The optical rule was made to be broken

The optical rule was made to be broken

September 19, 2022

(News from Nanowerk) If you’re going to break a rule in style, make sure everyone sees it. That’s the goal of Rice University engineers who hope to improve displays for virtual reality, 3D displays, and optical technologies in general.

Gururaj Naik, an associate professor of electrical and computer engineering at Rice’s George R. Brown School of Engineering, and Applied Physics graduate program alumnus Chloe Doiron have found a way to manipulate light at scale. nanoscale that violates Moss’ rule, which describes a trade-off between a material’s optical absorption and how it refracts light.

Apparently this sounds more like a guideline than an actual rule, since a number of “super-mossian” semiconductors exist. Fool’s gold, aka iron pyrite, is one of them. A scanning electron microscope image of an iron pyrite metasurface created at Rice University to test its ability to transcend Moss’s rule, which describes a trade-off between a material’s optical absorption and how it refracts light. The research shows the potential for improving screens for virtual reality and 3D displays as well as optical technologies in general. (Image: The Naik Lab, Rice University)

For their study in Advanced optical materials (“Super-Mossian Dielectrics for Nanophotonics”), Naik, Doiron and co-author Jacob Khurgin, professor of electrical and computer engineering at Johns Hopkins University, find that iron pyrite works particularly well as a nanophotonic material and could lead to better and thinner screens for portable devices.

More importantly, they have established a method for finding materials that exceed Moss’ rule and provide useful light-processing properties for displays and sensing applications.

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

“That’s what we wanted to show: there’s physics that can be applied here to pre-screen materials and then help us find the ones that can allow us to meet any industrial need,” he said. declared.

“Let’s say I want to design an LED or a waveguide that operates at a given wavelength, say 1.5 micrometers,” Naik said. “For this wavelength, I want the smallest possible waveguide that has the smallest loss, which means it can best confine the light.”

Choosing a material with the highest possible refractive index at that wavelength would normally guarantee success, according to Moss. “That’s generally the requirement for all nanoscale optical devices,” he said. “Materials need to have a bandgap slightly above the wavelength of interest, because that’s where we start to see less light passing through.

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

This prompted their search for other optical options. For this, they developed their formula to identify super-Mossian dielectrics.

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

The researchers settled on experiments with iron pyrite after applying their theory to a database of 1,056 compounds, searching three bandgap ranges for those with the highest refractive indices. 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 is commonly found in interstellar debris,” Naik said. “But in the context of optics, it’s little known.”

He noted that iron pyrite has been studied for use in solar cells. “In this context, they showed optical properties in the visible wavelengths, where there are really losses,” he said. “But that was a clue for us, because when something is extremely lossy in the visible frequencies, it’s probably going to have a very high refractive index in the near infrared.”

So the lab fabricated optical-grade iron pyrite films. Testing of the material revealed a refractive index of 4.37 with a band gap of 1.03 electron-volts, exceeding the performance predicted by Moss’ rule by about 40%.

That’s great, Naik said, but the research protocol could — and probably will — find even better materials. “There are a lot of candidates, some of whom haven’t even been selected,” he said.

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