A recently published theory has suggested that a cloak of invisibility is in principle possible, at least over a narrow frequency band. We describe here the first practical realization of such a cloak; in our demonstration, a copper cylinder was "hidden" inside a cloak constructed according to the previous theoretical prescription. The cloak was constructed with the use of artificially structured metamaterials, designed for operation over a band of microwave frequencies. The cloak decreased scattering from the hidden object while at the same time reducing its shadow, so that the cloak and object combined began to resemble empty space.

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Metamaterial Electromagnetic Cloak at Microwave Frequencies

We present the design for an absorbing metamaterial (MM) with near unity absorbance A(omega). Our structure consists of two MM resonators that couple separately to electric and magnetic fields so as to absorb all incident radiation within a single unit cell layer. We fabricate, characterize, and analyze a MM absorber with a slightly lower predicted A(omega) of 96%. Unlike conventional absorbers, our MM consists solely of metallic elements. The substrate can therefore be optimized for other parameters of interest. We experimentally demonstrate a peak A(omega) greater than 88% at 11.5 GHz.

Perfect metamaterial absorber.

Metamaterials are artificially fabricated materials that allow for the control of light and acoustic waves in a manner that is not possible in nature. This Review covers the recent developments in the study of so-called metasurfaces, which offer the possibility of controlling light with ultrathin, planar optical components. Conventional optical components such as lenses, waveplates and holograms rely on light propagation over distances much larger than the wavelength to shape wavefronts. In this way substantial changes of the amplitude, phase or polarization of light waves are gradually accumulated along the optical path. This Review focuses on recent developments on flat, ultrathin optical components dubbed 'metasurfaces' that produce abrupt changes over the scale of the free-space wavelength in the phase, amplitude and/or polarization of a light beam. Metasurfaces are generally created by assembling arrays of miniature, anisotropic light scatterers (that is, resonators such as optical antennas). The spacing between antennas and their dimensions are much smaller than the wavelength. As a result the metasurfaces, on account of Huygens principle, are able to mould optical wavefronts into arbitrary shapes with subwavelength resolution by introducing spatial variations in the optical response of the light scatterers. Such gradient metasurfaces go beyond the well-established technology of frequency selective surfaces made of periodic structures and are extending to new spectral regions the functionalities of conventional microwave and millimetre-wave transmit-arrays and reflect-arrays. Metasurfaces can also be created by using ultrathin films of materials with large optical losses. By using the controllable abrupt phase shifts associated with reflection or transmission of light waves at the interface between lossy materials, such metasurfaces operate like optically thin cavities that strongly modify the light spectrum. Technology opportunities in various spectral regions and their potential advantages in replacing existing optical components are discussed.

Flat Optics With Designer Metasurfaces

The unprecedented ability of nanometallic (that is, plasmonic) structures to concentrate light into deep-subwavelength volumes has propelled their use in a vast array of nanophotonics technologies and research endeavours. Plasmonic light concentrators can elegantly interface diffraction-limited dielectric optical components with nanophotonic structures. Passive and active plasmonic devices provide new pathways to generate, guide, modulate and detect light with structures that are similar in size to state-of-the-art electronic devices. With the ability to produce highly confined optical fields, the conventional rules for light-matter interactions need to be re-examined, and researchers are venturing into new regimes of optical physics. In this review we will discuss the basic concepts behind plasmonics-enabled light concentration and manipulation, make an attempt to capture the wide range of activities and excitement in this area, and speculate on possible future directions.

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Plasmonics for extreme light concentration and manipulation.

Artificially engineered metamaterials are now demonstrating unprecedented electromagnetic properties that cannot be obtained with naturally occurring materials. In particular, they provide a route to creating materials that possess a negative refractive index and offer exciting new prospects for manipulating light. This review describes the recent progress made in creating nanostructured metamaterials with a negative index at optical wavelengths, and discusses some of the devices that could result from these new materials.

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Optical negative-index metamaterials

Microwave propagation through an electric-field-coupled metamaterial lens with a refractive index gradient is experimentally investigated. A gradient in the refractive index of the metamaterial is introduced by continuous tuning of a single parameter in the metamaterial elements. Experimental field maps, acquired in a planar waveguide, demonstrate that the planar gradient index lens can be used to collimate radiation from a line source, thereby forming a beam.

Electric-field-coupled metamaterials for microwave beam formation

Source radio frequency energy (RF) is coupled wirelessly, with no direct physical contact, between two compound loop (CPL) antennas across a variety of barriers such as plastic, human tissues, glass, and air. The compound coupling interface is highly efficient in transferring the RF energy from a source including one CPL antenna to a destination including a second CPL antenna. A re-radiating structure including a further CPL antenna or a different type of antenna may be connected on the destination side to completely physically isolate the source side from the destination side. When the destination coupling antenna is removed, the source coupling antenna may operate as an efficient radiator at the desired operating frequencies. Likewise, the destination coupling antenna may operate as an efficient radiator in the absence of the source coupling antenna.

Compound coupling to re-radiating antenna solution

We have demonstrated that effective gradient index devices can be fabricated to collimate and steer microwave beams by designing the spatial profile of positive effective permittivity across the slab. Resonant losses in the devices is reduced dramatically by operating at frequency ranges far from the unit cell resonance, and substrate losses is reduced by utilizing low loss material. Using these approaches, we demonstrate large steering angles of up to 22 degrees with only 0.7 dB transmission loss. These pave the way for practical implementation of low-cost microwave beam steering devices using metamaterials.

Electrically Coupled Graded Index Metamaterial

Gradientenindexlinse fur Mikrowellenstrahlung umfassend: mehrere elektronische induktive kapazitive Resonatoren jeweils mit einer Resonanzfrequenz, wobei die Resonatoren in einem planaren Array mit voneinander beabstandeten Seitenkanten und voneinander beabstandeten Ober- und Unterkanten angeordnet sind, wobei sich die Resonanzfrequenz der Resonatoren zwischen mindestens zwei der voneinander beabstandeten Kanten des Arrays andert.

B.J. Justice

Papers

Electric-field-coupled metamaterials for microwave beam formation

Compound coupling to re-radiating antenna solution

Electrically Coupled Graded Index Metamaterial

Gradienten-Index-Linse für Mikrowellenstrahlung