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Metamaterials

Many of the technologies that underpin our economy and enable our standard of living depend on advanced materials. Therefore, the engine for progress in many scientific disciplines is the discovery and understanding of new materials. Metamaterials are novel artificial materials that enable the realization of innovative properties unattainable in naturally existing materials. This research project will explore the theoretical understanding, analysis, development, fabrication, and experimental characterization of metamaterials, and investigate their feasibility for applications. In view of the complexity of electromagnetic interactions in metamaterials, state-of-the-art computational techniques to understand these materials, and collaboration with experimentalists to fabricate and characterize them, are essential. Finding and understanding mechanisms that minimize loss and increasing the operating frequency will be critical for future applications, such as solar energy harvesting and biomedical/terahertz imaging. We will develop new 3D nanofabrication techniques such as direct laser writing and experimentally realize dynamic and tunable metamaterials employing nonlinear and gain materials. Finally, we will characterize the physical properties of metamaterials and develop unique optical characterization techniques.

This research is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering.

A novel ultrafast terahertz probe has traced dark composite particles and confirmed, for the first time, that their formation occurs on the femtosecond timescale in single-walled carbon nanotubes. Dark particles, known as dark excitons, are composite pairs of quasielectrons and electron holes. Single-walled carbon nanotubes have unique electrical properties governed by the presence of dark and bright particles. Because dark excitons cannot emit a photon like their bright counterparts, they do not interact with visible light, rendering them difficult to trace.

Broadband terahertz light emitters have been designed and fabricated using nanoscale U-shaped building blocks. The terahertz spectral range sits between infrared and typical radar frequencies, and the challenges of efficiently generating and detecting terahertz radiation has limited its use. However, broadband terahertz sources offer exciting possibilities to study fundamental physics principles, to develop non-invasive material imaging and sensing, and make possible terahertz information, communication, processing and storage.

Graphene — a one layer thick sheet of carbon atoms — has special properties that make it a desirable material for manipulating terahertz waves. Terahertz applications operate at frequencies between microwave and far infrared. Some metamaterials, which are engineered structures that can manipulate light in ways not seen in conventional materials, could benefit by replacing the metals currently used to build them with graphene.

Researchers have found a way to enhance the force of light on matter. Most of the time the momentum of light and the associated forces are too small to notice, but at the nanoscale the effect can be quite large, and researchers have used these forces to dynamically manipulate optical waveguides at the nanoscale. Optical forces decay significantly, however, as the distance between the waveguides increases and become too small for all-optical device actuation at larger separations.

Designing methods to slow down electromagnetic signals just got easier with a new model that predicts how light will absorb and scatter from devices made from metamaterials. Metamaterials are built from small, engineered structures that, in some ways, mimic the role of atoms, yet can manipulate light in ways not seen in conventional materials. Slowing down light can arise in metamaterials through a process known as electromagnetically induced transparency, when destructive coupling occurs between a bright resonator and a dark resonator.

Researchers now understand why artificially engineered materials, known as metamaterials, can sometimes perform better than expected. Metamaterials are built from small, engineered structures that manipulate light in ways not found in nature. Unfortunately, energy is typically lost by theconversion of light to heat in the metallic components and typical support materials; this is a key challenge for application development.

Designing the building blocks of artificially engineered materials, known as metamaterials, just got easier. Metamaterials are built from small engineered structures that, in some ways, mimic the role of atoms, and can manipulate light in ways not seen in nature. The conducting materials used to make them are central to their efficiency. Energy is lost by conversion of light to heat in the metallic components and the support materials. Gold and silver are known to be relatively good building block materials and now we have a way to predict which other materials could work even better.

Scientists have designed a device to achieve the seeming-impossibility of confining light to a space with dimensions smaller than its wavelength. The deceptively simple device is a pipe with a tiny bore, and walls made of so-called transformation optical materials. To understand how these materials work, consider first what happens when light hits water. Light changes directions, because of the difference in the refractive index of water versus air; it hits the water at one angle and travels through it at a different angle.

First predicted by distinguished Dutch physicist, H.B.G. Casimir in 1948, the Casimir force arises when two uncharged, parallel metallic plates are brought in close proximity. Noticeable at a distance of a few microns, this attracting force becomes dominant at a length scale of tens of nanometers. Classically, no force exists between the plates; the Casimir force is proportional to the surface’s area, a consequence of the quantum nature of the electromagnetic field. Not restricted to parallel plates, the Casimir force exists between any two objects for microscopic separations.

After years of doubt, the scientific community now embraces the almost paradoxical properties of metamaterials, also known as negative index materials (NIMs). The unusual properties of fabricated NIMs include perfect lensing (beating the diffraction limit for electromagnetic waves), zero reflectance, and negative Snell’s law angles. Acceptance of these phenomena has come with recent design, fabrication, demonstration, and detailed first principles simulations for operation at microwave and THz frequencies from the Ames Laboratory group.