‘Hyperbolic metamaterials’ closer to reality

May 15, 2014

“Hyperbolic metamaterials” could bring optical advances including powerful microscopes, quantum computers and high-performance solar cells. The graphic at left depicts a metamaterial’s “hyperbolic dispersion” of light. At center is a high-resolution transmission electron microscope image showing the interface of titanium nitride and aluminum scandium nitride in a “superlattice” that is promising for potential applications. At right are two images created using a method called fast Fourier transform to see individual layers in the material. (Credit: Purdue University)

Researchers have taken a step toward practical applications for “hyperbolic metamaterials” — ultra-thin crystalline films that could bring optical advances including powerful microscopes, quantum computers, and high-performance solar cells.

Metamaterials have engineered surfaces that contain features, patterns or elements, such as tiny antennas or alternating layers of nitrides that enable unprecedented control of light. Under development for about 15 years, metamaterials owe their unusual potential to precision design on the scale of nanometers.

Optical metamaterials harness clouds of electrons called surface plasmons to manipulate and control light. However, some of the plasmonic components under development rely on the use of metals such as gold and silver, which are incompatible with the complementary metal–oxide–semiconductor (CMOS) manufacturing process used to construct integrated circuits, and they do not transmit light efficiently.

Superlattice crystals

Now researchers have shown how to create “superlattice” crystals from layers of the metal titanium nitride and aluminum scandium nitride, a dielectric (insulator). Superlattices are crystals that can be grown continuously by adding new layers, a requirement for practical applications.

“We believe this demonstration brings a paradigm shift to the field of metamaterials similar to developments that led to dramatic advances in silicon technology,” said Alexandra Boltasseva, a Purdue University associate professor of electrical and computer engineering.

Research findings are detailed in a paper appearing in Proceedings of the National Academy of Sciences.

The researchers created the superlattices using a method called epitaxy, growing the layers inside a vacuum chamber with a technique known as magnetron sputtering.

Boltasseva said they have developed both plasmonic and dielectric materials that can be grown epitaxially into ultra-thin and ultra-smooth layers with sharp interfaces.

The hyperbolic metamaterial behaves as a metal when light is passing through it in one direction and like a dielectric in the perpendicular direction. This “extreme anisotropy” leads to “hyperbolic dispersion” of light and the ability to extract many more photons from devices than otherwise possible, resulting in high performance.

Powerful optical microscopes, better solar collectors

The list of possible applications for metamaterials includes a “planar hyperlens” that could make optical microscopes 10 times more powerful and able to see objects as small as DNA, advanced sensors, more efficient solar collectors, and quantum computing.

The layers of titanium nitride and aluminum scandium nitride used in this study are each about 5 to 20 nanometers thick. However, researchers have demonstrated that such superlattices can also be developed where the layers could be as thin as 2 nanometers, a tiny dimension only about eight atoms thick.

The feat is possible by choosing a metal and dielectric with compatible crystal structures, enabling the layers to grow together as a superlattice. The researchers alloyed aluminum nitride with scandium nitride, meaning the aluminum nitride is impregnated with scandium atoms to alter the material’s crystal lattice to match titanium nitride’s.

A U.S. patent application has been filed through the Purdue Office of Technology Commercialization.

The material has been shown to work in a broad spectrum from near-infrared to visible light, potentially promising a wide array of applications. The near-infrared is essential for telecommunications and optical communications, and visible light is important for sensors, microscopes and efficient solid-state light sources. It also has applications in quantum information technology.

The research has been funded in part by the U.S. Army Research Office and the National Science Foundation.

Abstract of Proceedings of the National Academy of Sciences paper

Titanium nitride (TiN) is a plasmonic material having optical properties resembling gold. Unlike gold however, TiN is CMOS-compatible, mechanically strong, and thermally stable at higher temperatures. Additionally, TiN can be grown in smooth, ultra-thin crystalline films, which are useful in constructing many plasmonic and metamaterial devices including hyperbolic metamaterials (HMMs). Hyperbolic metamaterials have been shown to exhibit exotic optical properties, including extremely high broadband photonic densities of states (PDOS), which are useful in quantum plasmonics applications. However, the extent to which the exotic properties of HMMs can be realized has been seriously limited by fabrication constraints and material properties. Here, we address these issues by realizing an epitaxial superlattice as an HMM. The superlattice consists of ultra-smooth layers as thin as 5 nm and exhibits sharp interfaces, which are essential for high-quality HMM devices. Our study reveals that such a TiN-based superlattice HMM provides a higher PDOS enhancement than gold- or silver-based HMMs. Given the advantages of TiN as a CMOS compatible plasmonic material, this demonstration brings a paradigm shift to the field of metamaterials similar to the way heterostructures did to the field of solid-state light sources.