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 colour.
Now, in a paper in Nature Nanotechnology, researchers at the University of Rochester and Case Western Reserve University have described a new class of optical coatings, so-called Fano Resonance Optical Coatings (FROCs), that can be used on filters to reflect and transmit colours 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 for precise control of the wavelength. Before this technology, the only coating that could do this was a multi-layered dielectric mirror, that is much thicker, suffers from a strong angular dependence, and far more expensive to make. Thus, this new coating can be a low-cost and high-performance alternative.
The researchers envision a few applications for the new technology. For example, they showed 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 as directing only the useful band of the solar spectrum to a photovoltaic cell prevents its overheating.
The technology could also lead to a six-fold increase in the life of a photovoltaic cell. And the rest of the spectrum absorbed as thermal energy, 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.
Applying Fano Resonance to Optical Coatings
The High-Intensity Femtosecond Laser Laboratory - the research lab for FROC, 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.
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. Researchers behind the development of FROCs found a simpler way to take advantage of Fano Resonance in their optical coatings.
They applied a thin, 15 nanometre-thick film of germanium to a metal surface, creating a surface capable of 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.
This project was supported by funding from the Army Research Office, the National Science Foundation, and AlchLight.