Breaking an optical rule – engineers manipulate light at the nanoscale

A formula developed by Rice engineers identifies materials for 3D displays and virtual reality.

If you’re going to break a rule with style, make sure everyone sees it. That’s the goal of Rice University engineers as they seek to improve virtual reality screens, 3D displays and optical technology in general.

The Rule of thumb, which describes a trade-off between a material’s optical absorption and the way it refracts light, was broken down by Gururaj Naik, assistant professor of electrical and computer engineering at Rice’s George R. Brown School of Engineering, and Applied Physics Program alumnus Chloe Doiron. He did this by developing a method of manipulating light at the nanoscale that violates Moss’s law.

This appears to be more of a guideline than a rule, as there are a handful of “super-mushy” semiconductors. One of them is iron pyrite, known as fool’s gold.

Naik, Doiron and co-author Jacob Hurgin, 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 displays for wearable electronics.

A scanning electron microscope image of an iron pyrite metasurface created at Rice University to test its ability to exceed Moss’s law, which describes a trade-off between a material’s optical absorption and the way it refracts light. The research shows potential to improve virtual reality screens and 3D displays along with optical technology in general. Credit: The Naik Laboratory/Rice University

More importantly, they have developed a technique to discover materials that defy Moss’s law and provide advantageous light-controlling 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 any idea how to find them. That’s what we wanted to show: there’s physics that can be applied here to shortlist materials and then help us look for those that can lead us to whatever industrial needs we have,” said he.

“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 that wavelength, I want the smallest possible waveguide that has the least loss, which means it can confine the light the best.”

Choosing a material with the highest possible refractive index at that wavelength usually guarantees success, according to Moss. “This is typically the requirement for all nanoscale optical devices,” he said. “Materials have to have a band gap just above the wavelength we’re interested in, because that’s where we start to see less light getting 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.”

This led them to look for other optical options. For this, they developed their formula for identifying super-Mossian dielectrics.

“In this work, we give people a prescription that can be applied to publicly available database with materials for their identification,said Nike.

The researchers settled on experiments with iron pyrite after applying their theory to a database of 1,056 compounds, searching across three distance ranges for those with the highest refractive index. Three compounds along with pyrite were identified as super-mosque candidates, but pyrite’s low cost and continued use in photovoltaic and catalytic applications made it the best choice for experiments.

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

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

So the lab made optical films from iron pyrite. Tests of the material reveal a refractive index of 4.37 with a band gap of 1.03 electron volts, surpassing the efficiency predicted by Mohs’ rule by about 40%.

That’s great, Naik said, but the search protocol could — and probably will — find material that’s even better.

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

Reference: “Super-Mosque Dielectrics for Nanophotonics” by Chloe F. Doiron, Jacob B. Khurgin, and Gururaj V. Naik, 6 Sep 2022, Advanced optical materials.
DOI: 10.1002/adom.202201084

The study was funded by the National Science Foundation and the Army Research Office.