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.

/pdf/metamaterial-electromagnetic-cloak-at-microwave-frequencies-1qj4zdgl8e.pdf

Metamaterial Electromagnetic Cloak at Microwave Frequencies

The possibility of cloaking an object from detection by electromagnetic waves has recently become a topic of considerable interest. The design of a cloak uses transformation optics, in which a conformal coordinate transformation is applied to Maxwell's equations to obtain a spatially distributed set of constitutive parameters that define the cloak. Here, we present an experimental realization of a cloak design that conceals a perturbation on a flat conducting plane, under which an object can be hidden. To match the complex spatial distribution of the required constitutive parameters, we constructed a metamaterial consisting of thousands of elements, the geometry of each element determined by an automated design process. The ground-plane cloak can be realized with the use of nonresonant metamaterial elements, resulting in a structure having a broad operational bandwidth (covering the range of 13 to 16 gigahertz in our experiment) and exhibiting extremely low loss. Our experimental results indicate that this type of cloak should scale well toward optical wavelengths.

http://science.sciencemag.org/content/sci/323/5912/366.full.pdf

Broadband Ground-Plane Cloak

Silveirinha and Engheta have recently proposed that electromagnetic waves can tunnel through a material with an electric permittivity ($ϵ$) near zero (ENZ). An ENZ material of arbitrary geometry can thus serve as a perfect coupler between incoming and outgoing waveguides with identical cross-sectional area, so long as one dimension of the ENZ is electrically small. In this Letter we present an experimental demonstration of microwave tunneling between two planar waveguides separated by a thin ENZ channel. The ENZ channel consists of a planar waveguide in which complementary split ring resonators are patterned on the lower surface. A tunneling passband is found in transmission measurements, while a two-dimensional spatial map of the electric field distribution reveals a uniform phase variation across the channel---both measurements in agreement with theory and numerical simulations.

/pdf/experimental-demonstration-of-electromagnetic-tunneling-4pnumtskxe.pdf

Experimental demonstration of electromagnetic tunneling through an epsilon-near-zero metamaterial at microwave frequencies.

Invisibility is a notion that has long captivated the popular imagination. However, in 2006, invisibility became a practical matter for the scientific community as well, with the suggestion that artificially structured metamaterials could enable a new electromagnetic design paradigm, now termed transformation optics. Since the advent of transformation optics and subsequent initial demonstration of the microwave cloak, the field has grown rapidly. However, the complexity of the transformation optics material prescription has continually forced researchers to make simplifying approximations to achieve even a subset of the desired functionality. These approximations place profound limitations on the performance of transformation optics devices in general, and cloaks especially. Here, we design and experimentally characterize a two-dimensional, unidirectional cloak that makes no approximations to the underlying transformation optics formulation, yet is capable of reducing the scattering of an object ten wavelengths in size. We demonstrate that this approximation-free design regains the performance characteristics promised by transformation optics.

A full-parameter unidirectional metamaterial cloak for microwaves

In a recent theoretical work by Narimanov and Kildishev (2009 Appl. Phys. Lett. 95 041106) an optical omnidirectional light absorber based on metamaterials was proposed, in which theoretical analysis and numerical simulations showed that all optical waves hitting the absorber are trapped and absorbed. Here we report the first experimental demonstration of an omnidirectional electromagnetic absorber in the microwave frequency. The proposed device is composed of non-resonant and resonant metamaterial structures, which can trap and absorb electromagnetic waves coming from all directions spirally inwards without any reflections due to the local control of electromagnetic fields. It is shown that the absorption rate can reach 99 per cent in the microwave frequency. The all-directional full absorption property makes the device behave like an 'electromagnetic black body', and the wave trapping and absorbing properties simulate, to some extent, an 'electromagnetic black hole.' We expect that such a device could be used as a thermal emitting source and to harvest electromagnetic waves.

https://iopscience.iop.org/article/10.1088/1367-2630/12/6/063006/pdf

An omnidirectional electromagnetic absorber made of metamaterials

We perform an experimental study of the phase and amplitude of microwaves interacting with and scattered by two-dimensional negative index metamaterials. The measurements are performed in a parallel plate waveguide apparatus at X-band frequencies (8–12 GHz), thus constraining the electromagnetic fields to two dimensions. A detection antenna is fixed to one of the plates, while a second plate with a fixed source antenna or waveguide is translated relative to the first plate. The detection antenna is inserted into, but not protruding below, the stationary plate so that fields internal to the metamaterial samples can be mapped. From the measured mappings of the electric field, the interplay between the microstructure of the metamaterial lattice and the macroscopic averaged response is revealed. For example, the mapped phase fronts within a metamaterial having a negative refractive index are consistent with a macroscopic phase—in accordance with the effective medium predictions—which travels in a direction opposite to the direction of propagation. The field maps are in excellent agreement with finite element numerical simulations performed assuming homogeneous metamaterial structures.

/pdf/spatial-mapping-of-the-internal-and-external-electromagnetic-4uwlxmaa25.pdf

Liheng Guo

Papers

Spatial mapping of the internal and external electromagnetic fields of negative index metamaterials.