Researchers create first supermirrors in mid-infrared range

The first mid-infrared supermirrors have been developed by an international team of researchers from Austria, the US, and Switzerland. These mirrors are an essential component of many industrial laser cutting and welding systems, as well as optical spectroscopy of greenhouse gases. The findings were just released in the journal Nature Communications.

In the realm of high-performance mirrors, the pursuit of flawless reflectivity coatings is the stuff of legends. Advanced metal mirrors may reach a reflectivity of up to 99% in the visible wavelength range (wavelengths between 380 and 700 nm), which means that one photon is lost for every 99 reflected photons. While it might sound like a lot, specialist mirror coatings have previously achieved 99.9997% reflectivity in the near-infrared band, which is about between 780 nm and 2.5 μm. This indicates that just three reflected photons are lost out of a million.

The goal of this supermirror technology has always been to reach the mid-infrared (wavelengths between 2.5 µm and 10 µm and beyond). This would enable important advancements in several fields, such as the analysis of biofuels and the measurement of trace gases linked to climate change. Furthermore, a lot of industrial and medicinal uses, such laser scalpels and cutting lasers, might be enhanced. The greatest mid-infrared mirrors available today, however, lose one in 10,000 photons, which is around 33 times poorer than near-infrared supermirror performance.

An international group of scientists has now produced the first mid-infrared supermirrors in the recently published study. The researchers were able to create mirrors that only lose eight out of a million photons thanks to the leadership of the Christian Doppler Laboratory for Mid-Infrared Spectroscopy and Semiconductor Optics (CDL Mid-IR) at the University of Vienna and the industrial partner Thorlabs Crystalline Solutions (Santa Barbara, California). This indicates that the reflectivity of these super mirrors is 99.99923%. The materials, the mirror design, and the production process all required exact analysis and control on the part of the researchers in order to set this record.

A novel coating technique was created.

The scientists had to create a new coating procedure first. They blended cutting-edge semiconductor materials and processes with traditional thin-film coating techniques. This allowed the material constraints in the challenging mid-infrared region to be overcome. The head of the University of Vienna's CDL Mid-IR, Oliver H. Heckl, stated, "This breakthrough shows the enormous potential in successful collaboration between innovative basic research and needs-oriented product development."

Thorlabs Crystalline Solutions (TCS) Technology Manager Garrett Cole says, "This work builds on our pioneering work in substrate-transferred crystalline coatings."

But manufacturing was only one aspect of the problem. To make certain that the mirrors performed as promised, the scientists also had to measure them carefully. The two initial authors, Gar-Wing Truong from TCS and Lukas Perner from the University of Vienna, said that was their primary responsibility. They add, "As co-inventors of this novel form of coating, it was exciting to put these mirrors through their paces and thus confirm their outstanding performance."

These new supermirrors have the immediate benefit of greatly increasing the mid-infrared gas analysis optical device sensitivity. These instruments are capable of precisely detecting and measuring minute concentrations of significant environmental indicators, such carbon monoxide.

The team invited specialists from the National Institute of Standards and Technology (NIST) to illustrate these potentials. They verified that the mid-IR spectral range offers a significant advantage for ultrasensitive spectroscopy, which includes the detection of radioisotopes crucial for carbon dating and nuclear forensics.

Provided by University of Vienna