A new kind of optical coating

a gloved hand holding four different surfaces coated with colored optical coatings

Case Western Reserve researchers, collaborators create new class of thin film coating; innovative technology could solve major hurdles for solar energy generation.

Researchers at Case Western Reserve University and the University of Rochester have developed a new class of optical coatings they say could make photovoltaic (PV) cells used for solar energy last six times longer, dramatically decreasing the cost of replacing worn out cells.

Optical coatings are layers of material deposited on a surface, usually the lens of an instrument, to either reflect or filter light into different wavelengths. They are an integral component in almost every optical instrument, from eyeglasses to telescopes to simple or very complex mirrors.

These new coatings can be made to fully reflect only a very narrow wavelength range that corresponds to the range which efficiently produces electricity by a PV cell.

professor Giuseppe Strangi talking at his desk,
Giuseppe Strang

That means they could lead to a high-performance, low-cost solution to a major hurdle facing solar energy generation, said Giuseppe Strangi, professor of physics and Ohio Research Scholar at Case Western Reserve.

The PV problem is two-fold: PV panels produce energy efficiently during day, when energy demand is relatively low, but not after sunset, when demand is high-before and after work. Additionally, storing energy generated from PV panels to overcome that dilemma is expensive.

These new coatings, in addition to an ability to be fine-tuned to reflect the perfect PV wavelength, could also be made to absorb the rest of the solar spectrum.

That means they could efficiently transfer those particular wavelengths to heat. And storing that thermal energy is relatively inexpensive, compared to storing electricity.

This approach would also protect the PV cell from overheating, leading to the promise of that six-fold increase in how long PV cells last, Strangi said.

While this solar energy application will take some time to develop, the scientists say they have taken an important first step in this new process of manipulating wave-optics at the nanoscale.

Strangi worked on the project with Mohamed Elkabbash, a Case Western Reserve alumnus now a post-doctoral researcher at MIT, Mike Hinczewski, associate professor of physics at CWRU, and Chunlei Guo, professor at Rochester’s Institute of Optics.

Their work is detailed in a paper in the journal Nature Nanotechnology.

Researchers say that for the first time they have created an optical coating that can reflect and transmit the same color (wavelength) of light.

Reflect and transmit

For more than a century, optical coatings have been used to do two things: reflect certain wavelengths of light from lenses and other devices or transmit themby allowing those wavelengths to pass through.

But the researchers said no optical coating had been developed that could simultaneously reflect and transmit the same wavelength, or color-until now. Strangi and his collaborators created a coating made from a 15 nanometer-thick film of germanium (similar to silicon or tin)-that can do both with exceptional clarity.

It is an advance in optical coatings that both Strangi and Guo said could readily lead to other applications.

“This is important because it opens up a whole, wide range of possibilities,” Strangi said. “We’ve done something that hasn’t been done in a century of development of optical coatings-and even we don’t know all of the applications that could come from this.”

The researchers call this new class of coatings Fano Resonance Optical Coatings (FROCs) in a nod to Ugo Fano, an Italian-born American physicist who worked with Enrico Fermi, known as the “architect of the nuclear age.”

Shaped resonances-essentially, a phenomenon in which two light waves are superimposed-is the foundation behind lightwave transport and behind the development of this new class of optical coatings.

It was Fano who realized that some unusual shape resonances were present in some experimental data in Fermi’s lab.

“Interest in harnessing Fano resonances in optical systems increased recently as they can be used for sensing, spectroscopy and imaging,” Strangi said. “Our work advances those possibilities in these areas as well as in the photovoltaic application.”

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