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. The coatings on tinted eyeglasses, for example, 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.
Researchers developed 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. Previously, 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.
The researchers envision several applications for the new technology. The 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. 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 at night, electricity generation, solar-driven water sanitation, or heating a supply of water.
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.
The team applied a thin, 15-nanometer-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.