For the first time, physicists from the University of Vienna have successfully demonstrated high-performance laser mirrors in the sensing-relevant mid-infrared wavelength range. These mirrors absorb below 10 out of a million photons.
The study was performed in global cooperation with research and industry partners, which also involved Thorlabs, the National Institute of Standards and Technology (NIST), and the University of Kansas.
These low-loss mirrors, produced in a novel process based on crystalline materials, could pave the way to entirely new application fields, such as in optical respiratory gas analysis for the detection of greenhouse gases or the early detection of cancer. The study will be published in the current issue of the Optica journal.
Back in 2016, scientists from the LIGO laser interferometer effectively achieved the first direct observation of gravitational waves, which had initially been forecast by Albert Einstein early in 1916.
The laser mirrors of the kilometer-long interferometer assembly considerably contributed to the observation of this wave-like propagation of disturbances in space-time, which later earned the Nobel Prize in 2017.
Optimizing these mirrors for very low optical absorption losses was a major development in achieving the sensitivity required to make these measurements.
Low-loss mirrors are a key technology for many different research fields. They are the link for such diverse research fields as cancer diagnosis and gravitational wave detection.
Oliver H. Heckl, Head of Christian Doppler Laboratory for Mid-IR Spectroscopy and Semiconductor Optics, Thorlabs Crystalline Solutions
As a matter of fact, similar mirror properties are also potential technological innovations for considerably more viable applications. This comprises, among other things, sensitive molecular spectroscopy, that is, the detection of the tiniest amounts of substances in gas mixtures—a focus of study at the Christian Doppler Laboratory (CDL).
Some examples can be seen in the early detection of cancer by identifying the smallest levels of marker molecules in patients’ breath, or by accurately detecting methane leaks in large-scale production systems of natural gas to reduce the contribution of these greenhouse gases to climate change.
But unlike the experiments performed at LIGO, these analyses are carried out much beyond the spectrum of visible light, that is, in the mid-infrared range. And in this wavelength region, also called the 'fingerprint region,' several structurally analogous molecules can be clearly differentiated based on their typical absorption lines.
Hence, the photonics community has historically wished to achieve analogously low loss levels in this wavelength range, which is technically challenging.
This is precisely what the researchers, headed by Oliver H. Heckl, have currently achieved in global cooperation. In this example, low loss indicates that the latest type of mirror absorbs below 10 out of a million photons.
For the sake of comparison: a bathroom mirror available in the market 'destroys' about 10,000 times more photons, and even the mirrors employed in leading studies have losses that are 10 to 100 times higher.
This major enhancement was achieved by using an entirely new optical coating technology: Single-crystal stacks of high-purity semiconductor materials are initially deposited through an epitaxial growth process.
Such monocrystalline multilayers are subsequently transferred onto curved silicon optical substrates through a proprietary bonding process, finishing the mirrors that were tested at both the NIST and CDL.
This exclusive 'crystalline coating’ technology was designed and performed by the industrial associate of the Christian Doppler Laboratory, Thorlabs Crystalline Solutions. This firm was initially established under the name Crystalline Mirror Solutions (CMS) back in 2013 as a spin-off from the University of Vienna by Garrett Cole and Markus Aspelmeyer.
Thorlabs Inc. later acquired CMS in December 2019. This industry association was made possible via the support of the Federal Ministry for Digital and Economic Affairs, through the internationally special model of the Christian Doppler Research Association (CDG) to support application-oriented fundamental studies.
A group of researchers headed by Adam Fleisher from the National Institute for Standards and Technology (NIST) based in Gaithersburg, Maryland, the United States, which is well-known for precision measurements, also contributed significantly to this success.
Georg Winkler, the co-author of the new study expressed his enthusiasm: “Precise measurement technology is much more than just pedantry. Wherever you can take a closer look by an order of magnitude, you usually discover completely new phenomena, just think of the invention of the microscope and telescope!”
This assessment has indeed already demonstrated to be true in the comprehensive characterization of the novel mirrors themselves, when a formerly unfamiliar effect of polarization-dependent absorption was identified in the semiconductor layers and hypothetically investigated by collaborator Professor Hartwin Peelaers from the University of Kansas.
These results open up great opportunities regarding further refinement of these mirrors. Thanks to the extremely low losses can we now further optimize the bandwidth and reflectivity.
Lukas Perner, Study Co-Author, University of Vienna
Keeping this aspect in mind, the research associates are already working to further enhance the technology—expanding the optical bandwidth of these mirrors will enable them to be used efficiently with the supposed optical frequency combs. This will help analyze specifically complex gas mixtures with unparalleled precision.
Winkler, G., et al. (2021) Mid-infrared interference coatings with excess optical loss below 10 ppm. Optica. doi.org/10.1364/OPTICA.405938.