For more than a century, optical coatings have been used to better reflect certain wave­lengths of light from lenses and other devices or, conversely, to better transmit certain wavelengths through them. For example, the coatings on tinted eyeglasses reflect, or block out, harmful blue light and ultraviolet rays. But until now, no optical coating had ever been developed that could simul­taneously reflect and transmit the same wavelength, or color.

Now, researchers at the University of Rochester and Case Western Reserve University describe a new class of optical coatings, Fano Reso­nance Optical Coatings (FROCs), that can be used on filters to reflect and transmit colors of remarkable purity. In addition, the coating can be made to fully reflect only a very narrow wavelength range.

“The narrowness of the reflected light is important because we want to have a very precise control of the wavelength,” says Chunlei Guo, professor at Rochester's Institute of Optics. “Before our technology, the only coating that could do this was a multilayered dielec­tric mirror, that is much thicker, suffers from a strong angular depen­dence, and is far more expensive to make. Thus, our coating can be a low-cost and high-performance alter­native.”

The researchers envision a few applications for the new technology. For example, they show how FROCs could be used to separate thermal and photovoltaic bands of the solar spectrum. Such capa­bility could improve the effec­tiveness of devices that use hybrid thermal-electric power generation as a solar energy option. “Directing only the useful band of the solar spectrum to a photo­voltaic cell prevents its overheating,” says Guo.

The technology could also lead to a six-fold increase in the life of a photo­voltaic cell. And the rest of the spectrum “is absorbed as thermal energy, which could be used in other ways, including energy storage for night-time, elec­tricity genera­tion, solar-driven water sani­tation, or heating up a supply of water,” Guo says. “These optical coatings can clearly do a lot of things that other coatings cannot do,” Guo adds. But as with other new disco­veries, it will take a little bit of time for us or other labs to further study this and come up with more appli­cations. Even when the laser was invented, people were initially confused about what to do with it. It was a novelty looking for an application.

Guo's lab, the High-Intensity Femto­second Laser Laboratory, is noted for its pioneering work in using femto­second lasers to etch unique properties into metal surfaces. The FROC project resulted from a desire to explore parallel ways to create unique surfaces that do not involve laser etching. “Some appli­cations are easier with laser, but others are easier without them,” Guo says.

Fano resonance, named after the physicist Ugo Fano, is a widespread wave scattering pheno­menon first observed as a funda­mental principle of atomic physics involving electrons. Later, researchers discovered that the same phenomenon can also be observed in optical systems. “But this involved very complex designs,” Guo says. Guo and his colleagues found a simpler way to take advantage of Fano resonance in their optical coatings.

They applied a thin, 15 nano­meter-thick film of germanium to a metal surface, creating a surface capable absorbing a broad band of wavelengths. They combined that with a cavity that supports a narrowband resonance. The coupled cavities exhibit Fano resonance that is capable of reflecting a very narrow band of light. (Source: U. Rochester)

Reference: M. ElKabbash et al.: Fano-resonant ultrathin film optical coatings, Nat. Nano., online 4 February 2021; DOI: 10.1038/s41565-020-00841-9

Link: High-Intensity Femtosecond Laser Laboratory, Institute of Optics, University of Rochester, Rochester, USA