Plasmonic material bridges photonics, electronics gap

Ashley N. Paddock, ashley.paddock@photonics.com

A thin film of titanium nitride was coaxed into transporting plasmons, becoming the first nonmetal to be added to the short list of surface-plasmon-supporting materials and bridging the gap between photonics and electronics. The nonmetal could pave the way to a new class of optoelectronic devices that have unprecedented efficiency and speed.

Until recently, the best candidates for plasmonic materials were gold and silver. However, these noble metals are not compatible with standard silicon manufacturing technologies, limiting their use in commercial products. Of the two metals, silver has the best optical and surface plasmon properties; however, it forms semicontinuous or grainy thin films and degrades in air, which results in optical signal loss. Because of these properties, its application in plasmon technologies is limited.

(a) Excitation by light of a surface plasmon polariton on a thin film of titanium nitride. (b) Atomic force microscopy image of the surface of titanium nitride film. (c) Scanning electron microscopy image of titanium nitride thin film on sapphire. Courtesy of Alexandra Boltasseva, Purdue University/Optical Materials Express.

Now, researchers at Purdue University are studying the plasmonic capabilities of titanium nitride, a ceramic material used to coat metal surfaces such as medical implants or machine tooling parts. Titanium nitride was chosen as a test material because it is easy to manipulate in manufacturing, and it can be easily integrated into silicon semiconductor devices. The nonmetal also can be grown one crystal at a time, allowing it to form highly uniform, ultrathin films.

“When we started to think about alternative plasmonic materials, we turned to several promising candidates, among which are highly doped oxides (but these will be operating in the near-infrared, not visible, region) and intermetallics,” Alexandra Boltasseva, the lead researcher, told Photonics Spectra. “The choice of titanium nitride as the first studied material was not accidental. This material is known to have a golden luster. And since it looks like gold, we expected to find optical properties that resemble those of gold.”

To measure its plasmonic capabilities, Boltasseva’s team laid a very thin film of titanium nitride evenly over a sapphire surface and discovered that titanium nitride transmits plasmons about as well as gold does, but not as efficiently as silver under ideal conditions. The scientists are now seeking to improve the performance of titanium nitride using molecular beam epitaxy, a manufacturing technique that enables the crystal-by-crystal growth of superlattices.

The investigators believe that titanium nitride could outperform noble metals in certain devices based on transformation optics and metamaterials, such as those with hyperbolic dispersion, in the visible and near-IR regions.

“Titanium nitride could provide performance that is comparable to that of gold for plasmonic applications (including plasmonic waveguides and nanoparticles) and can significantly outperform gold and silver for transformation optics and some metamaterial applications in the visible and near-infrared regions,” Boltasseva said.

Next, the scientists plan to move from material to devices to better understand metamaterial structures that have optical performance acceptable for real-life applications and that are compatible with standard semiconductor processing lines, she said.

“We have found that titanium nitride is a promising candidate for an entirely new class of technologies based on plasmonics and metamaterials,” Boltasseva said. “This is particularly compelling because surface plasmons resolve a basic mismatch between wavelength-scale optical devices and the much smaller components of integrated electronic circuits.”

The research appeared in Optical Materials Express (http://dx.doi.org/ 10.1364/ OME.2.000478).