Super-Mossian Dielectrics for Nanophotonics

High refractive index dielectrics enable nanoscale integration of optical components with practically no absorption loss. Hence, high index dielectrics are promising for many emerging applications in nanophotonics. However, the lack of a complete library of high index dielectric materials poses a significant challenge to understanding the full potential for dielectric nanophotonics.

If you’re going to break a rule with style, make sure everybody sees it. That’s the goal of engineers at Rice University who hope to improve screens for virtual reality, 3D displays and optical technologies in general. Researchers found a way to manipulate light at the nanoscale that breaks the Moss rule, which describes a trade-off between a material’s optical absorption and how it refracts light. The research team 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. 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. 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. Choosing a material with the highest possible refractive index at that wavelength would normally guarantee success.

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. The authors 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%.