Crystalline Super Mirrors For Trace Gas Detection In Medicine And Environmental Sciences

In an international cooperation with partners from industry and research, physicists from the University of Vienna, together with Thorlabs, the National Institute of Standards and Technology (NIST) and the University of Kansas, succeeded for the first time in creating high-power laser mirrors in the important wavelength range of demonstrate mid-infrared, which absorb less than ten out of a million photons. Manufactured in a new process based on crystalline materials, these low-loss mirrors promise completely new fields of application, for example in optical respiratory gas analysis for early cancer detection or the detection of greenhouse gases. The work on this will be published in the current issue of the journal "Optica".

In 2016, researchers at the LIGO laser interferometer succeeded in detecting gravitational waves for the first time, as predicted by Albert Einstein in 1916. The laser mirrors of the kilometer-long interferometer structure made a significant contribution to the observation of this wave-like propagation of disturbances in space-time, which was rewarded with the Nobel Prize a year later. Optimizing the mirrors for extremely low absorption losses was an essential step towards realizing the sensitivity required for such measurements. "Low-loss mirrors are a key technology for many different research fields," explains Oliver H. Heckl, Head of the Christian Doppler Laboratory for Mid-IR Spectroscopy and Semiconductor Optics, "

In fact, comparable mirror properties are also promising technological breakthroughs for significantly more practical applications. This includes, among other things, sensitive molecular spectroscopy, i.e. the detection of the smallest amounts of substances in gas mixtures - a research focus of the Christian Doppler Laboratory (CDL). Possible examples can be found in the early detection of cancer through the detection of the smallest concentrations of marker molecules in the breath of patients, or in the precise detection of methane leaks in large-scale natural gas production facilities in order to curb the contribution of such greenhouse gases to climate change.

Unlike the experiments at LIGO, however, such investigations take place much further outside the visible light spectrum, in the mid-infrared range. In this wavelength range, also known as the "fingerprint region", many structurally similar molecules can be clearly distinguished on the basis of their characteristic absorption lines. It is therefore a long-cherished wish of science to realize similarly low-loss mirrors in this technically demanding wavelength range.

This is exactly what the team led by Oliver H. Heckl has now achieved in an international cooperation. In this case, low-loss means that the new type of mirror absorb less than 10 out of a million photons. By way of comparison: a commercially available bathroom mirror "destroys" around ten thousand times more photons, and even the mirrors used in cutting-edge research have losses that are ten to a hundred times higher.

This drastic improvement was made possible through the use of a completely new technology: First, stacks of high-purity semiconductor materials are grown in an epitaxial process. As a result, the monocrystalline layers are transferred to curved silicon substrates using a proprietary contacting process in order to complete the mirrors that were tested both in the CDL and at NIST. The unique manufacturing technology was developed by the industrial partner of the Christian Doppler Laboratory, Thorlabs Crystalline Solutions. This company was originally founded by Garrett Cole and Markus Aspelmeyer under the name Crystalline Mirror Solutions (CMS) as a spin-off from the University of Vienna. This industrial cooperation was made possible with the support of the Federal Ministry for Digitization and Business Location, through the internationally unique model of the Christian Doppler Society (CDG) for the promotion of application-oriented basic research. A research group from the National Institute for Standards and Technology (NIST) in Gaithersburg, Maryland (USA), which is renowned for precision measurements, played a key role in this success. Georg Winkler, co-author of the current study, is also enthusiastic: "Precise measurement technology is much more than just pedantry. Wherever you can look an order of magnitude closer, you usually discover completely new phenomena, just think of the invention of the microscope and telescope! " A research group from the National Institute for Standards and Technology (NIST) in Gaithersburg, Maryland (USA), which is renowned for precision measurements, played a key role in this success. Georg Winkler, co-author of the current study, is also enthusiastic: "Precise measurement technology is much more than just pedantry. Wherever you can look an order of magnitude closer, you usually discover completely new phenomena, just think of the invention of the microscope and telescope! " A research group from the National Institute for Standards and Technology (NIST) in Gaithersburg, Maryland (USA), which is renowned for precision measurements, played a key role in this success. Georg Winkler, co-author of the current study, is also enthusiastic: "Precise measurement technology is much more than just pedantry. Wherever you can look an order of magnitude closer, you usually discover completely new phenomena, just think of the invention of the microscope and telescope! "

In fact, this assessment has already proven true during the detailed characterization of the new mirrors, when a previously unknown effect of polarization-dependent absorption was discovered in the semiconductor layers and was further theoretically investigated by Prof. Hartwin Peelaers at the University of Kansas. "These results give us great opportunities to further develop the mirrors," says co-author Lukas Perner, "thanks to the extremely low losses, we can now further optimize the bandwidth and reflectivity."

With this in mind, the project partners are already working on a further improvement of the technology: The expansion of the optical bandwidth of the mirrors allows them to be used efficiently with so-called optical frequency combs. This enables the analysis of particularly complex gas mixtures with unprecedented accuracy.