Materials get their optical properties based on how much they reflect, absorb or transmit different wavelengths (colors) of light. Usually optical coatings either reflect or transmit a given color of light, but now researchers at the University of Rochester have developed a new class of optical coating that can both transmit and reflect the same wavelengths at the same time.
A plain piece of glass will transmit light – ie, let it pass through – of all colors. But for different purposes we may need to reflect or absorb certain wavelengths, and that’s where optical coatings come in. Sunglasses are made with coatings that absorb UV wavelengths to protect the wearer’s eyes, and increasingly common are glasses that block harmful blue light from computer and phone screens.
Usually, optical coatings either reflect or transmit a particular wavelength, not both. But now, the Rochester researchers have developed a new class of thin optical coatings that can do just that.
“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 far more expensive to make,” says Chunlei Guo, corresponding author of the study. “Thus, our coating can be a low-cost and high-performance alternative.”
The team calls the new coatings Fano resonance optical coatings (FROCs), because, as the name suggests, they make use of a particular wave scattering phenomenon called Fano resonance. The coating starts with a thin film of germanium on a metal surface, which creates a surface that can absorb a wide range of wavelengths. This is paired with a cavity that resonates in a narrow band, and together the coating can reflect light in a very narrow range of wavelengths.
FROCs could be useful for a range of applications, the team says. Their ability to separate specific wavelengths could, for example, improve photovoltaic or thermal solar cells, by preventing them from overheating and boosting their working lifetimes up to sixfold.
Beyond that, the team admits that it’s only begun to scratch the surface of what they could do.
“These optical coatings can clearly do a lot of things that other coatings cannot do,” says Guo. “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.”
The research was published in the journal Nature Nanotechnology. The team demonstrates the FROCs in the video below.
Source: University of Rochester
