Reviewed by Mila PereraSep 13 2022
Rice University engineers aim to break a rule by enhancing screens for three-dimensional (3D) displays, virtual reality, and a broad range of optical technologies.
A scanning electron microscope image of an iron pyrite metasurface created at Rice University to test its ability to transcend the Moss rule, which describes a trade-off between a material’s optical absorption and how it refracts light. The research shows potential to improve screens for virtual reality and 3D displays along with optical technologies in general. Image Credit: The Naik Lab/Rice University.
Gururaj Naik, an associate professor of electrical and computer engineering at Rice’s George R. Brown School of Engineering, and Chloe Doiron, an Applied Physics Graduate Program alumna, formulated a method to exploit light at the nanometer scale that breaks the Moss rule, which illustrates a trade-off between the optical absorption of a material and how it refracts light.
Supposedly, it is more a guideline than a definite rule because several “super-Mossian” semiconductors exist, such as Fool’s gold or iron pyrite.
Their study has been published in the journal Advanced Optical Materials. Naik, Doiron, and co-author Jacob Khurgin, a professor of electrical and computer engineering at Johns Hopkins University, discovered that iron pyrite functions very well as a nanophotonic material and could result in optimized and thinner displays for wearable gadgets.
Furthermore, they have established a technique for detecting materials that overshadow the Moss rule and deliver beneficial light-handling features for sensing applications and displays.
In optics, we’re still limited to a very few materials. Our periodic table is really small. But there are so many materials that are simply unknown, just because we haven't developed any insight on how to find them. That’s what we wanted to show: There are physics that can be applied here to short-list the materials, and then help us look for those that can get us to whatever the industrial needs are.
Gururaj Naik, Associate Professor, Electrical and Computer Engineering, George R. Brown School of Engineering, Rice University
“Let’s say I want to design an LED or a waveguide operating at a given wavelength, say 1.5 micrometers. For this wavelength, I want the smallest possible waveguide, which has the smallest loss, meaning that can confine light the best,” Naik added.
According to Moss, selecting a material possessing the maximum possible refractive index at that wavelength would ensure success.
That’s generally the requirement for all optical devices at the nanoscale. The materials must have a bandgap slightly above the wavelength of interest, because that’s where we begin to see less light getting through. Silicon has a refractive index of about 3.4, and is the gold standard. But we started asking if we could go beyond silicon to an index of 5 or 10.
Gururaj Naik, Associate Professor, Electrical and Computer Engineering, George R. Brown School of Engineering, Rice University
This triggered their hunt for more optical options, so they fashioned their formula to detect super-Mossian dielectrics to aid their search.
“In this work, we give people a recipe that can be applied to the publicly available database of materials to identify them,” Naik said.
After using their theory on a database of 1,056 compounds, the team decided to conduct experiments with iron pyrite, looking at three bandgap ranges for those with the maximum refractive indices.
Three compounds together with pyrite were selected as super-Mossian options. Still, the economic and extended use of pyrite in catalytic and photovoltaic applications rendered it the ideal 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, it’s little-known.”
He observed that iron pyrite had been explored for application in solar cells.
In that context, they showed optical properties in the visible wavelengths, where it’s really lossy. But that was a clue for us, because when something is extremely lossy in the visible frequencies, it’s likely going to have a very high refractive index in the near-infrared.
Gururaj Naik, Associate Professor, Electrical and Computer Engineering, George R. Brown School of Engineering, Rice University
Thus, the laboratory created optical-grade iron pyrite films. Tests of the material showed a refractive index of 4.37 with a band gap of 1.03 electron volts, exceeding the performance projected by the Moss rule by approximately 40%.
Although that is significant, the search procedure can — and probably will — discover much better materials, Naik stated.
“There are many candidates, some of which haven’t even been made,” he added.
The Army Research Office (W911NF2120031) and the National Science Foundation (1935446) supported the study.
Journal Reference
Doiron, F. C., et al. (2022) Super-Mossian Dielectrics for Nanophotonics. Advanced Optical Materials. doi.org/10.1002/adom.202201084.