Science & Technology

Metal-Dielectric Epitaxial Superlattice Brings ‘Hyperbolic Metamaterials’ Closer to Reality

“Hyperbolic metamaterials” might convey optical advances together with highly effective microscopes, quantum computer systems and high-performance photo voltaic cells. The graphic at left depicts a metamaterial’s “hyperbolic dispersion” of sunshine. At middle is a high-resolution transmission electron microscope picture displaying the interface of titanium nitride and aluminum scandium nitride in a “superlattice” that’s promising for potential functions. At proper are two photographs created utilizing a technique referred to as quick Fourier remodel to see particular person layers within the materials.

Researchers from Purdue College have created superlattices from layers of metallic titanium nitride and aluminum scandium nitride, bringing “hyperbolic metamaterials” nearer to actuality.

West Lafayette, Indiana – Researchers have taken a step towards sensible functions for “hyperbolic metamaterials,” ultra-thin crystalline movies that would convey optical advances together with highly effective microscopes, quantum computer systems and high-performance photo voltaic cells.

New developments are paying homage to advances that ushered in silicon chip know-how, mentioned Alexandra Boltasseva, a Purdue College affiliate professor {of electrical} and pc engineering.

Optical metamaterials harness clouds of electrons referred to as floor plasmons to manipulate and management mild. Nevertheless, among the plasmonic elements below growth depend on the usage of metals akin to gold and silver, that are incompatible with the complementary metallic–oxide–semiconductor (CMOS) manufacturing course of used to assemble built-in circuits and don’t transmit mild effectively.

Now researchers have proven how to create “superlattice” crystals from layers of the metallic titanium nitride and aluminum scandium nitride, a dielectric, or insulator. Superlattices are crystals that may be grown repeatedly by including new layers, a requirement for sensible utility.

“This work is an important step by way of basic contributions in supplies science and optics in addition to paving the best way to some fascinating functions,” Boltasseva mentioned. “We imagine this demonstration brings a paradigm shift to the sphere of metamaterials comparable to developments that led to dramatic advances in silicon know-how.”

Analysis findings are detailed in a paper appearing this week in the online Early Edition of Proceedings of the National Academy of Sciences.

Researchers created the superlattices utilizing a technique referred to as epitaxy, “rising” the layers inside a vacuum chamber with a method often called magnetron sputtering. It’s troublesome to use the approach to create constructions which have sharply outlined, ultra-thin and ultra-smooth layers of two completely different supplies.

“This is among the first stories of a metal-dielectric epitaxial superlattice,” mentioned Purdue doctoral scholar Bivas Saha, co-lead writer of the PNAS paper with Gururaj V. Naik, a former Purdue doctoral scholar and now a postdoctoral scholar at Stanford College.

The record of attainable functions for metamaterials features a “planar hyperlens” that would make optical microscopes 10 occasions extra highly effective and in a position to see objects as small as DNA, superior sensors, extra environment friendly photo voltaic collectors, and quantum computing.

“Plasmonic and metamaterial units require good materials constructing blocks, each plasmonic and dielectric, so as to be helpful in any real-world utility,” Boltasseva mentioned. “Right here, we develop each plasmonic and dielectric supplies that may be grown epitaxially into ultra-thin and ultra-smooth layers with sharp interfaces.”

Metamaterials have engineered surfaces that include options, patterns or components, akin to tiny antennas or alternating layers of nitrides that allow unprecedented management of sunshine. Underneath growth for about 15 years, the metamaterials owe their uncommon potential to precision design on the dimensions of nanometers.

The PNAS paper was authored by Naik; Saha; doctoral college students Jing Liu and Sammy M. Saber; Eric Stach, a researcher at Brookhaven Nationwide Laboratory; Joseph Irudayaraj, a professor in Purdue’s Division of Agricultural and Organic Engineering; Timothy D. Sands, government vice chairman for tutorial affairs and provost and Basil S. Turner Professor of Engineering within the Faculties of Supplies Engineering and Electrical and Pc Engineering; Vladimir M. Shalaev, scientific director of nanophotonics at Purdue’s Birck Nanotechnology Middle and a distinguished professor {of electrical} and pc engineering; and Boltasseva.

“This work outcomes from a singular collaboration between nanophotonics and supplies science,” Boltasseva mentioned.

The hyperbolic metamaterial behaves as a metallic when mild is passing by it in a single route and like a dielectric within the perpendicular route. This “excessive anisotropy” leads to “hyperbolic dispersion” of sunshine and the flexibility to extract many extra photons from units than in any other case attainable, leading to excessive efficiency.

The layers of titanium nitride and aluminum scandium nitride used on this examine are every about 5 to 20 nanometers thick. Nevertheless, researchers have demonstrated that such superlattices will also be developed the place the layers might be as skinny as 2 nanometers, a tiny dimension solely about eight atoms thick.

“Folks have tried for greater than 50 years to mix metals and semiconductors with atomic-scale precision to construct superlattices,” Saha mentioned. “Nevertheless, this is among the first demonstrations of attaining that step. The fascinating optical properties we see listed here are a manifestation of extraordinary structural management that we’ve achieved.”

The feat is feasible by selecting a metallic and dielectric with appropriate crystal constructions, enabling the layers to develop collectively as a superlattice. The researchers alloyed aluminum nitride with scandium nitride, that means the aluminum nitride is impregnated with scandium atoms to alter the fabric’s crystal lattice to match titanium nitride’s.

“The opportunity of rising each metallic and dielectric materials elements as an entire epitaxial system is indispensable for realizing high-performance metamaterials,” Saha mentioned. “One of many obstacles is the truth that widespread dielectrics akin to silica, alumina and different oxides can’t be utilized in mixture with metallic elements akin to metallic nitrides as a result of the deposition processes will not be appropriate with one another.”

Each of the supplies ought to possess the identical or appropriate crystal constructions.

“Basically, a lattice mismatch of lower than 5 % is critical for rising epitaxial high quality movies,” he mentioned.

A U.S. patent utility has been filed by the Purdue Workplace of Know-how Commercialization.

The fabric has been proven to work in a broad spectrum from near-infrared to seen mild, probably promising a big selection of functions.

“That’s a novel a part of this work – that we will create a superlattice metamaterial displaying hyperbolic dispersion within the seen spectrum vary,” Boltasseva mentioned.

The near-infrared is crucial for telecommunications and optical communications, and visual mild is necessary for sensors, microscopes and environment friendly solid-state mild sources.

“Most fascinating is the realm of quantum data know-how,” she mentioned.

Computer systems primarily based on quantum physics would have quantum bits, or “qubits,” that exist in each the on and off states concurrently, dramatically growing the pc’s energy and reminiscence. Quantum computer systems would reap the benefits of a phenomenon described by quantum principle referred to as “entanglement.” As an alternative of solely the states of 1 and 0 utilized in standard pc processing, there are a lot of attainable “entangled quantum states” in between one and 0, growing the capability to course of data.

The analysis has been funded partially by the U.S. Military Analysis Workplace and the Nationwide Science Basis.

Publication: Gururaj V. Naik, et al., “Epitaxial superlattices with titanium nitride as a plasmonic element for optical hyperbolic metamaterials,” PNAS, 2014; doi: 10.1073/pnas.1319446111

Picture: Purdue College

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