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Heterogeneous photocatalysis is an important research problem relevant to a variety of sustainable energy technologies. However, obtaining high photocatalytic efficiency from visible light absorbing semiconductors is challenging due to a combination of weak absorption, transport losses, and low activity. Aspects of this problem have been addressed by multilayer approaches, which provide a general scheme for engineering surface reactivity and stability independent of electronic considerations. However, an analogous broad framework for optimizing light–matter interactions has not yet been demonstrated. Here, we establish a photonic approach using semiconductor metasurfaces that is highly effective in enhancing the photocatalytic activity of GaAs, a high-performance semiconductor with a near-infrared bandgap. Our engineered pillar arrays with heights of ∼150 nm exhibit Mie resonances near 700 nm that result in near-unity absorption and exhibit a field profile that maximizes charge carrier generation near the solid–liquid interface, enabling short transport distances. Our hybrid metasurface photoanodes facilitate oxygen evolution and exhibit enhanced incident photon-to-current efficiencies that are ∼22× larger than a corresponding thin film for resonant excitation and 3× larger for white light illumination. Key to these improvements is the preferential generation of photogenerated carriers near the semiconductor interface that results from the field enhancement profile of magnetic dipolar-type modes. [ABSTRACT FROM AUTHOR]

Metasurfaces are ushering in an era of multifunctional control over optical wavefronts realized with ultrathin planarized devices. Recent advances have been enabling unprecedented control over the frequency response of these surfaces, suggesting that the future of flat optics may tailor both spectral and spatial degrees of freedom in highly multispectral and multifunctional devices. Diffractive nonlocal metasurfaces are opening new opportunities in this direction: they leverage symmetry‐protected scattering from quasi‐bound states in the continuum and, by spatially manipulating controlled geometric perturbations, they support ultrasharp optical responses with wavefront‐manipulating features. Encoded in nonlocal (i.e., spatially extended) resonant modes, the resulting response is observed exclusively within the bandwidth of the resulting Fano resonance, affording ideal features for a wide range of applications. In this perspective, this novel class of metasurfaces are discussed in the broader context of flat optics, highlighting their peculiar operation in contrast to relevant predecessors, and highlighting the opportunities for future advancement and applications. In particular, it is emphasized that nonlocality and selectivity are inherently related, but that spectral and spatial selectivity can be independently tuned in suitably tailored metasurfaces. In turn, this freedom allows the design and implementation of both wavefront‐shaping and wavefront‐selective devices. The novel optical responses, combined with the compatibility with rational design, herald new prospects for active, nonlinear and quantum metasurfaces, ultrathin devices for augmented reality, and compact tailored optical sources. [ABSTRACT FROM AUTHOR]

In the last few years, highly anisotropic metamaterials have been explored in various geometries, showcasing interesting routes to achieve better control of sound propagation. As an extreme example, hyperbolic metasurfaces have been shown to offer broadband enhanced sound-matter interactions and diffraction-less propagation of acoustic waves, providing opportunities for sub-diffraction imaging and enhanced sound emission. In this study, we show that structure design of a locally resonant metamaterial enables extreme anisotropic responses, ranging from elliptic to hyperbolic propagation of acoustic surface waves, offering interesting opportunities for extreme sound guiding and steering at the subwavelength scale well compatible with a wide range of additive manufacturing techniques. [ABSTRACT FROM AUTHOR]

The quality factor of a passive, linear, small acoustic radiator is fundamentally limited by its volume normalized to the emitted wavelength, imposing severe constraints on the bandwidth and efficiency of compact acoustic sources and of metamaterials composed of arrangements of small acoustic resonators. We demonstrate that these bounds can be overcome by loading a piezoelectric transducer with a non-Foster active circuit, showing that its radiation bandwidth and efficiency can be largely extended beyond what is possible in passive radiators, fundamentally limited only by stability considerations. Based on these principles, we experimentally observe a threefold bandwidth enhancement compared to its passive counterpart, paving the way toward non-Foster acoustic radiation and more broadly active metamaterials that overcome the bandwidth constraints hindering passive systems. [ABSTRACT FROM AUTHOR]

We theoretically explore signatures of plasmonic Fano interferences in a subwavelength plasmonic metamolecule consisting of closely packed asymmetric gold nanodimers, which lead to the possibility of generating multiple Fano resonances in the scattering spectrum. This spectral feature is attributed to the interference between bright and dark plasmonic modes sustained by the constituent nanodimers. The excited Fano dips are highly sensitive in both wavelength and amplitude to geometry and background dielectric medium. The tunability of induced Fano resonances associated with enhanced electric fields from the visible to infrared region provides promising applications, particularly in refractive index sensing, light-trapping, and photon up-converting. [ABSTRACT FROM AUTHOR]

Originally discovered in condensed matter systems, topological insulators (TIs) have been ubiquitously extended to various fields of classical wave physics including photonics, phononics, acoustics, mechanics, and microwaves. In the bulk, like any other insulator, electronic TIs exhibit an excessively high resistance to the flow of mobile charges, prohibiting metallic conduction. On their surface, however, they support one-way conductive states with inherent protection against certain types of disorder and defects, defying the common physical wisdom of electronic transport in presence of impurities. When transposed to classical waves, TIs open a wealth of exciting engineering-oriented applications, such as robust routing, lasing, signal processing, switching, etc., with unprecedented robustness against various classes of defects. In this article, we first review the basic concept of topological order applied to classical waves, starting from the simple onedimensional example of the Su–Schrieffer–Heeger (SSH) model. We then move on to two-dimensional wave TIs, discussing classical wave analogues of Chern, quantum Hall, spin-Hall, Valley-Hall, and Floquet TIs. Finally, we review the most recent developments in the field, including Weyl and nodal semimetals, higherorder topological insulators, and self-induced non-linear topological states. [ABSTRACT FROM AUTHOR]

Following our recent idea of using plasmonic and nonplasmonic nanoparticles as nanoinductors and nanocapacitors in the infrared and optical domains [N. Engheta et al., Phys. Rev. Letts. 95, 095504 (2005)], in this work we analyze in detail some complex circuit configurations involving series and parallel combinations of these lumped nanocircuit elements at optical frequencies. Using numerical simulations, it is demonstrated that, after a proper design, the behavior of these nanoelements may closely mimic that of their lower-frequency [i.e., radio frequency (rf) and microwave] counterparts, even in relatively complex configurations. In addition, we analyze here in detail the concepts of nanoinsulators and nanoconnectors in the optical domain, demonstrating how these components may be crucial in minimizing the coupling between adjacent optical nanocircuit elements and in properly connecting different branches of the nanocircuit. The unit nanomodules for lumped nanoelements are introduced as building blocks for more complex nanocircuits at optical frequencies. Numerical simulations of some complex circuit scenarios considering the frequency response of these nanocircuits are presented and discussed in detail, showing how practical applications of such optical nanocircuit concepts may indeed be feasible within the current limits of nanotechnology. [ABSTRACT FROM AUTHOR]

Leaky waves are complex eigenmodes compatible with radiation in terms of their momentum, which can be supported by open waveguiding structures and can be effective at modeling radiative phenomena from such systems. Here, a "flatland" analog of leaky‐wave radiation is introduced to describe a novel mechanism for in‐plane radiation leakage that can occur in artificial or natural low‐dimensional materials. Possible platforms are illustrated for its physical realization, in the form of suitably designed planar junctions of isotropic impedance surfaces that implement surface‐wave or line‐wave guiding structures. A simple and insightful semi‐analytical model for the radiation mechanism is developed and validated against full‐wave numerical simulations. These results provide a new tool for the advanced manipulation of surface waves at the nanoscale, which relies entirely on interface‐optics effects, and can find interesting applications in the emerging field of polaritonics. [ABSTRACT FROM AUTHOR]

Unusual scattering effects from tiny spherical particles may be obtained when concentric shells are designed by pairing together “complementary” double-negative, single-negative, and/or standard double-positive materials. By embedding these highly polarizable scatterers in a host medium one can achieve a bulk medium with interesting effective parameters. Some physical insights and justifications for the anomalous polarizability of these concentric spherical nanoparticles and the effective parameters of the bulk composite medium are discussed. [ABSTRACT FROM AUTHOR]

Wediscuss resonant states with divergingQfactor within the radiation continuum based on the anomalous interaction of leaky guided modes and diffracted waves in suitably designed reflection gratings. We show that these trapped optical states can be understood within the framework of leakywave theory, which unveils their generation process and dynamics. Our findings reveal an interesting mechanism to realize embedded eigenstates in periodic structures, shedding light on their electromagnetic properties, and offering the possibility to quantitatively predict their occurrence and systematically design optimal structures that support them. The realization of extraordinary optical trapping in open structures may be important for applications that require strongly confined and enhanced fields and high selectivity in angle and frequency. [ABSTRACT FROM AUTHOR]

Ebrahim I et al. i [3] investigate the temporal dynamics in strongly coupled ENZ plasmonic systems, illustrating the dynamics of field enhancement processes and highlighting the impact of dissipation loss. Beruete I et al. i [14] demonstrate experimentally the unusual waveguiding features of zero-permittivity structures with applications for enhanced sensing, and in parallel, Nicolussi I et al. i [15] discuss unusual unidirectional transparency in waveguides featuring zero-permittivity materials obeying parity-time symmetry. Zero-index metamaterials (photonic materials and artificial media with a near-zero refractive index in different parts of the electromagnetic spectrum) make possible novel forms of light-matter interactions where light behaves in a qualitatively different way than in conventional materials. [Extracted from the article]

Breaking acoustic reciprocity is essential to create robust one-way propagation where sound waves or elastic vibrations are permitted to travel in only one direction. This unidirectional response forms the basis for devices such as acoustic isolators and circulators, and it also unlocks new functionalities for complex systems such as acoustic topological insulators. After reviewing the principles of acoustic reciprocity, we look at techniques to achieve large reciprocity breaking, including nonlinearities, moving media, spatiotemporal modulation, and nonlinear bianisotropy. We then discuss the recent surge of progress in nonreciprocal surface acoustic wave devices and topological acoustic systems, areas which we predict will continue to flourish in the coming years. We anticipate that these and other applications of nonreciprocity will continue to enhance acoustic technology and form the basis for new acoustic devices. Reciprocity is a fundamental principle in wave manipulation, and techniques for breaking its symmetry will continue to be discovered, refined, optimized, and applied to several acoustic domains as the understanding of the underlying principles and new technologies mature. [ABSTRACT FROM AUTHOR]

B Diffractive Nonlocal Metasurfaces b In article number 2100633, Adam Overvig and Andrea Alù present a perspective on the emerging field of diffractive nonlocal metasurfaces, which are able to tailor both the spatial and spectral properties of light through deliberately engineered perturbations in high symmetry lattices. Highly customizable scattering rate, phase, and polarization properties, tailored over subwavelength scales, enable highly selective optical responses, offering new avenues for rationally designing ultrathin photonic devices. [Extracted from the article]

The study of light at the nanoscale has become a vibrant field of research, as researchers now master the flow of light at length scales far below the optical wavelength, largely surpassing the classical limits imposed by diffraction. Using metallic and dielectric nanostructures precisely sculpted into two-dimensional (2D) and 3D nanoarchitectures, light can be scattered, refracted, confined, filtered, and processed in fascinating new ways that are impossible to achieve with natural materials and in conventional geometries. This control over light at the nanoscale has not only unveiled a plethora of new phenomena but has also led to a variety of relevant applications, including new venues for integrated circuitry, optical computing, solar, and medical technologies, setting high expectations for many novel discoveries in the years to come. [ABSTRACT FROM AUTHOR]

Broadband impedance matching and zero reflection of acoustic waves at a planar interface between two natural materials is a rare phenomenon, unlike its optical counterpart, frequently observed for polarized light incident at the Brewster angle. In this article, it is shown that, by inserting a metamaterial layer between two acoustic materials with different impedance, it is possible to artificially realize an extremely broadband Brewster-like acoustic intromission angle window, in which energy is totally transmitted from one natural medium to the other. The metamaterial buffer, composed of acoustically hard materials with subwavelength tapered apertures, provides an interesting way to match the impedances of two media in a broadband fashion, different from traditional methods like quarter-wave matching or Fabry-Pérot resonances, inherently narrowband due to their resonant nature. This phenomenon may be interesting for a variety of applications including energy harvesting, acoustic imaging, ultrasonic transducer technology, and noise control. [ABSTRACT FROM AUTHOR]

We explore the enhancement mechanisms associated with combined Fano resonances in metasurfaces, demonstrating enhanced absorption efficiency of thin-film organic solar cells over a moderately broad bandwidth of operation. An integrated AM1.5G (air mass 1.5 global solar energy) absorption enhancement of up to 44.6% is obtained, corresponding to an increase in solar absorption at normal incidence from 48% to 69.4%. Fano resonances are induced in arrays of four asymmetric-arranged nanodisks (nanopillars) with small inter-disk gaps, incorporated into an organic thin-film. They are shown to play a dominant role in light-trapping enhancement, mediated by combined localized surface plasmon resonances. [ABSTRACT FROM AUTHOR]

Analog computing based on wave interactions with metamaterials has been raising significant interest as a low-energy, ultrafast platform to process large amounts of data. Engineered materials can be tailored to impart mathematical operations of choice on the spatial distribution of the impinging signals, but they also require extended footprints and precise large-area fabrication, which may hinder their practical applicability. Here we show that the nonlocal response of a compact scatterer can be engineered to impart operations of choice on arbitrary impinging waves, and even to solve integro-differential equations, whose solution is observed in the scattered fields. The lack of strongly resonant phenomena makes the response robust, and the compact nature opens to scalability and cascading of these processes, paving the way to efficient, compact analog computers based on engineered microstructures. [ABSTRACT FROM AUTHOR]

We study the possibility of controlling the propagation of ballistic electrons using an electronic metamaterial with zero quantum index of refraction. This is obtained by tailoring the band structure of a semiconductor superlattice to exhibit a Dirac cone at the center of the Brillouin zone, creating an artificial quantum medium with zero refractive index for electrons. We demonstrate attractive functionalities, such as anomalous tunneling, ideal matching, electronic routing, cloaking, and focusing. We envision applications in nanoelectronics and sensing. [ABSTRACT FROM AUTHOR]

In this Letter, a bilaminate acoustic cloak designed using scattering cancellation methods is applied to the case of an acoustic sensor consisting of a hollow piezoelectric shell with mechanical absorption. The bilaminate cloak provides 20-50 dB reduction in scattering strength relative to the uncloaked configuration over the typical range of operation for an acoustic sensor, retains its ability to sensing acoustic pressure signals, and remains within the physical bounds of a passive absorber. Further, the cloak is shown to increase the range of frequencies over which there is nearly perfect phase fidelity between the acoustic signal and the voltage generated by the sensor. The feasibility of achieving the necessary fluid layer properties is demonstrated using sonic crystals with the use of readily available acoustic materials. [ABSTRACT FROM AUTHOR]

We propose a comprehensive scheme for the efficient design of graded optical metasurfaces capable of rerouting the impinging energy at will in a flexible way. We show that carefully designed conjoined optical nanoelements may be used as basic building blocks to arbitrarily shape the reflected phase front, while providing large controllability and low loss over deeply subwavelength thicknesses. The metasurface elements are designed using transmission line concepts combined with nanocircuit theory, and they can be realized with conventional lithographic techniques. Based on these concepts, we put forward a fast and accurate analytical model to design the optical analog of reflectarrays for light bending, steering, and focusing. The proposed designs show large efficiency over a broad angular spectrum, accompanied by broad bandwidths of operation. Our technique may lead to significant advances in the field of planarized nanophotonics and light manipulation over a surface, with potential applications in light trapping and efficient photonic couplers. [ABSTRACT FROM AUTHOR]

Confinement of light is usually achieved by reducing the photonic density of states of the surrounding environment, as in waveguides and photonic crystal cavities. In contrast, optical bound states have been recently shown to exist within the radiation continuum in large periodic arrays, in analogy with embedded eigenvalues in certain quantum systems. Here we demonstrate that under special conditions it may be possible to induce three-dimensional photonic embedded eigenvalues in a subwavelength open scattering system. The striking properties of these optical states without radiation loss may lead to unprecedented light localization in nanophotonic structures. [ABSTRACT FROM AUTHOR]

The rise of plasmonic metamaterials in recent years has unveiled the possibility of revolutionizing the entire field of optics and photonics, challenging well-established technological limitations and paving the way to innovations at an unprecedented level. To capitalize the disruptive potential of this rising field of science and technology, it is important to be able to combine the richness of optical phenomena enabled by nanoplasmonics in order to realize metamaterial components, devices, and systems of increasing complexity. Here, we review a few recent research directions in the field of plasmonic metamaterials, which may foster further advancements in this research area. We will discuss the anomalous scattering features enabled by plasmonic nanoparticles and nanoclusters, and show how they may represent the fundamental building blocks of complex nanophotonic architectures. Building on these concepts, advanced components can be designed and operated, such as optical nanoantennas and nanoantenna arrays, which, in turn, may be at the basis of metasurface devices and complex systems. Following this path, from basic phenomena to advanced functionalities, the field of plasmonic metamaterials offers the promise of an important scientific and technological impact, with applications spanning from medical diagnostics to clean energy and information processing. [ABSTRACT FROM AUTHOR]

Absorption and scattering are inherently related, as it is not possible to absorb power without creating a far-field shadow. We show, however, that properly overlapped resonant modes in a suitably designed system may in principle lead to arbitrarily large absorption levels, while at the same time minimizing the total scattering. We discuss the fundamental limits on scattering and absorption of an arbitrary receiving system and envision a composite nanoparticle that demonstrates the concept of a minimum-scattering superabsorber, with potential applications in energy harvesting, sensing, and imaging. [ABSTRACT FROM AUTHOR]

We propose the concept of a graphene-based nanoantenna-enhanced photomixer to realize wideband-tunable terahertz (THz) frequency generation. When two laser beams are focused on the graphene nanoemitter of a planar field-emission diode, THz current oscillations can be created at the emitter tip through the optical heterodyne. Graphene’s optical transparency allows suitably designed plasmonic nanoantennas to boost the mixing of laser radiation at the emitter tip, significantly increasing the overall produced photomixing current. The THz wave generated at the graphene emitter is then coupled to a loading circuit, thanks to the THz wave confinement in the graphene nanostructures. Our design is ideally suited for THz sources that may be tuned from DC to 10 THz by simply shifting the frequency offset of two pumping lasers. [ABSTRACT FROM AUTHOR]

We introduce the acoustic equivalent of "supercoupling" by studying the anomalous sound transmission and uniform energy squeezing through ultranarrow acoustic channels filled with zero-density metamaterials. As a realistic example, we propose their realization by inserting transverse membranes with a subwavelength period along the channel, and we prove a novel form of acoustic tunneling based on impedance matching and infinite phase velocity at the zero-density operation. We envision applications in sensing, noise control, cloaking, and energy harvesting. [ABSTRACT FROM AUTHOR]

Based on the recently introduced homogenization theory developed by Alù [ Phys. Rev. B 84 075153 (2011)], we propose a generalized retrieval method that allows extracting physically meaningful bulk effective parameters from conventional scattering measurements of periodic metamaterial samples composed of subwavelength inclusions. We show that, compared to conventional approaches, our method is able to capture the anomalous physics in the wave interaction with resonant metamaterials and return physically meaningful homogenized parameters that retain local properties in the long-wavelength limit. As a byproduct, we are also able to retrieve the polarizabilities of the constituent inclusions, which are shown to satisfy expected dispersion properties for passive inclusions, in contrast with conventional retrieval approaches. [ABSTRACT FROM AUTHOR]

Anomalous resonances in properly shaped plasmonic nanostructures can in principle lead to infinite absorption/gain efficiencies over broad bandwidths of operation. By developing a closed-form analytical solution for the fields scattered by conjoined hemicylinders, we outline the fundamental physics behind these phenomena, associated with broadband adiabatic focusing of surface plasmons at the nanoscale. Over a continuous frequency range, our proposed composite nanostructure shows finite amount of absorption/amplification even in the limit of infinitesimally small intrinsic material loss/gain. Detailed physical insights are provided to justify the nature of this apparent paradox, and its counterintuitive behavior is discussed for potential applications in nonlinear optics, spasing, sensing, and energy-harvesting devices. [ABSTRACT FROM AUTHOR]

We introduce the concept of furtive quantum sensing, demonstrating the possibility of concealing quantum objects from matter waves while maintaining their ability to interact and to get excited by the impinging particles. This is obtained by cloaking, with tailored homogeneous metamaterial layers, quantum systems having internal degrees of freedom. The effect is a low-observable quantum sensor with drastically reduced elastic scattering and optimized absorption levels, a concept that opens interesting venues in particle detection, high-efficiency electrical pumping, and quantum information processing. [ABSTRACT FROM AUTHOR]

We describe the possibility of drastically boosting the spontaneous emission of a collection of two-level quantum emitters by embedding them in an epsilon-near-zero (ENZ) environment, consisting of a plasmonic waveguide operated at cutoff. This phenomenon relies on the combination of Purcell enhancement and Dicke superradiance effects, exploiting the large and uniform local density of states in ENZ channels, which is shown to significantly extend the spatial extent of these quantum effects. We envision exciting applications in optical sensing, molecular detection, and low-threshold lasing. [ABSTRACT FROM AUTHOR]

We set the theoretical basis for scattering cancellation and cloaking of quantum particle waves. We discuss the possibility of cloaking a spherical object from an impinging matter wave by using a homogeneous monolayer with properly tailored effective mass and potential. The method, based on cancellation of scattered s and p waves, is the quantum analog of plasmonic cloaking of electromagnetic waves. We show that it may be possible to use this cloaking mechanism to make semiconductor quantum dots effectively invisible to impinging electrons, providing interesting venues in nano-optics, quantum computing, and furtive quantum sensing. [ABSTRACT FROM AUTHOR]

We discuss here potential venues for applications and exotic features of quantum metamaterials. We explore the coupling of conventional electromagnetic metamaterials with quantum emitters and the wave properties of quantum metamaterials obtained by tailoring their effective band structure. We discuss anomalous enhancement effects in the quantum emission properties of individual and collections of small emitters in the presence of metamaterials, as well as matter-wave cloaking and anomalous tunneling phenomena for quantum mechanical waves in artificial materials with exotic band structures. [ABSTRACT FROM AUTHOR]

Energy confinement plays an important role in improving wave–matter interactions, enabling applications such as sensing, lasing, and filtering. One convenient solution for achieving large energy-confinement is based on embedded eigenstates, i.e., non-radiating eigenmodes supported by open structures. While the analysis of these modes is quite consolidated in electromagnetics and optics, their relevance in acoustics has been less explored, despite their wide application potential. In this work, we explore acoustic embedded scattering eigenstates within the radiation continuum enabled by resonant metasurface pairs. At resonance, each metasurface strongly reflects the input wave, but as their surface resonances interfere with each other through coupling with a longitudinal resonance, an embedded eigenstate emerges. Through both theoretical and numerical analysis, we show the conditions for such an exotic resonant state to emerge and its implication for acoustic systems. [ABSTRACT FROM AUTHOR]

Topology is the branch of mathematics studying the properties of an object that are preserved under continuous deformations. Quite remarkably, the powerful theoretical tools of topology have been applied over the past few years to study the electronic band structure of crystals. Topological band theory can explain and predict topological phase transitions in a material, and the unusual robustness of certain band structure shapes, such as Dirac cones, against small perturbations. These findings have also unveiled a new phase of matter—topological insulators—whose exotic transport properties at their boundaries are topologically protected against imperfections and disorder. The fascinating features of topological boundary states have triggered the search for their analogs in classical wave physics. Here, we focus on the peculiar features of two‐dimensional topological insulators for sound and mechanical waves. Two‐dimensional Dirac cones and phononic topological insulators can emerge under certain conditions in periodic acoustic metamaterials, demonstrating great potential for acoustic and mechanical systems to demonstrate, over a tabletop platform, complex fundamental phenomena driven by topological concepts. In addition, these discoveries offer a direct path toward new technologies for enhanced sound control and manipulation. [ABSTRACT FROM AUTHOR]

We analyze the design of ultrathin quarter-wave plates based on plasmonic metasurfaces. After exploring the general theoretical possibilities offered by thin surfaces to manipulate the impinging polarization, we propose optimal designs to realize quarter-wave metasurface plates, analyzing their frequency and angular response. Our designs may provide a large degree of linear polarization output for circularly polarized input over a broad bandwidth in the optical regime. The geometry may be implemented within currently available lithographic techniques and easily integrated with other optical devices for polarization manipulation, detection, and sensing at the nanoscale. [ABSTRACT FROM AUTHOR]

We explore in detail the concept of mantle cloaking, showing that a single, ultrathin, conformal metasurface may drastically suppress the scattering from planar [one-dimensional (1D)], cylindrical [two-dimensioqal (2D)], and/or spherical [three-dimensional (3D)] objects. We propose realistic designs that may realize this concept at radio (RF) or far-infrared (FIR) frequencies. In addition we discuss properties and physical insights of this cloaking functionality compared to other available techniques involving bulk metamaterials, showing exciting potentials in terms of ultralow-profile, light-weight, and bandwidth of operation. These results may be of great interest for low-observability, camouflaging, noninvasive probing, and low-noise communication applications. [ABSTRACT FROM AUTHOR]

We derive from first principles an accurate homogenized description of periodic metamaterials made of magnetodielectric inclusions, highlighting and overcoming relevant limitations of standard homogenization methods. We obtain closed-form expressions for the effective constitutive parameters, pointing out the relevance of inherent spatial dispersion effects, present even in the long-wavelength limit. Our results clarify the limitations of quasistatic homogenization models, restore the physical meaning of homogenized metamaterial parameters, and outline the reasons behind magnetoelectric coupling effects that may arise also in the case of centersymmetric inclusions. [ABSTRACT FROM AUTHOR]

Metamaterial homogenization is often based on implicit assumptions inspired by naturalmaterial models. However, retrieved permittivity and permeability frequently retain nonphysical values, especially near the metamaterial resonances where most interesting features are expected. We explain here the nature of typical homogenization artifacts, relating them to an inherent form of magnetoelectric coupling associated with the finite phase velocity along metamaterial arrays. Our findings allow restoring the proper definition and physical meaning of local constitutive parameters for metamaterials. [ABSTRACT FROM AUTHOR]

Enhancement of molecular emission is usually obtained by coupling small optical emitters with external resonant structures and systems, as first established by Purcell several decades ago, and verified in several recent investigations using molecules or quantum dots coupled with plasmonic nanoantennas. Here we theoretically investigate in detail a different mechanism for emission enhancement, based on our recent idea of a plasmonic nanolauncher [Phys. Rev. Lett. 2009, 103, 043902], i.e., a metamaterial-inspired ultranarrow waveguide channel operating near its cut-off frequency. Such system is not necessarily at resonance, but its peculiar operation may provide enhanced emission over a relatively broad physical area, which may allow enhancement of emission independent of the position of an individual or of a group of molecules along such plasmonic channel, and the possibility to bend and route the emitted energy with large flexibility. We present here extensive theoretical and numerical results that confirm this intuition and may envision a novel method for molecular emission enhancement at the nanoscale, with more flexibility than the conventional Purcell resonance techniques. [ABSTRACT FROM AUTHOR]

The main objective of this paper is to provide a self-consistent analytical model for the approximate current distribution induced on an arbitrarily loaded wire antenna of moderate length operating in its receiving mode. The results of our analysis shed light on a debated issue regarding the circuit model of a receiving antenna and show that currently available simplified analytical models for receiving dipoles fail to satisfy basic power conservation requirements. These general findings are applied to the proper homogenization of composite materials and surfaces, and to minimum-scattering receivers and absorbers. [ABSTRACT FROM AUTHOR]

We propose the use of metamaterial-inspired ultranarrow channels at cutoff to realize an interesting matching between a coaxial antenna and a waveguide. The anomalous properties of a channel at cutoff, analogous to those of zero permittivity materials, allow a simple matching design, valid for arbitrary waveguides, with large degrees of freedom in terms of geometry, length and possible bending of the connecting channel. Moreover, the static-like properties of the channel allow such matching, independent of the relative position of the antenna and possible bending and abruptions along the channel. [ABSTRACT FROM AUTHOR]

We have recently suggested that suitably designed plasmonic layers may cloak a given object simultaneously at multiple frequencies (Alù and Engheta 2008 Phys. Rev. Lett. 100 113901). Here, we extend our theory and fully analyze this possibility, highlighting the potentials of this plasmonic cloaking technique and its fundamental limitations dictated by the passivity and causality of the materials involved. The cloaking mechanism relies on the scattering cancellation properties of plasmonic materials. By exploiting their inherent frequency dispersion, it is possible to reduce the 'visibility' of a given object by several orders of magnitude simultaneously at multiple frequencies, such that any of the particular layers composing the cloak is responsible for noticeable reduction of scattering at each frequency of operation. [ABSTRACT FROM AUTHOR]

In this article, we analyze the dispersion features of cloaking structures formed by plasmonic materials and/or metamaterials. Following our recent proposal for realizing a cloaking metamaterial shell capable of drastically suppressing the total scattering of moderately-sized objects, we investigate the frequency response and dispersion behavior of this anomalous phenomenon. Insights into the scattering reduction and possibility of multi-frequency response are presented, considering the inherent dispersion constraints of passive materials. [ABSTRACT FROM AUTHOR]

The detailed theory of a single subwavelength conformal radiator that exploits the resonant properties of thin cylindrical metamaterial shells supporting leaky waves is presented. It is shown and reviewed analytically and numerically how a circularly symmetric resonant leaky mode may be supported by a properly designed subwavelength homogenous cylindrical shell of low negative permittivity. Some physical insights are provided and numerical simulations with a feed point, considering also the finiteness of the antenna in the longitudinal direction, are presented and discussed. Moreover, possibilities and limitations of a practical realization of this setup are mentioned, considering in details the possible anisotropies in the metamaterials. [ABSTRACT FROM AUTHOR]

The potential use of metamaterial planar bilayers for synthesizing compact subwavelength leaky-wave radiators in the microwave regime is analyzed in detail. In particular, the possibility of pairing "complementary" metamaterials in order to reduce the dimensions of microwave components is explored for the leaky-wave operation of an open waveguide consisting of a grounded pair of planar layers. In connection with our similar findings in other setups employing such complementary pairings, here we show how the compact resonance at the interface between "negative" and "positive" materials may also be properly exploited in this context. Choosing materials with low constitutive parameters, moreover, shows to be effective for enhancing the directivity of these components. We explore in detail the notable guidance and radiation properties of the anomalous natural modes supported by these bilayered structures, giving some physical insights into the anomalous phenomenon and considering the possible limitations in some realistic setups. [ABSTRACT FROM AUTHOR]

Here we solve the transmission-line equations for a metamaterial having shunt and series inductances electromagnetically coupled together. We derive and analyze the anomalous wave properties that such a metamaterial may have, envisioning its realization both as a transmission-line metamaterial and as a bulk complex material whose electromagnetic relations may be read in terms of the present theory. The dependence of the wave impedances on the coupling factor between the inductive elements is derived in closed form, showing an anomalous distinct behavior between the incident and reflected waves. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 94–97, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22028 [ABSTRACT FROM AUTHOR]

We analyze here the performance of μ-negative metamaterials in different configurations at microwave and optical frequencies that employ variable values of capacitances or tunable varactors, with the purpose of increasing their bandwidth of operation. The proposed method shows moderately good results both in resonant-like and in transmission-line configurations. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 55–59, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22036 [ABSTRACT FROM AUTHOR]

Recently, there has been an increased interest in the problem of wave transmission through sub-wavelength apertures, following successful experimental demonstration by several groups for enhancing optical power transmission through nano-scale holes in metallic screens due to properly designed periodic corrugation. Oliner, Jackson, and their co-workers explained and justified this phenomenon as the result of the excitation of the leaky waves supported by the corrugated screen. Here we discuss in detail the mechanism and analysis for another setup we have recently proposed, in which metamaterial layers with special parameters may be utilized as covers over a single sub-wavelength aperture in a perfectly electric conducting (PEC) flat screen in order to increase the wave transmission through this aperture, and we provide a detailed physical insights and analytical explanation for this aperture setup that may lead to similar, potentially even more pronounced effects when the proper metamaterial layers are used in the entrance and the exit face of the hole in the flat PEC screen with no corrugation. Some numerical results confirming this theory are presented and discussed. We also investigate the sensitivity of the transmission enhancement to the geometrical and electromagnetic parameters of this structure. [ABSTRACT FROM AUTHOR]

An effective design of polygonal patch antennas with multifrequency or broad-band operation modes for wireless communications is presented in this paper. It is shown how polygonal patches with suitable features may be obtained after a proper perturbation of conventional rectangular geometries, which inherently present poor bandwidth performances. These perturbed irregular geometries may support multiple resonances and, thus, may present a broad-band or multifrequency operation mode, even employing conventional patch antennas with a single dielectric substrate. These polygonal patches are efficiently analyzed through a numerical code based on the method of moments, with entire domain basis functions that accurately describe the radiation mechanism. After the presentation of the analysis and design techniques, some antenna layouts for modern wireless communication systems will be proposed. Such antennas are designed for both universal mobile telecommunication system and wireless local area network portable equipment with real-life finite ground planes. [ABSTRACT FROM AUTHOR]

This paper presents a generalization of the method of lines (MoL) numerical algorithm for the analysis of integrated structures with a finite local curvature. Starting from the derivation of the generalized transmission line equations in a generic orthogonal reference system and in a bianisotropic material, it is shown the possibility of solving the electromagnetic problem associated to an integrated structure with arbitrary curvature and/or loaded by complex dielectric materials via a MoL approach. The introduction of new algorithms that extend and complete the original method is required in the most general case. A detailed description of this new extended version of the MoL is presented through the paper and its full coincidence with the original algorithm in the case of structures with simple loading materials is also shown. Finally, some numerical results obtained analyzing microwave conformal antennas and resonators are presented and compared with analyzes performed employing other numerical methods and available in the literature. [ABSTRACT FROM AUTHOR]

Presents the results of a theoretical analysis for guided modes in parallel-plate waveguides filled with pairs of parallel layers. Salient properties of the guided modes; Ways in which materials and their parameters are chosen to be paired; Contrast with those of the guided modes in conventional waveguides; Physical insights and intuitive justifications for the mathematical findings.