A multi-institutional team led by professor Chunlei Guo from the University of Rochester’s Institute of Optics has developed an optical coating capable of simultaneously reflecting and transmitting the same wavelength. The new class of optical coatings has been dubbed Fano Resonance Optical Coatings (FROCs). Previously, optical coatings were limited to reflecting certain wavelengths of light, or, conversely, to better transmit certain wavelengths through them. With the new technology, thermal and photovoltaic bands of the solar spectrum can be separated. The advance could significantly improve the efficiency of devices using hybrid thermal-electric power generation as a solar energy option. Separating those bands can prevent photovoltaic cells from overheating, Guo said. Guo’s lab, the High-Intensity Femtosecond Laser Laboratory, is known for its work in etching unique properties into metal surfaces with femtosecond lasers. However, this work stems from an interest in “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 said. To that end, Guo and his colleagues took a look at Fano resonance. Named for physicist Ugo Fano, the phenomenon is a widespread wave scattering, first observed as a fundamental principle of atomic physics involving electrons. Later, scientists discovered that the same phenomenon was observable in optical systems, albeit in highly complex designs. Seeking a simpler method, Guo and his team explored the use of optical coatings to take advantage of Fano resonance. They applied a 15-nm-thick germanium film to a metal surface, which enabled the surface to absorb a broad band of wavelengths. They combined the design with a cavity supporting a narrow-band resonance. The coupled cavities showed Fano resonance capable of reflecting a very narrow band of light. “The narrowness of the reflected light is important because we want to have a very concise control of the wavelength,” Guo said. “Before our technology, the only coating that could do this was a multilayered dependence, and far more expensive to make. Thus, our coating can be a low-cost and high-performance alternative.” Researchers in the lab of Chunlei Guo developed an optical coating that exhibits the same color in reflection (pictured) and transmission. Courtesy of University of Rochester/Adam Fenster. The technology may have the potential to enable a sixfold increase in the life of a photovoltaic cell, the researchers reported. In application, the remaining spectra is absorbed as thermal energy that could be used in other ways, including energy storage, electricity generation, solar-driven water sanitation, or heating up a supply of water, Guo said. As is the case with many new technologies, it will take some time for it to be studied further to develop and determine 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 said. The research was published in Nature Nanotechnology (www.doi.org/10.1038/s41565-020-00841-9).