This is an artist's rendering of an exploded view of the crystalline coating cavity. One of the bonded mirror discs, the key element of the low noise reflectors, can be seen here separated from the glass substrate, along with a magnified view of a unit cell of the AlGaAs crystal structure.

Credit: Brad Baxley, JILA Scientific Reports Office (SRO)

For the past decade, 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. Now an international collaboration of scientists from Vienna, Austria and Boulder, Colorado, USA has demonstrated a novel technology for producing mirrors with a tenfold reduction in mechanical loss. The work, reported in Nature Photonics, represents an entirely 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 advance nanofabrication processes, and an in-depth knowledge of mechanical losses gained from the field of cavity optomechanics, the researchers in Vienna realized a novel "crystalline coating" technology. The unprecedented improvement in mechanical quality, verified by the world-renowned experts in precision measurement in Boulder, arises from the intrinsic order of the high-quality semiconductor materials used to fabricate the mirrors. The development of such materials was historically driven by advances in microelectronics and photonics, enabling the technologies we interact with every day: high-speed integrated circuits, diode lasers for telecommunications, etc. Previously, the major impediment to utilizing such materials in general optics applications was two-fold:

On the one hand, optical surfaces are in many cases curved, which presents a problem for direct crystal growth techniques, and on the other hand,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 microfabrication process to separate and then bond high-quality single-crystal films onto curved glass substrates.

The mirror technology described in the manuscript promises to accelerate progress in the development of narrow linewidth laser sources for use in precision measurement systems, spanning time keeping with optical atomic clocks, as well as fundamental physics research involving precision tests of relativity, cavity quantum electrodynamics, and quantum optomechanics. Moreover, leveraging advanced semiconductor production techniques, there is a clear path to implementing large area crystalline coatings in astronomical endeavors, such as gravitational wave detectors. According to Professor Ye, "The development of highly phase coherent optical sources is a key technology that impacts a vast range of scientific explorations. In our own lab, we are able to demonstrate the most stable optical atomic clock thanks to these narrow linewidth lasers, and the progress is marching on!"

"The collaboration with Jun's group was fantastic," states Garrett Cole, who, along with Wei Zhang, is the lead author on the work. "They not only had the courage to take on an unproven technology, but also the ability to tackle a tremendously difficult task: quickly achieving thermally limited noise performance with their characterization system and verifying the high mechanical quality of our mirrors." Following this 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 PTB in Braunschweig, Germany (see "The World's Most Stable Laser," September 2012: ). In combination, an all crystalline cavity (comprising crystalline coatings, substrates, and spacer) would enable world-record stability and hence a new milestone in laser technology.


G. D. Cole, W. Zhang, M. J. Martin, J. Ye, and M. Aspelmeyer, "Tenfold reduction of Brownian noise in optical interferometry," Nature Photonics, vol. 7, no. 8, August 2013.

Organizations involved in the research effort:

University of Vienna, Vienna Center for Quantum Science and Technology (VCQ) The "Vienna Center for Quantum Science and Technology" (VCQ) is a joint initiative of the University of Vienna, the Vienna University of Technology, and the Austrian Academy of Sciences, which unites quantum physicists of Vienna's research institutions in one collaborative center. The VCQ sets new standards for research and teaching through its unique spectrum of research topics - from fundamental quantum physics to novel quantum technologies.

Crystalline Mirror Solutions

The aptly named venture "Crystalline Mirror Solutions" (CMS) is a spin-off of the VCQ at the University of Vienna and a manufacturer of low-noise reflective optics using a proprietary substrate-transfer coating technology. Monocrystalline multilayers fabricated with this technique enable unprecedented advancements in precision interferometry.


JILA is a joint institute of the University of Colorado at Boulder and the National Institute of Standards and Technology. JILA supports an eclectic and innovative research program that fosters creative collaborations among its scientists.

Collaborations play a key role in the pioneering research JILA and its Center for AMO Physics are known for around the world.

The Work at the University of Vienna is supported by the Austrian Science Fund (FWF), the European Commission and the European Research Council (ERC) Starting Grant Program. The work at CMS is supported by the Austria Wirtschaftsservice GmbH (AWS) and the ERC Proof of Concept Initiative.Work at JILA is supported by the USNational Institute of Standards and Technology (NIST), the DARPA QuASAR Program, and the US National Science Foundation (NSF) Physics Frontier Center at JILA. Microfabrication was carried out at the Center for Micro- and Nanostructures (ZMNS) of the Vienna University of Technology.

Scientific Contacts:

University of Vienna
Univ.-Prof. Markus Aspelmeyer
VCQ, Faculty of Physics
University of Vienna
Boltzmanngasse 5
A-1090 Wien
T +43-1-4277-72530
University of Colorado, Boulder
Prof. Jun Ye
440 UCB
University of Colorado
Boulder, CO 80309-0440
T +1-303-735-3171
Crystalline Mirror Solutions GmbH
Dr. Garrett Cole
Strudlhofgasse 4
A-1090 Vienna
Press contacts:
Alexandra Frey
Press Office
University of Vienna
Research and teaching
Universitätsring 1
1010 Vienna
T +43-1-4277-175 33
Julie Phillips
Director of Communications, JILA
440 UCB
University of Colorado
Boulder, CO 80309-0440
T +1-303-492-7809

Markus Aspemeyer | EurekAlert!
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