Acoustic metamaterials are artificially fabricated materials designed to control, direct, and manipulate sound waves as these might occur in gases, liquids, and solids. Dr. Steven Cummer's group develops metmaterials that cover a large range of effective mass densities, bulk moduli, positive and negative refractive indexes [Xie, Y. et al, "Measurement of broadband negative index with space-coiling acoustic metamaterials", Phys. Rev. Lett. 110, 175501 (2013)], high anisotropy factors, as well as media having highly non-linear responses [LINK: Popa, B.-I. and S. A. Cummer, "Non-reciprocal and highly nonlinear active acoustic metamaterials", Nature Communications 5, 3398 (2014)]. This wealth of available material parameters is employed in a wide range of applications hard to implement using other methods. In this regard, Dr. Steven Cummer's team has demonstrated scattering reducing shells known as "acoustic cloaks" working in air over a very broad band of frequencies in the audio spectrum [L. Zigoneanu, B.-I. Popa, S.A.

One of the most intriguing properties of optical metamaterials formed from conducting or semiconducting materials is their ability to localize and enhance optical fields at selected “hotspots” within the metamaterial structure. Such optical hotspots can be used to enhance the nonlinearities associated with materials embedded within the hotspots—or even enhance the intrinsic nonlinearity of the metals that form the metamaterial or plasmonic structure. Understanding the underlying mechanisms of optical nonlinearity in metals as well as nonlinear enhancement is one of the major thrust areas within CMIP, requiring a combination of theory, modeling and experimental investigation. The exploitation of nonlinear enhancement and wave propagation will lead to new photonic devices, such as all-optical switching and computing, as well as many other applications relevant to imaging, communications and networking.

With metamaterials, unusual and extraordinary material properties can be created and manipulated with unprecedented flexibility and control. To harness these emerging capabilities, a new methodology was needed that would enable the rapid design of devices based on these complex materials. Working with collaborator Sir John Pendry (Imperial College, London), the Duke team has developed the technique of Transformation Optics, in which bending space with coordinate transformations is used as an intuitive means of creating unique devices. The now well-known “invisibility cloak” was the first example of a transformation optical device, first suggested by Pendry and the Duke team in 2006 [J. B. Pendry, D. Schurig, D. R. Smith, “Controlling electromagnetic fields,” Science 312, 1780 (2006)] and subsequently demonstrated at Duke [D. Schurig et al., “Metamaterial electromagnetic cloak at microwave frequencies,” Science 314, 977 (2006)].

Very thin layers of patterned metamaterials can be used to create unique low-profile, flat, optical devices, such as lenses, filters and other optical components. Metamaterials can achieve a large range of effective index-of-refraction values, enabling a new realm of diffractive and gradient index optical devices. To produce metamaterial optics or metasurfaces, the shape and size of each metamaterial element is controlled, allowing nearly arbitrary distributions of refractive index and other optical properties to be controlled with tremendous precision. As an example, we have applied the metamaterial approach to demonstrate the design and fabrication of a dual-polarization hologram—a device that produces two different images when illuminated by two different polarizations. Holographic metamaterials have particular relevance for infrared optical components, where traditional materials have many more limitations.

Active Metamaterials

Passive metamaterials, while still able to create many exotic effects, are limited by losses and constrained bandwidth due to their constitutive structures. By changing these inclusions from passive to active, the limitations created by passivity can be overcome and new effects can also be discovered. Prof. Steven Cummer's team has shown that through the use of embedded active circuits and devices, active metamaterials can be created. Examples of these active metamaterials are metamaterial structures with zero loss [Y. Yuan, B-I. Popa, S. A. Cummer, "Zero loss magnetic metamaterials using powered active unit cells," Optics Express, vol. 18, no. 19, 2009] and "smarter" metamaterials that can self-tune [J. P. Barrett, S. A. Cummer, "Roadmap to electrically self-tuning metamaterials: Design and experimental validation," ICEAA, 2014]. Recently, this approach has been extended to acoustic metamaterials in order to implement material functionality hard to access in passive media.

Metamaterials are composite electromagnetic materials that enable the development of new imaging modalities in the long-wavelength regime. Imaging at long wavelengths, for example at terahertz and millimetre-wave frequencies, is a highly sought-after goal of researchers because of the great potential for applications ranging from security screening and skin cancer detection to all-weather navigation and biodetection. Dr. Willie Padilla’s single-pixel terahertz imaging system is based on dynamic metamaterial absorbers (MMAs), which have demonstrated near-unity absorption across much of the electromagnetic spectrum. A modulation technique permits imaging with negative mask values, which is typically difficult to achieve with intensity-based components. The Padilla lab demonstrates techniques allowing the acquisition of high-frame-rate, high-fidelity images. Details about Dr. Padilla’s work with Terahertz can be found in the article “Terahertz compressive imaging with metamaterial spatial light modulators” published in Nature Photonics 8, 605–609 (2014).

Metamaterials can be used to form a unique imaging device, capable of forming images without moving parts or arrays of detectors. The metamaterial imager consists of one or a few receivers that connects to a metamaterial surface antenna. One potential application of the metamaterial imager is security screening applications, in which microwaves or millimeter waves are used to detect potential threats. The metamaterial imager can form an image using just a frequency sweep across the measurement bandwidth. As frequency is swept, the metamaterial aperture produces complex radiation patterns that illuminate the target and provide information that can be used to reconstruct the scene. The metamaterial imager is an example of a computational imaging system, in that information processing approaches are used for scene reconstruction.

Details about the metamaterial imager can be found in the article by J. Hunt et al., “Compressive metamaterial imager,” published in Science, 339 100 (2013)

Emitters of light such as molecules and semiconductor quantum dots have relatively long emission lifetimes (~10 ns) and non-directional emission. Unfortunately, these intrinsic optical properties are not well suited to the demands of nanophotonic devices such as ultrafast LEDs, nanoscale lasers, and single photon sources. At CMIP we have developed a platform based on metal nanostructures that allows us to dramatically enhance the radiative properties of emitters. The approach involves trapping and squeezing light into nanometer sized gaps between a metal nanocube and a metal surface, a structure we call a nanopatch antenna. Molecules placed in this gap interact with the intensified light, causing them to emit light more than 1000 times faster, with higher efficiency, and in the desired direction. Details about the work can be found in G. M. Akselrod et al., Nature Photonics (2014).

Remote powering of devices is fast becoming one of the most important technological pursuits in engineering. An efficient means of transferring power from a source to a receiver wirelessly is through electromagnetic near-fields. The near-field of a source consists of electromagnetic fields that do not radiate, but rather build up around a source and fall off quickly with distance from the source. Being mostly magnetic, near-fields are considered safe for human exposure and can be used in highly efficient wireless power transfer schemes; however, the distance between the source and the item receiving power must be very small—usually on the order of a few feet at most. Metamaterials have a potentially interesting role to play in wireless power transfer, as they can manipulate and focus near-fields, much as a lens can focus or modify visible light. In recent work, CMIP researchers have shown that the distance between a source/receiver pair of coils can be increased with the insertion of a metamaterial near-field ‘lens’ between them. More about this work can be found in the article by G.