An optical coating like no other

February 04, 2021

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 color.

In a paper in Nature Nanotechnology, researchers at the University of Rochester and Case Western Reserve University describe a new class of optical coatings, so-called 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.

"The narrowness of the reflected light is important because we want to have a very precise control of the wavelength," says corresponding author Chunlei Guo, professor at Rochester's Institute of Optics. "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 is far more expensive to make. Thus, our coating can be a low-cost and high-performance alternative."

The researchers envision a few applications for the new technology. For example, they show 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. "Directing only the useful band of the solar spectrum to a photovoltaic cell prevents its overheating," says Guo.

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 for night-time, electricity generation, solar-driven water sanitation, or heating up a supply of water," Guo says.

"These optical coatings can clearly do a lot of things that other coatings cannot do," Guo adds. But as with other new discoveries, "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."

Guo's lab, the High-Intensity Femtosecond Laser Laboratory, 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. "Some applications are easier with laser, but others are easier without them," Guo says.

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," Guo says.

Guo and his colleagues found a simpler way to take advantage of Fano resonance in their optical coatings.

They applied a thin, 15 nanometer-thick film of germanium to a metal surface, creating a surface capable 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.
-end-
Other coauthors at the University of Rochester include lead author Mohamed ElKabbash and Jihua Zhang, both postdoctoral associates; Sohail Jalil and Chun-Hao Fann, both graduate students; and James Rutledge '19, who worked on the project as an undergraduate major in optical engineering, all in the Guo lab. Coauthors at Case Western Reserve University include Giuseppe Strangi, professor of physics; Michael Hinczewski, associate professor of physics, and, from the Strangi lab, Andrew Lininger, PhD student, and Theodore Letsou and Nathaniel Hoffman, former undergraduate research assistants.

The project was supported by funding from the Army Research Office, the National Science Foundation, and AlchLight.

University of Rochester

Related Laser Articles from Brightsurf:

Laser technology: New trick for infrared laser pulses
For a long time, scientists have been looking for simple methods to produce infrared laser pulses.

Sensors get a laser shape up
Laser writing breathes life into high-performance sensing platforms.

Laser-powered nanomotors chart their own course
The University of Tokyo introduced a system of gold nanorods that acts like a tiny light-driven motor, with its direction of motion is determined by the orientation of the motors.

What laser color do you like?
Researchers at the National Institute of Standards and Technology (NIST) and the University of Maryland have developed a microchip technology that can convert invisible near-infrared laser light into any one of a panoply of visible laser colors, including red, orange, yellow and green.

Laser technology: The Turbulence and the Comb
While the light of an ordinary laser only has one single, well-defined wavelength, a so-called ''frequency comb'' consists of different light frequencies, which are precisely arranged at regular distances, much like the teeth of a comb.

A laser for penetrating waves
The 'Landau-level laser' is an exciting concept for an unusual radiation source.

Laser light detects tumors
A team of researchers from Jena presents a groundbreaking new method for the rapid, gentle and reliable detection of tumors with laser light.

The first laser radio transmitter
For the first time, researchers at Harvard School of Engineering have used a laser as a radio transmitter and receiver, paving the way for towards ultra-high-speed Wi-Fi and new types of hybrid electronic-photonic devices.

The random anti-laser
Scientists at TU Wien have found a way to build the 'opposite' of a laser -- a device that absorbs a specific light wave perfectly.

Laser 'drill' sets a new world record in laser-driven electron acceleration
Combining a first laser pulse to heat up and 'drill' through a plasma, and another to accelerate electrons to incredibly high energies in just tens of centimeters, scientists have nearly doubled the previous record for laser-driven particle acceleration at Berkeley Lab's BELLA Center.

Read More: Laser News and Laser Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.