Researchers Tout New Non-metal Plasmonic Material

In an effort to bridge the gap between photonics and electronics, researchers from Purdue University (West Lafayette, Indiana) have coaxed a thin film of titanium nitride into transporting plasmons, tiny electron excitations coupled to light that can direct and manipulate optical signals on the nanoscale. Titanium nitride’s addition to the short list of surface-plasmon-supporting materials, formerly comprised only of metals, could lead to a new class of optoelectronic devices with unprecedented speed and efficiency.

In a statement published in the Optical Society’s open-access journal, Optical Materials Express, Alexandra Boltasseva, a researcher at Purdue said “We have found that titanium nitride is a promising candidate for an entirely new class of technologies based on plasmonics and metamaterials. This is particularly compelling because surface plasmons resole a basic mismatch between wavelength-scale optical devices and the much smaller components of integrated electric circuits.”

Until now, the best candidates for plasmonic materials were gold and silver. These metals, however, are not compatible with standard silicon manufacturing technologies, limiting their use in commercial products. Silver is the metal with the best optical and surface plasmon properties, but it forms grainy, or semi-continuous, thin films. Silver also easily degrades in air, which causes loss of optical signal, making it a less-attractive material in plasmon technologies.

To overcome these drawbacks, Boltasseva and her team chose to study titanium nitride because they could manipulate its properties in the manufacturing process. It also could be easily integrated into silicon products, and grown crystal-by-crystal, forming highly uniform, ultrathin films—properties that metals do not share.

To test its plasmonic capabilities, the researchers deposited a very thin, very even film of titanium nitride on a sapphire surface. They were able to confirm that titanium nitride supported the propagation of surface plasmons almost as efficiently as gold. Silver, under perfect conditions, was still more efficient for plasmonic applications, but its acknowledged signal loss limited its practical applications.

To further improve the performance of titanium nitride, the researchers are now looking into a manufacturing method known as molecular beam epitaxy, which would enable them to grow the films and layered structures known as superlattices crystal-by-crystal.

In addition to plasmonics, the researchers also speculate that titanium nitride may have applications in metamaterials, which are engineered materials that can be tailored for almost any application because of their extraordinary response to electromagnetic, acoustic, and thermal waves. Recently proposed applications of metamaterials include invisibility cloaks, optical black holes, nanoscale optics, data storage, and quantum information processing.