The Pursuit of Perfection: The Ultimate Mid-Infrared Mirror Achieves 99.99923% Reflectivity

A collaborative international effort has led to the creation of the first mid-infrared supermirrors with exceptional reflectivity, as reported in Communication in Nature. This innovation is expected to further improve gas sensitivity in the environment and industrial processes, marking a major leap forward in mirror technology. Source:

Advanced infrared mirrors improve climate and biofuel research through precise trace gas sensing.

An international team of researchers from the United States, Austria, and Switzerland has demonstrated the first true supermirrors in the mid-infrared spectral region. These mirrors are key for many applications, such as optical spectroscopy for environmental sensing, as well as laser cutting and welding for manufacturing.

Achieving Near-Perfect Meditation

In the field of high-performance mirrors, everyone is chasing the impossible: coatings with perfect reflectivity. In the visible range of wavelengths (ie, between 380 nm and 700 nm), advanced metallic mirrors achieve reflectivity as high as 99%, which means 1 photon lost every 99 shown. That sounds impressive, but in the near-infrared region (ie, between ~780 nm and 2.5 μm), mirror coatings exhibit 99.9997% reflectance, losing only 3 photons out of 1 million reflected .

Silicon Substrate Interference Coating

One inch diameter silicon substrate coated with a custom deposited interference coating. Source: Valentin Wittwer

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 improvements can be applied to trace gas sensing. tasks related to climate change and biofuels, as well as industrial applications such as laser machining and nanofabrication. Until now, the best mid-infrared mirrors lost about 1 in every 10,000 photons, or about 33 times worse than near-infrared.

International Collaboration Leads to Progress

As described in the article published in Communication in Nature, an international collaboration of researchers from Thorlabs’ Crystalline Solutions (Santa Barbara, CA), the Christian Doppler Laboratory for Mid-Infrared Spectroscopy at the University of Vienna (Austria), the US National Institute of Standards and Technology (NIST ), and The University of Neuchâtel (Switzerland) has now demonstrated the first true mid-infrared supermirrors. These mirrors lose only 8 photons in 1 million, achieving a reflectivity of 99.99923%. Achieving such intense reflections requires a combined expertise in materials, mirror design, and manufacturing processes.

Formed Four-inch GaAs Wafer

A patterned four-inch GaAs wafer with a monocrystalline GaAs/AlGaAs die will eventually be fusion-bonded to coated silicon substrates. Source: Georg Winkler

New Paradigm in Mirror Coatings

To realize this first generation of mid-infrared (MIR) supermirrors, researchers have conceived and demonstrated a new paradigm of coatings. They combine conventional thin-film coating techniques and novel semiconductor materials and methods to overcome material constraints in the challenging mid-infrared region.

According to Garrett Cole, Technology Manager of Thorlabs’ Crystalline Solutions team, “This work builds on our pioneering efforts in substrate-transferred crystalline coatings. Expanding this platform to longer wavelengths, our international collaboration is the first to demonstrate a MIR coating method with unwanted absorption and scattering losses of less than 5 parts per million.

These mirrors use the extremely pure and excellent structural quality of molecular beam epitaxy, an advanced process used to manufacture many different semiconductor devices, to produce monocrystalline GaAs/AlGaAs. multilayers with less absorption and scattering. This starting material is then made into high-performance mirrors using advanced microfabrication techniques including direct “fusion” bonding of a high-quality conventional non-crystalline thin-film interference coating deposited at the University of Neuchâtel.

Measuring and Validating Superior Performance

Creating these groundbreaking mirrors is only half the challenge. Scientists also need to measure the mirrors to verify their optimal performance. Gar-Wing Truong, Lead Scientist at Thorlabs Crystalline Solutions, said, “It was a great team effort to combine the equipment and expertise to fully demonstrate the total loss below 7.7 parts per million, which 6 times better than previously achieved with any conventional MIR coating technique.”

Co-lead author Lukas Perner, Scientist at the University of Vienna, added, “As a co-inventor of this novel coating paradigm, it was exciting and rewarding to put these mirrors to the test. Our joint efforts in new mirror technology and advanced characterization methods allow us to demonstrate their unique performance, breaking new ground in MIR.

Impact on Environmental Sensing and Spectroscopy

The immediate application of these novel MIR supermirror is to greatly improve the sensitivity of optical devices used to measure the amount of gases. These devices, called cavity ringdown spectrometers (CRDS), can detect and quantify minuscule amounts of important environmental markers, such as carbon monoxide. The team turned to NIST research chemists, Adam Fleisher and Michelle Bailey, who have been working on this technique for a long time. In a proof-of-concept experiment that puts these mirrors through their paces, Fleisher and Bailey show that the mirrors are beyond cutting-edge.

“Low mirror losses make it possible to achieve very long optical pathlengths in a small device – in this case, it’s like compressing the distance from Philadelphia to NYC to the length of on a meter,” said Bailey. “This is a significant advantage for ultra-sensitive spectroscopy in the MIR spectral range, including the measurement of radioisotopes important for nuclear forensics and carbon dating.”

Reference: “Mid-infrared supermirrors with finesse exceeding 400 000” by Gar-Wing Truong, Lukas W. Perner, D. Michelle Bailey, Georg Winkler, Seth B. Cataño-Lopez, Valentin J. Wittwer, Thomas Südmeyer, Catherine Nguyen, David Follman, Adam J. Fleisher, Oliver H. Heckl and Garrett D. Cole, 6 December 2023, Communication in Nature.
DOI: 10.1038/s41467-023-43367-z

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