A patterned four-inch GaAs wafer with monocrystalline GaAs/AlGaAs dies, which will eventually be fusion-bonded onto coated silicon substrates. Image: Georg Winkler.An international team of researchers from the US, Austria and Switzerland has developed the first true supermirror in the mid-infrared spectral region. Such mirrors are crucial for many applications, including optical spectroscopy for environmental sensing, as well as laser cutting and welding for manufacturing. The researchers report their advance in a paper in Nature Communications.
In the field of high-performance mirrors, everyone is chasing the impossible – coatings with perfect reflectivity. For visible wavelengths of light (i.e. between 380nm and 700nm), advanced metallic mirrors can achieve reflectivities as high as 99%, which means one photon is lost for every 99 reflected. That may seem impressive, but for infrared wavelengths (i.e. between 780nm and 2.5μm), mirror coatings have demonstrated 99.9997% reflectivity, losing only three photons out of 1 million reflected.
Researchers have thus had a long-standing desire to extend this supermirror level of performance to mid-infrared wavelengths (from 2.5µm to 10µm and beyond), which would lead to advances in trace-gas sensing, as well as in industrial applications such as laser machining and nanofabrication. However, the best current mid-infrared mirrors lose roughly one out of every 10,000 photons, a performance that is about 33 times worse than in the near-infrared.
Now, a team of researchers from Thorlabs’ Crystalline Solutions, the Christian Doppler Laboratory for Mid-Infrared Spectroscopy at the University of Vienna in Austria, the US National Institute of Standards and Technology (NIST) and the University of Neuchâtel in Switzerland have demonstrated the first true mid-infrared supermirrors. These mirrors lose only eight photons out of 1 million, meaning a reflectivity of 99.99923%. Achieving such extreme reflectivities required a combined mastery of materials, mirror design and manufacturing processes.
To realize this first generation of mid-infrared (MIR) supermirrors, the researchers conceived and demonstrated a new paradigm in coatings. They combined conventional thin-film coating techniques with novel semiconductor materials and methods to overcome the material constraints in the challenging mid-infrared region.
"This work builds upon our pioneering efforts in substrate-transferred crystalline coatings,” explained Garrett Cole, technology manager of Thorlabs’ Crystalline Solutions team. “Extending this platform to longer wavelengths, our international collaboration is the first to demonstrate a MIR coating method with undesirable absorption and scatter losses below 5 parts per million."
These mirrors leverage the extreme purity and excellent structural quality obtained with molecular beam epitaxy, an advanced process used to manufacture many different semiconductor devices, to produce monocrystalline gallium arsenide/aluminum gallium arsenide multilayers with negligible absorption and scatter. This starting material is then turned into a high-performance mirror using advanced microfabrication techniques, including direct ‘fusion’ bonding onto a high-quality, non-crystalline, thin-film interference coating deposited at the University of Neuchâtel.
Fabricating these groundbreaking mirrors was only half the challenge, though. The scientists also needed to methodically measure the mirrors to prove their superior performance.
"It was a tremendous team effort to bring together the equipment and expertise to definitively show total losses as low as 7.7 parts per million, which is six times better than previously achieved with any conventional MIR coating technique," said Gar-Wing Truong, lead scientist at Thorlabs Crystalline Solutions.
"As a co-inventor of this novel coating paradigm, it was both exciting and rewarding to put these mirrors to the test,” said Lukas Perner, a scientist at the University of Vienna and co-lead author of the paper. “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."
One immediate application of these novel MIR supermirrors is greatly improving the sensitivity of optical devices used to measure trace amounts of gases. These devices, called cavity ringdown spectrometers (CRDS), can detect and quantify miniscule amounts of important environmental markers, such as carbon monoxide.
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 out-perform the state-of-the-art.
"Low-loss mirrors make it possible to achieve exceptionally long optical pathlengths in a small device – in this case it’s like compressing the distance from Philadelphia to NYC down to the span of a single meter," Bailey said. "This is a key advantage for ultra-sensitive spectroscopy in the MIR spectral range, including for measurement of radioisotopes which are important for nuclear forensics and carbon dating."
This story is adapted from material from the University of Vienna, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.