Metamaterials, artificial electromagnetic media that are structured on the subwavelength scale, were initially suggested for the negative-index 'superlens'. Later metamaterials became a paradigm for engineering electromagnetic space and controlling propagation of waves: the field of transformation optics was born. The research agenda is now shifting towards achieving tunable, switchable, nonlinear and sensing functionalities. It is therefore timely to discuss the emerging field of metadevices where we define the devices as having unique and useful functionalities that are realized by structuring of functional matter on the subwavelength scale. In this Review we summarize research on photonic, terahertz and microwave electromagnetic metamaterials and metadevices with functionalities attained through the exploitation of phase-change media, semiconductors, graphene, carbon nanotubes and liquid crystals. The Review also encompasses microelectromechanical metadevices, metadevices engaging the nonlinear and quantum response of superconductors, electrostatic and optomechanical forces and nonlinear metadevices incorporating lumped nonlinear components.

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From metamaterials to metadevices.

Transformation optics describes the capability to design the path of light waves almost at will through the use of metamaterials that control effective materials properties on a subwavelength scale. In this review, the physics and applications of transformation optics are discussed.

Transformation optics and metamaterials

Metamaterials based on effective media can be used to produce a number of unusual physical properties (for example, negative refraction and invisibility cloaking) because they can be tailored with effective medium parameters that do not occur in nature. Recently, the use of coding metamaterials has been suggested for the control of electromagnetic waves through the design of coding sequences using digital elements ‘0’ and ‘1,' which possess opposite phase responses. Here we propose the concept of an anisotropic coding metamaterial in which the coding behaviors in different directions are dependent on the polarization status of the electromagnetic waves. We experimentally demonstrate an ultrathin and flexible polarization-controlled anisotropic coding metasurface that functions in the terahertz regime using specially designed coding elements. By encoding the elements with elaborately designed coding sequences (both 1-bit and 2-bit sequences), the x- and y-polarized waves can be anomalously reflected or independently diffused in three dimensions. The simulated far-field scattering patterns and near-field distributions are presented to illustrate the dual-functional performance of the encoded metasurface, and the results are consistent with the measured results. We further demonstrate the ability of the anisotropic coding metasurfaces to generate a beam splitter and realize simultaneous anomalous reflections and polarization conversions, thus providing powerful control of differently polarized electromagnetic waves. The proposed method enables versatile beam behaviors under orthogonal polarizations using a single metasurface and has the potential for use in the development of interesting terahertz devices. An artificial material that controls electromagnetic waves of different polarization independently has been demonstrated by a team in China. Tie Jun Cui from the Southeast University and co-workers have created a metamaterial that can, for example, split incoming unpolarized radiation so that horizontally polarized light goes one way while vertically polarized light goes the other. Metamaterials are structures that can be engineered to have optical properties not found in natural materials, and they consist of a repeated pattern of elements that are smaller than the wavelength of light. The researchers used two types of element, simple squares and dumbbells, which enabled them to independently control beams of long-wavelength radiation known as terahertz waves having differing polarizations. By reducing the size of the metamaterial elements, the same idea could also be applied to visible light.

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Anisotropic coding metamaterials and their powerful manipulation of differently polarized terahertz waves

Invisibility to electromagnetic fields has become an exciting theoretical possibility. However, the experimental realization of electromagnetic cloaks has only been achieved starting from simplified approaches (for instance, based on ray approximation, canceling only some terms of the scattering fields, or hiding a bulge in a plane instead of an object in free space). Here, we demonstrate, directly from Maxwell equations, that a specially designed cylindrical superconductor-ferromagnetic bilayer can exactly cloak uniform static magnetic fields, and we experimentally confirmed this effect in an actual setup.

https://science.sciencemag.org/content/sci/335/6075/1466.full.pdf

Experimental Realization of a Magnetic Cloak

DOI: 10.1002/adma.201104012 Isolation from external magnetic fields is a fundamental requirement that appears in various scientific experiments today. In particular, for applications where static or slowly varying magnetic fields need to be screened, enclosures made of superconductors or high magnetic permeability metal alloys are typically utilized. These are called “shields” as they isolate the inner space from the external fields. They achieve this interior shielding by either expelling the fields from or drawing the fields into the shield materials, which inevitably results in the distortion of the external field profile by the presence of the shields themselves. In this communication, we report the first experimental realization of a “cloak” for dc magnetic fields. We construct a hollow cylindrical cloak with a material consisting of an artificially patterned network of superconducting and soft ferromagnetic elements. We show that, when an external dc magnetic field is applied, the interior of the cloak is completely shielded while the field outside remains unperturbed. A dc magnetic cloak that shields the inner region without affecting the exterior space requires a highly anisotropic medium characterized with a response that is diamagnetic in one direction and paramagnetic in the perpendicular directions.[1,2] For the former diamagnetic property, arrays of superconducting strips have been investigated,[1–10] and we incorporate such structures into our cloaking material. See Figure 1(a). We thermally evaporate 200 nm-thick Pb film onto 77m-thick polyimide sheets and pattern the film into an array network using photolithography. When the magnetic field is applied perpendicularly to the superconducting structures, the Meissner effect funnels the field into the regions between the adjacent plates. When the field encounters the next layer of superconducting planes, it becomes expelled, distorted, and then guided in a direction perpendicular to the original incoming direction. The field that penetrates the second layer through the gap between superconducting structures again sees the third layer to repeat the same process, until all applied field has been guided in-plane. In this stacked superconducting meta-material, we cascade the layers so that the positions of superconducting structures do not align perfectly from layer to layer to encourage field distortion within the material. The strength of diamagnetism (for fields perpendicular to the plates) depends on the dimension of material building blocks (i.e., superconducting structures) and the lattice spacing for their network.[1–5] This provides us with a knob to tune the field path by using the “graded” geometry with variations in lattice parameters. The spatial variation that we prescribe is that the gap between the superconducting plates narrows as the field penetrates deeper into the material. This allows the magnetic field to enter the cloak freely with little field perturbation while forcing it to be more distorted and guided within the material as it penetrates further into it. The outermost layer has 2000 m 2000 m superconducting plates with a 600 m gap. The innermost layer has the plates of the same size with 4 m spacing. The gap is reduced in five discrete steps. See Table 1 for the physical parameters of different superconducting layers. The superconducting plate thickness (200 nm) is chosen to be smaller than the lattice spacing but larger than the London penetration depth. The second requirement to realizing a dc magnetic cloak is to engineer a material with a tendency to shunt the magnetic field that is in plane. To achieve this specific profile of permeability, we deposit a soft ferromagnetic material between the neighboring superconducting layers and turn the space into a low reluctance path for guided magnetic field lines. Permalloy Supradeep Narayana and Yuki Sato* DC Magnetic Cloak

DC magnetic cloak.

Electromagnetic metamaterials are a class of materials that have been artificially structured on a subwavelength scale. They are currently the focus of a great deal of interest because they allow access to previously unrealizable properties such as a negative refractive index. Most metamaterial designs have so far been based on resonant elements, such as split rings, and research has concentrated on microwave frequencies and above. Here, we present the first experimental realization of a non-resonant metamaterial designed to operate at zero frequency. Our samples are based on a recently proposed template for an anisotropic magnetic metamaterial consisting of an array of superconducting plates. Magnetometry experiments show a strong, adjustable diamagnetic response when a field is applied perpendicular to the plates. We have calculated the corresponding effective permeability, which agrees well with theoretical predictions. Applications for this metamaterial may include non-intrusive screening of weak d.c. magnetic fields.

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A d.c. magnetic metamaterial

Electromagnetic metamaterials are a class of materials which have been artificially structured on a subwavelength scale. They are currently the focus of a great deal of interest because they allow access to previously unrealisable properties like a negative refractive index. Most metamaterial designs have so far been based on resonant elements, like split rings, and research has concentrated on microwave frequencies and above. In this work, we present the first experimental realisation of a non-resonant metamaterial designed to operate at zero frequency. Our samples are based on a recently-proposed template for an anisotropic magnetic metamaterial consisting of an array of superconducting plates. Magnetometry experiments show a strong, adjustable diamagnetic response when a field is applied perpendicular to the plates. We have calculated the corresponding effective permeability, which agrees well with theoretical predictions. Applications for this metamaterial may include non-intrusive screening of weak DC magnetic fields.

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A DC magnetic metamaterial

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Contrasts in critical current

The pinning by twin planes and an isotropic pinning disorder was investigated on single crystals of NdBa 2 Cu 3 O 7− δ and YBa 2 Cu 3 O 7− δ studying the angular dependence of total magnetic moment measured quasistatically by means of SQUID and dynamically by means of a vibrating sample magnetometer. The field dependence of the normalized relaxation rate of the vortex system pinned by twin planes, and by a random isotropic pinning disorder was determined. The qualitative difference of the two field profiles is clearly reflected by the shapes of the corresponding magnetic hysteresis loops. Material aspects of the pinning and the effect of the measurement method are discussed.

David Caplin

Papers

A d.c. magnetic metamaterial

A DC magnetic metamaterial

Contrasts in critical current

Flux pinning by twin planes in Nd-123 and Y-123 single crystals

High-temperature anomalies