Metamaterials are typically engineered by arranging a set of small scatterers or apertures in a regular array throughout a region of space, thus obtaining some desirable bulk electromagnetic behavior. The desired property is often one that is not normally found naturally (negative refractive index, near-zero index, etc.). Over the past ten years, metamaterials have moved from being simply a theoretical concept to a field with developed and marketed applications. Three-dimensional metamaterials can be extended by arranging electrically small scatterers or holes into a two-dimensional pattern at a surface or interface. This surface version of a metamaterial has been given the name metasurface (the term metafilm has also been employed for certain structures). For many applications, metasurfaces can be used in place of metamaterials. Metasurfaces have the advantage of taking up less physical space than do full three-dimensional metamaterial structures; consequently, metasurfaces offer the possibility of less-lossy structures. In this overview paper, we discuss the theoretical basis by which metasurfaces should be characterized, and discuss their various applications. We will see how metasurfaces are distinguished from conventional frequency-selective surfaces. Metasurfaces have a wide range of potential applications in electromagnetics (ranging from low microwave to optical frequencies), including: (1) controllable “smart” surfaces, (2) miniaturized cavity resonators, (3) novel wave-guiding structures, (4) angular-independent surfaces, (5) absorbers, (6) biomedical devices, (7) terahertz switches, and (8) fluid-tunable frequency-agile materials, to name only a few. In this review, we will see that the development in recent years of such materials and/or surfaces is bringing us closer to realizing the exciting speculations made over one hundred years ago by the work of Lamb, Schuster, and Pocklington, and later by Mandel'shtam and Veselago.

An Overview of the Theory and Applications of Metasurfaces: The Two-Dimensional Equivalents of Metamaterials

Huygens' principle is a well-known concept in electromagnetics that dates back to 1690. Here, it is applied to develop designer surfaces that provide extreme control of electromagnetic wave fronts across electrically thin layers. These reflectionless surfaces, referred to as metamaterial Huygens' surfaces, provide new beam shaping, steering, and focusing capabilities. The metamaterial Huygens' surfaces are realized with two-dimensional arrays of polarizable particles that provide both electric and magnetic polarization currents to generate prescribed wave fronts. A straightforward design methodology is demonstrated and applied to develop a beam-refracting surface and a Gaussian-to-Bessel beam transformer. Metamaterial Huygens' surfaces could find a wide range of applications over the entire electromagnetic spectrum including single-surface lenses, polarization controlling devices, stealth technologies, and perfect absorbers.

Metamaterial Huygens' surfaces: tailoring wave fronts with reflectionless sheets.

Magnetodielectric small spheres present unusual electromagnetic scattering features, theoretically predicted a few decades ago. However, achieving such behaviour has remained elusive, due to the non-magnetic character of natural optical materials or the difficulty in obtaining low-loss highly permeable magnetic materials in the gigahertz regime. Here we present unambiguous experimental evidence that a single low-loss dielectric subwavelength sphere of moderate refractive index (n=4 like some semiconductors at near-infrared) radiates fields identical to those from equal amplitude crossed electric and magnetic dipoles, and indistinguishable from those of ideal magnetodielectric spheres. The measured scattering radiation patterns and degree of linear polarization (3–9 GHz/33–100 mm range) show that, by appropriately tuning the a/λ ratio, zero-backward (‘Huygens’ source) or almost zero-forward (‘Huygens’ reflector) radiated power can be obtained. These Kerker scattering conditions only depend on a/λ. Our results open new technological challenges from nano- and micro-photonics to science and engineering of antennas, metamaterials and electromagnetic devices. The absence of forward or backward scattered radiation by magnetodielectric spheres was predicted decades ago, yet direct measurements have remained elusive. Geffrin et al. present unambiguous evidence of such scattering effects in the gigahertz range for a sub-wavelength dielectric sphere.

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Magnetic and electric coherence in forward- and back-scattered electromagnetic waves by a single dielectric subwavelength sphere

Metamaterials are materials typically engineered with novel or artificial structures to produce electromagnetic properties that are unusual or difficult to obtain in nature. Because of their promise to provide engineerable permittivity, permeability, and index of refraction, metamaterials have drawn broad interest and have led to possible utilization in many electromagnetic applications from the microwave to optical regime, especially for the radiated-wave devices. This paper presents a detailed review upon the most recent research efforts associated with those metamaterial-based small antennas. They are discussed and classified in several different categories such as the antennas based on dispersion engineering, metaresonators, and metamaterial loadings. Some practical difficulties or limitations for the development of metamaterial-based small antennas are pointed out and the possible approaches to resolve these problems are also illustrated. A wide variety of antenna examples are included to facilitate the understanding for general readers.

Metamaterial-Based Antennas

Metamaterial Huygens’ surfaces manipulate electromagnetic wavefronts without reflection. A broadband Huygens’ surface that efficiently refracts normally incident light at the telecommunication wavelength of 1.5 μm is reported. The electric and magnetic responses of the surface are independently controlled by cascading three patterned, metallic sheets with a subwavelength overall thickness of 430 nm. The peak efficiency of the device is significantly enhanced by reducing the polarization and reflection losses that are inherent to earlier single-layer designs.

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Efficient Light Bending with Isotropic Metamaterial Huygens' Surfaces

A variety of antennas have been engineered with metamaterials (MTMs) and metamaterial-inspired constructs to improve their performance characteristics. Examples include electrically small, near-field resonant parasitic (NFRP) antennas that require no matching network and have high radiation efficiencies. Experimental verification of their predicted behaviors has been obtained. Recent developments with this NFRP electrically small paradigm will be reviewed. They include considerations of increased bandwidths, as well as multiband and multifunctional extensions.

Metamaterial-Inspired Engineering of Antennas

Two compact, printed, ultrawideband (UWB) monopole antennas with tri-band notched characteristics are reported. The notched filters are achieved by introducing printed, electrically small, capacitively-loaded loop (CLL) resonators. The directly driven elements consist of printed top-loaded CLL-based monopoles and 50 Ω microstrip feed lines. By adding three CLL elements close to the feed line, band-notch properties in the WiMAX (3.3-3.6GHz), lower WLAN (5.15-5.35GHz) and higher WLAN (5.725-5.825GHz) bands are achieved. Each antenna system is contained on a 27×34mm2 sheet of Rogers Duroid 5880 substrate. One is designed with three additional CLL elements; the other is achieved with only two. Comparisons between the simulation and measurement results show that these UWB antennas have broadband matched impedance values and stable radiation patterns for all radiating frequencies.

Single, Dual and Tri-Band-Notched Ultrawideband (UWB) Antennas Using Capacitively Loaded Loop (CLL) Resonators

Electrically small electric and magnetic dipole antennas are introduced that are based on electric and magnetic near-field resonant parasitic (NFRP) elements that are electrically coupled to the driven element. By properly combining these two NFRP antenna types, electrically small Huygens sources are realized. The proposed Huygens sources have one feed point, are well matched to 50 Ω, and have high radiation efficiencies. Their predicted directivity patterns approach the known Huygens source theoretical limit.

Metamaterial-Inspired, Electrically Small Huygens Sources

Several metamaterial-inspired, electrically small, near-field resonant parasitic antennas are presented. Both electric and magnetic couplings to the parasitic are compared and contrasted. The electric-coupled versions are shown to be more efficient and to have more bandwidth. The evolution of circular polarized designs from their linear counterparts by introducing multiple parasitics having different resonant frequencies is demonstrated. Single L1 and dual band L1/L2 GPS systems are emphasized for practical illustrations of the resulting performance characteristics. Preliminary experimental results for a dual band, circularly polarized GPS L1/L2 antenna are provided and underscore several practical aspects of these designs.

Multi-Frequency, Linear and Circular Polarized, Metamaterial-Inspired, Near-Field Resonant Parasitic Antennas

The possibility of using an active internal matching element in several types of metamaterial-inspired, electrically small antennas (ESAs) to overcome their inherent narrow bandwidths is demonstrated. Beginning with the Z antenna, which is frequency tunable through its internal lumped element inductor, a circuit model is developed to determine an internal matching network, i.e., a frequency dependent inductor, which leads to the desired enhanced bandwidth performance. An analytical relation between the resonant frequency and the inductor value is determined via curve fitting of the associated HFSS simulation results. With this inductance-frequency relation defining the inductor values, a broad bandwidth, electrically small Z antenna is established. This internal matching network paradigm is then confirmed by applying it to the electrically small stub and canopy antennas. An electrically small canopy antenna with k? = 0.0467 that has over a 10% bandwidth is finally demonstrated. The potential implementation of the required frequency dependent inductor is also explored with a well-defined active negative impedance converter circuit that reproduces the requisite inductance-frequency relations.

Peng Jin

Papers

Metamaterial-Inspired Engineering of Antennas

Single, Dual and Tri-Band-Notched Ultrawideband (UWB) Antennas Using Capacitively Loaded Loop (CLL) Resonators

Metamaterial-Inspired, Electrically Small Huygens Sources

Multi-Frequency, Linear and Circular Polarized, Metamaterial-Inspired, Near-Field Resonant Parasitic Antennas

Broadband, Efficient, Electrically Small Metamaterial-Inspired Antennas Facilitated by Active Near-Field Resonant Parasitic Elements