The materials could enable better control of light at the nanoscale, opening up new possibilities for display technologies.
A formula developed by Rice engineers identifies materials for 3D displays and virtual reality.
If you’re going to break a rule in style, make sure everyone sees it. This is the goal of engineers at Rice University, who seek to improve screens for virtual reality, 3D screens and optical technologies in general.
Moss’s rule, which describes a trade-off between a material’s optical absorption and how it refracts light, was broken by Gururaj Naik, associate professor of electrical and computer engineering at the George R. Brown School of Engineering from Rice and a graduate in applied physics. Former student of the Chloé Doiron program. He did this by developing a method for manipulating light at the nanoscale that breaks Moss’ rule.
This seems to be more of a guideline than a rule since there are a handful of “super-mossian” semiconductors. One of them is iron pyrite, commonly known as fool’s gold.
Naik, Doiron and co-author Jacob Khurgin, a professor of electrical and computer engineering at Johns Hopkins University, found that iron pyrite works particularly well as a nanophotonic material. They recently published their findings in the journal Advanced optical materials which could lead to better and smaller screens for wearable electronics.
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. Credit: The Naik Lab/Rice University
More importantly, they have developed a technique to discover materials that defy Moss’s rule and provide advantageous 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. .
“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 approximately 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.
Reference: “Super-Mossian Dielectrics for Nanophotonics” by Chloe F. Doiron, Jacob B. Khurgin and Gururaj V. Naik, September 6, 2022, Advanced optical materials.
DOI: 10.1002/adom.202201084
The study was funded by the National Science Foundation and the Army Research Office.
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