Advances in low power design open the possibility to harvest ambient energies to power directly the electronics or recharge a secondary battery. The key parameter of an energy harvesting (EH) device is its efficiency, which strongly depends on the conversion medium. To address this issue, metamaterials, artificial materials and structures with exotic properties, have been introduced for EH in recent years. They possess unique properties not easily achieved using naturally occurring materials, such as negative stiffness, mass, Poisson's ratio, and refractive index. The goal of this paper is to review the fundamentals, recent progresses and future directions in the field of metamaterials-based enhanced energy harvesting. An introduction on EH followed by the classification of potential metamaterials for EH is presented. A number of theoretical and experimental studies on metamaterials-based EH are outlined, including phononic crystals, acoustic metamaterials, and electromagnetic metamaterials. Finally, we give an outlook on future directions of metamaterials-based energy harvesting research including but not limited to active metamaterials-based EH, metamaterials-based thermal EH, and metamaterials-based multifunctional EH capabilities.

Metamaterials-based enhanced energy harvesting: A review

In this article, we review definition, origin, terminology, fundamental properties, design concepts and procedures, basic applications, modeling, and numerical simulation of metamaterials. It is shown that metamaterial origin can be easily understood by placing metamaterials in the more general context of artificial electromagnetic materials and of the efforts performed by the scientific community working in complex materials to mimic and overcome the properties of natural materials. The basic properties of some classes of metamaterials as well as the related limitations are reviewed and discussed. A new application-oriented definition of metamaterials is given in terms of a functionalizing layer placed in between the traditional concepts of materials and devices. Metamaterial applications and the design steps of metamaterial-based and metamaterial-inspired components are also discussed, with particular emphasis on the numerical simulation of ideal metamaterials through available solvers. In this frame, finite-difference time-domain (FDTD) modeling and simulation of the electromagnetic properties of metamaterials are deeply discussed. The recently introduced virtual MTM-FDTD tool is used to simulate the interaction between the electromagnetic field and metamaterials and show some metamaterial effects, such as beam focusing, existence of negative refractive angle, and so forth, through various examples in different scenarios. © 2012 Wiley Periodicals, Inc. Int J RF and Microwave CAE 22:422–438, 2012. © 2012 Wiley Periodicals, Inc.

Metamaterials: Definitions, properties, applications, and FDTD-based modeling and simulation (Invited paper)

We designed, implemented, and experimentally characterized electrically thin microwave absorbers by using the metamaterial concept. The absorbers consist of (i) a metal back plate and an artificial magnetic material layer; (ii) metamaterial back plate and a resistive sheet layer. We investigated absorber performance in terms of absorbance, fractional bandwidth, and electrical thickness, all of which depend on the dimensions of the metamaterial unit cell and the distance between the back plate and metamaterial layer. As a proof of concept, we demonstrated a λ/4.7 thick absorber of type I, with a 99.8% absorption peak along with a 8% fractional bandwidth. We have shown that as the electrical size of the metamaterial unit cell decreases, the absorber electrical thickness can further be reduced. We investigated this concept by using two different magnetic metamaterial inclusions: the split-ring resonator (SRR) and multiple SSR (MSRR). We have also demonstrated experimentally a λ/4.7 and a λ/4.2 thick absorber...

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Experimental verification of metamaterial based subwavelength microwave absorbers

In this paper, we present the design of cylindrical and spherical electromagnetic cloaks working at visible frequencies. The cloak design is based on the employment of layered structures consisting of alternating plasmonic and non-plasmonic materials and exhibiting the collective behavior of an effective epsilon-near-zero material at optical frequencies. The design of a cylindrical cloak to hide cylindrical objects is firstly presented. Two alternative layouts are proposed and both magnetic and non-magnetic objects are considered. Then, the design of spherical cloaks is also presented. The full-wave simulations presented throughout the paper confirm the validity of the proposed setup, and show how this technique can be used to reduce the observability of cylindrical and spherical objects. The effect of the losses is also considered.

Plasmonic metamaterial cloaking at optical frequencies

In this paper, we investigate on the use of non-Foster active elements to increase the operation bandwidth of a split-ring resonator (SRR) for possible application in metamaterial-inspired components. First, we design the circuit topology of the active load required to compensate the intrinsic reactance of the SRR and get a broadband response. Then, we show that the same procedure can be successfully applied to the case of a SRR-based monopole antenna and, in principle, to any metamaterial-inspired device employing SRRs. Finally, integrating an electromagnetic and a circuit simulator, we propose a possible realistic implementation of the active load, based on the employment of commercially available circuit elements. The obtained results (seven times improvement of the impedance bandwidth of the SRR-based monopole antenna) prove that non-Foster active loads can be successfully used to overcome the inherent narrow-band operation of SRR-based passive metamaterials and metamaterial-inspired components. The implementation issues related to circuit element dispersion, parasitic effects, and stability of the active circuit are fully considered in the proposed design.

Design of a Non-Foster Actively Loaded SRR and Application in Metamaterial-Inspired Components

In this paper, we propose a number of resonant metamaterial-based approaches to enhance the power transmission through sub-wavelength apertures. The extraordinary transmission beyond the diffraction limit is usually obtained using setups that rely on covers supporting proper surface plasmon polaritons or leaky waves. However, the actual implementation in real-life applications of these structures is strongly limited by their large transverse extension. After briefly reviewing the aforementioned limitations, we present in the paper new setups based on resonant approaches, characterized by covers whose transverse dimensions are of the same order of magnitude as the aperture size. The phenomenology of the enhanced transmission, the related theoretical aspects, the results of the numerical simulations and the successful experimental verification using the newly proposed setups are then presented in detail.

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Enhanced transmission through a sub-wavelength aperture: resonant approaches employing metamaterials

The recent advances in UAV (Unmanned Aerial Vehicle) -related technologies has led to an increasing demand for small, lightweight and broadband antennas to be used as sensors for both airborne EW (Electronic Warfare) and communication systems. In this paper, the design guidelines of a compact printed wideband dipole antenna for UAV applications is presented, an experimental characterization has been performed through near-field measurements showing excellent agreement with numerical simulations.

A small lightweight wideband printed dipole for UAV applications

Small, lightweight, and broadband antennas are a growing need for unmanned aerial vehicle (UAV) platforms due to the increasing demand of highly integrated systems for both electronic warfare (EW) and communication-oriented applications. In this article, an experimental characterization of a compact printed passive wide-band dipole antenna is presented. The main advantages of this design are the reduction of both the height and the weight of the antenna while keeping the structure as tough as possible from an environmental point of view with the only drawback being a tradeoff with respect to voltage-standing wave ratio (VSWR) performances. The simplicity (no lumped passive components are used to match the antenna input impedance to 50 X) and performance effectiveness of the proposed antenna is such that it avoids a complicated screening process among different phase-matched sets of radiating elements. The experimental characterization was performed through a near-field measurement setup, showing excellent agreement with numerical fullwave simulations.

Experimental Investigations of a Novel Lightweight Blade Antenna for UAV Applications [Antenna Applications Corner]

In this contribution, a novel low-profile blade monopole is presented, working in the 500–2500 MHz frequency band. The simple yet effective design is such to grant both physical strength and stable performances in terms of radiation pattern over the entire working frequency band, confirming this antenna as an excellent candidate for high precision Direction-Of-Arrival (DOA) avionic systems.

http://ieeexplore.ieee.org/iel7/7442673/7447267/07447271.pdf

A novel ultra-wideband UHF low-profile monopole for UAV platforms

In this letter, we propose the employment of nano-rings made of silver spheres, to enhance the transmission through a subwavelength aperture drilled in a metallic screen at optical frequencies. We show that the increase in the transmission is due to the excitation of a strong magnetic resonance in close proximity of the aperture. Compared to similar configurations presented in the literature, the proposed layout allows us to significantly reduce the size of the setup. The effectiveness of the approach is demonstrated through proper full-wave simulations, taking into account material dispersion and losses. In addition, it is shown how the nano-ring response can be tuned by embedding the structure in a host material, doped with liquid crystals.

Luca Scorrano

Papers

Enhanced transmission through a sub-wavelength aperture: resonant approaches employing metamaterials

A small lightweight wideband printed dipole for UAV applications

Experimental Investigations of a Novel Lightweight Blade Antenna for UAV Applications [Antenna Applications Corner]

A novel ultra-wideband UHF low-profile monopole for UAV platforms

Achieving Power Transmission Enhancement by Using Nano-Rings Made of Silver Spheres