Hyperbolic Metamaterials Advance Practical Applications

Optical imaging of objects as small as DNA strands could be possible using hyperbolic metamaterials.

The ultrathin crystalline films act like metals when light passes through in one direction and like dielectrics when light passes in a perpendicular direction. Researchers exploring the metamaterials at Purdue University say practical applications could include quantum computing and high-performance solar cells.

“This work is a very important step in terms of fundamental contributions in materials science and optics, as well as paving the way to some interesting applications,” said researcher Alexandra Boltasseva, an associate professor of electrical and computer engineering at Purdue.

The metamaterial's hyperbolic dispersion of light. Images courtesy of Purdue University.

Hyperbolic metamaterials harness surface plasmons to control light. However, some plasmonic components use metals like gold and silver, which are incompatible with CMOS manufacturing processes and do not transmit light efficiently.

To alleviate that issue, the researchers created superlattice crystals by using magnetron sputtering to combine metals and semiconductors with atomic-scale precision using layers of titanium nitride and aluminum scandium nitride.

“Here, we develop both plasmonic and dielectric materials that can be grown epitaxially into ultrathin and ultrasmooth layers with sharp interfaces,” Boltasseva said.

In the study, the layers of titanium nitride and aluminum scandium nitride were each about 5 to 20 nm thick. The researchers said such superlattices could be developed in 2-nm thickness.

The material has been shown to work in a broad spectrum from NIR, essential for telecommunications and optical communications, to visible light, essential for sensors, microscopes and efficient solid-state light sources.

At left, a high-resolution transmission electron microscope image shows the interface of titanium nitride and aluminum scandium nitride in a superlattice. At right are images created with a technique that allows the material’s individual layers to be visible.

“That's a novel part of this work: that we can create a superlattice metamaterial showing hyperbolic dispersion in the visible spectrum range,” Boltasseva said.

A possible application for hyperbolic metamaterials is in planar hyperlenses, which could make optical microscopes 10 times more powerful and able to see objects as small as DNA. Advanced sensors could also be possible, in addition to more efficient solar collectors and quantum computing.

The work was funded by the U.S. Army Research Office and the National Science Foundation and is published in Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.1319446111).

For more information, visit www.purdue.edu.