In Search of the Perfect Mirror at Mid-Infrared Wavelengths

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The team turned to NIST research chemists Adam Fleisher and Michelle Bailey, who have long worked with this technique. In a proof-of-concept experiment that put these mirrors through their paces, Fleisher and Bailey showed that the mirrors already outperform the state of the art. “Low-loss mirrors make it possible to achieve exceptionally long optical path lengths in a small device—in this case, it’s like compressing the distance from Philadelphia to New York City down to the span of a single meter,” says Bailey. “This is a key advantage for ultrasensitive spectroscopy in the MIR spectral range, including for measurement of radioisotopes, which are important for nuclear forensics and carbon dating.”

According to Garrett Cole, technology manager of Thorlabs’ Crystalline Solutions team, “This work builds upon our pioneering efforts in substrate-transferred crystalline coatings. Extending this platform to longer wavelengths, our international collaboration is the first to demonstrate an MIR coating method with undesirable absorption and scatter losses below 5 parts per million.”


A patterned 4 in. GaAs wafer with monocrystalline GaAs/AlGaAs dies that will eventually be fusion-bonded onto the coated silicon substrates. Credit: Georg Winkler

Metrology

In Search of the Perfect Mirror at Mid-Infrared Wavelengths

Advanced infrared mirrors enhance climate and biofuel research via precise trace-gas sensing

As described in the article published in Nature Communications1, an international collaboration of researchers from Thorlabs’ Crystalline Solutions (Santa Barbara, California), the Christian Doppler Laboratory for Mid-Infrared Spectroscopy at the University of Vienna, the U.S. National Institute of Standards and Technology (NIST, Gaithersburg, Maryland), and the University of Neuchâtel (Switzerland) has now demonstrated the first true mid-infrared supermirrors. These mirrors lose only 8 photons out of 1 million, achieving a reflectivity of 99.99923%. Achieving such extreme reflectivities required a combined mastery of materials, mirror design, and manufacturing processes.

In the field of high-performance mirrors, everyone chases the impossible: coatings with perfect reflectivity. In the visible range of wavelengths (i.e., between 380 nm and 700 nm), advanced metallic mirrors achieve reflectivities as high as 99%, which means 1 photon is lost for every 99 reflected. That may seem impressive, but in the near-infrared region (i.e., between ~780 nm and 2.5 μm), mirror coatings have demonstrated 99.9997% reflectivity, losing only three photons out of one million reflected.

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Co-lead author Lukas Perner, a scientist at the University of Vienna, says, “As a co-inventor of this novel coating paradigm, it was both exciting and rewarding to put these mirrors to the test. Our combined efforts in innovative mirror technology and advanced characterization methods have allowed us to demonstrate their outstanding performance, breaking new ground in the MIR.”

There has been a long-standing desire to extend this supermirror level of performance into the mid-infrared (wavelengths from 2.5 µm to 10 µm and beyond), where advancements can be enabled in trace-gas sensing tasks related to climate change and biofuels, as well as in industrial applications such as laser machining and nanofabrication. Until now, the best mid-infrared mirrors lose roughly 1 out of every 10,000 photons, about 33 times worse than in the near-infrared.

1. G.-W. Truong, L. W. Perner, D.M. Bailey, G. Winkler, S. B. Cataño-Lopez, V. J. Wittwer, T. Südmeyer, C. Nguyen, D. Follman, A. J. Fleisher, O. H. Heckl, and G. D. Cole. “Mid-infrared supermirrors with finesse exceeding 400 000.” Nature Communications. 2023. DOI: 10.1038/s41467-023-43367-z.