“We are excited to bring the atomic physics and nanophotonic concepts to the thin-film technology and make these advancements”Chunlei Guo
Scientists at the University of Rochester and Case Western Reserve University have developed a new class of thin-film optical coating that can transmit and reflect the same color simultaneously. These new Fano-resonant optical coatings (FROC) could be used on filters to reflect and transmit colors of outstanding purity, as well as fully reflecting only a very narrow wavelength range.
Optical coatings are used to better reflect certain wavelengths of light from lenses and other devices or, conversely, to better transmit certain wavelengths through them. Although they are found on everyday objects such as camera lenses, sunglasses, mirrors and lightbulbs to reflect or block out harmful blue light and ultraviolet rays, no previous optical coating has been able to simultaneously reflect and transmit the same wavelength, or color.
In this new study, reported in Nature Nanotechnology [ElKabbash et al. Nat. Nanotechnol. (2021) DOI: 10.1038/s41565-020-00841-9], the lab of Chunlei Guo at the University of Rochester, known for using femtosecond lasers to etch unique properties into metal surfaces, was investigating “parallel” ways to develop unique surfaces not dependent on such etching. Fano resonance is a common wave scattering phenomenon taken from atomic physics, and although it has also been observed in optical systems, it involved very complex designs.
Here they developed a simpler way to use Fano resonance, where a thin, 15 nanometer-thick film of germanium was applied to a metal surface, producing a surface that could absorb a broad band of wavelengths. A cavity was then added to support a narrowband resonance to exhibit Fano resonance capable of reflecting a very narrow band of light. They demonstrated how their coating could be used to separate thermal and photovoltaic bands of the solar spectrum, allowing them to reflect only the useful wavelength to a photovoltaic cell, preventing the cell from overheating.
The breakthrough could therefore significantly increase the life of photovoltaic cells, and also enhance the efficiency of the hybrid thermal-electric power generation and solar energy harnessing. As Chunlei Guo told Materials Today, “We are excited to bring the atomic physics and nanophotonic concepts to the thin-film technology and make these advancements”.
The narrowness of the reflected light is key as they were looking for extremely precise control of the wavelength, helping them produce narrow-band high reflectors with a simple four-layered and nanoscale thin-film coating. This was only previously possible with a multilayered dielectric mirror coating, which is much thicker, suffers from a strong angular dependence, and is much more expensive to make. The team are now further investigating other properties and applications for their thin-film coating.