TiN-based metamaterials exhibit high photonic densities

A research on nano-engineered materials exhibiting hyperbolic dispersion in the visible spectrum range is cutting the first turf to optical advances that could yield quantum computers, high-performance solar cells, and powerful microscopes.

The artificial materials or 'metamaterials' have designed properties beyond those available in nature. They harness clouds of electrons called surface plasmons to manipulate and control light. In unusual cases, they display hyperbolic or indefinite dispersion that bears exotic optical properties including extremely high broadband photonic densities of states (PDOS). However, some of the plasmonic components currently being developed rely on the use of metals such as gold and silver—materials that do not transmit light efficiently, and are incompatible with the CMOS manufacturing process.

Purdue University researchers have developed a method to create superlattice crystals with titanium nitride (TiN) and a dielectric, aluminium scandium nitride. Having optical properties resembling gold, TiN is CMOS-compatible, mechanically strong, and thermally stable at higher temperatures. Furthermore, the metal can be grown in smooth, ultra-thin crystalline films, which are useful in constructing many plasmonic and metamaterial devices including hyperbolic metamaterials (HMMs).

The superlattices were created using epitaxy—the process of "growing" the layers inside a vacuum chamber with a technique known as magnetron sputtering. This technique proves difficult to employ in creating structures that have sharply defined, ultra-thin and ultra-smooth layers of two different materials. However, this feat was accomplished by choosing a metal and dielectric with compatible crystal structures, enabling the layers to grow together as a superlattice. The researchers alloyed aluminium nitride with scandium nitride to alter the material's crystal lattice to match TiN's.

"Plasmonic and metamaterial devices require good material building blocks, both plasmonic and dielectric, in order to be useful in any real-world application," said Alexandra Boltasseva, associate professor of electrical and computer engineering at Purdue, and one of the researchers. "Here, we develop both plasmonic and dielectric materials that can be grown epitaxially into ultra-thin and ultra-smooth layers with sharp interfaces."

1???2??