New materials for thinner displays

For their study, Naik, Doiron and Jacob Khurgin from Johns Hopkins University find iron pyrite works parti­cularly well as a nanophotonic material and could lead to better and thinner displays for wearable devices. More important is that they’ve established a method for finding materials that surpass the Moss rule and offer useful light-handling properties for displays and sensing appli­cations. “In optics, we’re still limited to a very few materials,” Naik said. “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,” he said. “Let’s say I want to design an LED or a waveguide operating at a given wavelength, say 1.5 micro­meters,” Naik said. “For this wavelength, I want the smallest possible waveguide, which has the smallest loss, meaning that can confine light the best.”

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 optical devices at the nanoscale,” he said. “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,” Naik said. “But we started asking if we could go beyond silicon to an index of 5 or 10.”

That prompted their search for other optical options. For that, 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 in three bandgap ranges for those with the highest refractive indices. Three compounds along with pyrite were identified as super-Mossian candidates, but pyrite’s low cost and long use 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, it’s little-known.” He noted iron pyrite has been studied for use in solar cells. “In that context, they showed optical properties in the visible wave­lengths, where it’s really lossy,” he said. “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.”

So the lab made optical-grade iron pyrite films. Tests of the material revealed a refrac­tive index of 4.37 with a band gap of 1.03 electron volts, surpassing the performance predicted by the Moss rule by about 40 %. That’s great, Naik said, but the search protocol could and likely will find materials that are even better. “There are many candi­dates, some of which haven’t even been made,” he said. (Source: Rice U.)

Reference: C. F. Doiron et al.: Super-Mossian Dielectrics for Nanophotonics, Adv. Opt. Mat., online 6 September 2022; DOI: 10.1002/adom.202201084

Link: Nanophotonic Materials Laboratory, Dept. of Electrical and Computer Engineering, Rice University, Houston, USA