Advance brings 'hyperbolic metamaterials' closer to reality

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.
 

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 do not transmit light efficiently.
 

Researchers created the superlattices using a method called epitaxy, "growing" the layers inside a vacuum chamber with a technique known as magnetron sputtering. It is difficult to use the technique to create structures that have sharply defined, ultra-thin and ultra-smooth layers of two different materials.
 

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.
 

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, the metamaterials owe their unusual potential to precision design on the scale of nanometers.
 

"This work results from a unique collaboration between nanophotonics and materials science."Alexandra Boltasseva, a Purdue University associate professor of electrical and computer engineering.
 

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. 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.
 

The material has been shown to work in a broad spectrum from near-infrared to visible light, potentially promising a wide array of applications.
 

"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. The near-infrared is essential for telecommunications and optical communications, and visible light is important for sensors, microscopes and efficient solid-state light sources. "Most interesting is the realm of quantum information technology," she said.
 

Computers based on quantum physics would have quantum bits, or "qubits," that exist in both the on and off states simultaneously, dramatically increasing the computer's power and memory. Quantum computers would take advantage of a phenomenon described by quantum theory called "entanglement." Instead of only the states of one and zero used in conventional computer processing, there are many possible "entangled quantum states" in between one and zero, increasing the capacity to process information.