Breaking an optical rule: Engineers find a way to manipulate light at the nanoscale

When you break a rule in style, make sure everyone sees it. That’s the goal of Rice University engineers who hope to improve virtual reality screens, 3D displays, and optical technologies in general.

Gururaj Naik, associate professor of electrical and computer engineering at Rice’s George R. Brown School of Engineering, and Chloe Doiron, graduate of the Applied Physics Graduate Program, found a way to manipulate light at the nanoscale that breaks Moss’ rule, which describes a compromise between the optical absorption of a material and its refraction.

Apparently it’s more of a guideline than an actual rule as there are a number of “super-Mossian” semiconductors. Fool’s gold, also called iron pyrite, is one of them.

For your studies in Advanced optical materialsNaik, Doiron and co-author Jacob Khurgin, professor of electrical and computer engineering at Johns Hopkins University, note that iron pyrite works particularly well as a nanophotonic material and could lead to better and thinner displays for wearable devices.

More importantly, they developed a method to find materials that surpass Moss’ rule and offer useful light-handling properties for display and sensor applications.

“In optics, we’re still limited to very few materials,” said Naik. “Our periodic table is really small. But there are so many materials that are simply unknown simply because we haven’t developed any insights into how to find them.”

“This is what we wanted to show: there is physics that can be applied here to shortlist the materials and then help us find the ones that can lead us to any industrial need,” he said.

“Let’s say I want to design an LED or waveguide that operates at a specific wavelength, say 1.5 microns,” Naik said. “For this wavelength, I want the smallest possible waveguide that has the lowest losses, i.e. that can best contain the light.”

According to Moss, choosing a material with the highest possible index of refraction at that wavelength would usually guarantee success. “That’s the general requirement for all nanoscale optical devices,” he said. “The materials have to have a bandgap that’s slightly above the wavelength of interest, because that’s where we start to let less light through.”

“Silicon has a refractive index of about 3.4 and is the gold standard,” Naik said. “But we started asking if we could go beyond silicon to an index of 5 or 10.”

That prompted them to look for other optical options. To do this, they developed their formula for identifying super-Mossian dielectrics.

“In this work, we give people a recipe that can be applied to the publicly available material database to identify them,” Naik said.

The researchers decided to experiment with iron pyrite after applying their theory to a database of 1,056 compounds and looking for those with the highest refractive indices in three bandgap regions. Three compounds along with pyrite have been identified as super-Mossian candidates, but the low cost and long use of pyrite in photovoltaic and catalytic applications made it the best choice for experiments.

“Fool’s gold has traditionally been studied in astrophysics because it’s commonly found in interstellar debris,” Naik said. “But in the context of optics, little is known about it.”

He noted that iron pyrite was being studied for use in solar cells. “In this context, they showed optical properties in the visible wavelength range, where it’s really lossy,” he said. “But that was a clue for us, because if something is extremely lossy in the visible frequency range, it will likely have a very high index of refraction in the near-infrared.”

So the lab made optical quality iron pyrite films. Tests of the material revealed a refractive index of 4.37 with a bandgap of 1.03 electron volts, which exceeds the performance predicted by Moss’ rule by about 40%.

That’s great, Naik said, but the search log could — and likely will — find even better materials.

“There are many candidates, some of which haven’t even been made,” he said.