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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.

This paper develops a quasi-analytical and self-consistent model to compute the polarizabilities of split ring resonators (SRRs). An experimental setup is also proposed for measuring the magnetic polarizability of these structures. Experimental data are provided and compared with theoretical results computed following the proposed model. By using a local field approach, the model is applied to the obtaining of the dispersion characteristics of discrete negative magnetic permeability and left-handed metamaterials. Two types of SRRs, namely, the so-called edge coupled- and broadside Coupled- SRRs, have been considered. A comparative analysis of these two structures has been carried out in connection with their suitability for the design of metamaterials. Advantages and disadvantages of both structures are discussed. [ABSTRACT FROM AUTHOR]

Method of line (MOL) procedure is very useful in the analysis of radiative and transmissive microwave components, but its standard version does not allow for the study of elements with complex substrates. In this work, we first show that components integrated into materials exhibiting the magneto-electric effect (biisotropic and general bianisotropic media) cannot be analyzed following a standard MoL algorithm. Next, we derive an extended MoL numerical tool, which allows for the analysis of components in the presence of any linear medium (even inhomogeneous, bianisotropic and lossy). Such an extension is based on the generalization of the transmission.line equations for a general linear medium, which, in the case in point, are not necessarily decoupled. Furthermore, we present the full coincidence of this new method with the standard MoL in the case of simpler media (i.e., not exhibiting the magneto-electric effect) and, finally, we show some numerical results, obtained analyzing microwave antennas and resonators with bianisotropic and chiral substrates. [ABSTRACT FROM AUTHOR]

Acoustic metamaterials and metasurfaces have been explored in the past few years to realize a wide range of extreme responses for sound waves. As one remarkable phenomenon, extreme anisotropy and hyperbolic sound propagation are particularly challenging to realize compared to electromagnetic waves because of the scalar nature of airborne acoustics. In parallel, moiré superlattices and the rapidly expanding domain of twistronics have shown that large anisotropy combined with tailored geometrical rotations can enable tantalizing emerging phenomena, such as tailored phase transitions in metamaterials. Connecting these areas of research, here, we explore the realization of acoustic hyperbolic metasurfaces and their combination to drive topological phase transitions from hyperbolic to elliptic sound propagation. The transition point occurring at a specific rotation angle between two acoustic metasurfaces supports highly directional canalization of sound, opening exciting opportunities for twisted acoustics metasurfaces for robust surface wave guiding and steering. [ABSTRACT FROM AUTHOR]

The article discusses the study on kerr optical nonlinearities in optical isolation, and details the new opportunities and limitations in time-invariant optical systems. The researchers have demonstrated that Kerr nonlinear devices can be designed to isolate strong signals, but they do not necessarily provide the same non-reciprocal response in superposition with large signals or noise. They suggest that nano-devices such as diodes and transistors should be given priority in photonics.

The article focuses on on-chip optical isolation by citing reference to various research studies demonstrating non-reciprocal optical response, which is based on a synthetic magnetic field in an all-silicon platform. Topics discussed include research study by L.D. Tzuang and team in which they used synthesized magnetic field to 43 percent isolation for showing non-reciprocal effect, Aharonov?Bohm effect, and use of optical isolation in fields including on-chip lasers and optical routing.

Hyperbolic metasurfaces, supporting extreme anisotropy of the surface impedance tensor, have recently been explored in nanophotonic systems for robust diffractionless propagation over a surface, offering interesting opportunities for subdiffraction imaging and enhanced Purcell emission. In acoustics, due to the longitudinal nature of sound transport in fluids, these phenomena are forbidden by symmetry, requiring the acoustic surface impedance to be inherently isotropic. Here we show that nonlocalities produced by strong coupling between neighboring impedance elements enable extreme anisotropic responses for sound traveling over a surface, supporting negative phase and energy velocities, as well as hyperbolic propagation for acoustic surface waves. [ABSTRACT FROM AUTHOR]

The analysis of patch antennas loaded by general inhomogeneous and lossy chiral media is presented in this paper, implementing a new method-of-lines (MoL) procedure, which considers inhomogeneous planar structures loaded by any bi-isotropic material. Presented results refer to the effect of a chiral cover on the resonance frequency of a simple squared-patch structure. Design charts are presented to simply analyze the separate effects of cover thickness and chirality. © 2002 John Wiley & Sons, Inc. Microwave Opt Technol Lett 32: 143–145, 2002. [ABSTRACT FROM AUTHOR]

A correction to the research "Polarizabilities and Effective Parameters for Collections of Spherical Nanoparticles Formed by Pairs of Concentric Double-Negative, Single-Negative, and/or Double-Positive Metamaterial Layers," that was published in the 2005 is "Journal of Applied Physics" is presented.

Topological boundary modes in electronic and classical-wave systems exhibit fascinating properties. In photonics, topological nature of boundary modes can make them robust and endows them with an additional internal structure--pseudo-spins. Here, we introduce heterogeneous boundary modes, which are based on mixing two of the most widely used topological photonics platforms--the pseudo-spin-Hall-like and valley-Hall photonic topological insulators. We predict and confirm experimentally that transformation between the two, realized by altering the lattice geometry, enables a continuum of boundary states carrying both pseudo-spin and valley degrees of freedom (DoFs). When applied adiabatically, this leads to conversion between pseudo-spin and valley polarization. We show that such evolution gives rise to a geometrical phase associated with the synthetic gauge fields, which is confirmed via an Aharonov-Bohm type experiment on a silicon chip. Our results unveil a versatile approach to manipulating properties of topological photonic states and envision topological photonics as a powerful platform for devices based on synthetic DoFs. [ABSTRACT FROM AUTHOR]

Optical metalenses offer a compact approach for the development of nanoscale optical devices with various imaging functionalities. Realizing high‐quality images using metalenses with large numerical aperture (NA) requires eliminating unwanted optical aberrations. Existing strategies have largely focused only on correcting chromatic aberrations, and a metalens capable of simultaneously correcting chromatic and spherical aberrations has not yet been developed. Here, this issue is addresed by realizing an aplanatic phase profile for a red–green–blue (RGB) achromatic metalens, thereby demonstrating an RGB‐achromatic aplanatic metalens. This device consists of crystalline silicon nanostructures and it enables RGB‐achromatic aplanatic focusing with an NA of 0.635. The focal length of metalens remains unchanged at the three RGB wavelengths, whereas the associated focal spots are significantly smaller than those of an RGB‐achromatic metalens with spherical aberrations. An enhanced RGB imaging resolution is demonstrated, offering exciting opportunities to power up various imaging and display applications using flat metalenses. [ABSTRACT FROM AUTHOR]

The article discusses exceptional points, or spectral singularities, within photonics and optics, including in non-Hermitian systems in photonics. Exceptional points in parameter spaces involving eigenvalues are discussed.

This Special Topic collection provides an opportunity for the broad readership of Applied Physics Letters to get a glimpse of the recent advances in the area of metastructures and their applications. The main idea, borrowed from condensed matter physics, is that the nontrivial topological features of the band diagram of a metamaterial can be translated into an unusually robust boundary propagation of waves, of great interest for various applications from microwaves, photonics to acoustics. Huang I et al. i [32] as well as Mi I et al. i [36] and Zaccherini I et al. i [37] discuss various metamaterial geometries supporting broadband absorption exploiting sophisticated sound-matter interactions. The exotic wave phenomena in metastructures and their wide applications are one of the most researched subjects in electromagnetic waves and photonics, from radio frequencies to optics, and extend into several other disciplines, such as acoustics, mechanics, thermodynamics, materials science, condensed matter, etc. [Extracted from the article]

Willis fluids, or more generally Willis materials, are homogenized composites that exhibit coupling between momentum and strain. This coupling is intrinsic to inhomogeneous media and can play a significant role in the overall response in acoustic metamaterials. In this paper, we draw connections between bianisotropy in electromagnetism and Willis coupling in elastodynamics to provide a qualitative understanding. Building upon these analogies, we introduce a homogenization technique for acoustic metamaterials based on a source-driven, multiple scattering approach that highlights the physical origins of Willis coupling. Moreover, through numerical examples, we compare several macroscopic material descriptions of acoustic metamaterials with non-negligible Willis coupling. The descriptions neglecting Willis coupling may not satisfy restrictions stemming from reciprocity, passivity, and causality, which suggests that including Willis coupling in macroscopic descriptions is necessary to realize physically meaningful macroscopic parameters. [ABSTRACT FROM AUTHOR]

We discuss the possibility of plasmon canalization, collimation, and tunneling over ultrathin metasurfaces, enabled by extreme anisotropy in their complex conductivity dyadic. The interplay between anisotropy, conductivity-near-zero, and loss is exploited here to derive general conditions for plasmon canalization and efficient energy transport. We also demonstrate how the intrinsic in-plane anisotropy of black phosphorus can provide a natural platform to engineer these conditions, exhibiting important advantages over isotropic plasmonic materials. Our findings have implications for plasmonic sensors, planar hyperlenses, and plasmon steering over a surface, and they highlight the potential of two-dimensional materials beyond graphene. [ABSTRACT FROM AUTHOR]

Synthetic magnetism has been recently realized using spatiotemporal modulation patterns, producing non-reciprocal steering of charge-neutral particles such as photons and phonons. Here, we design and experimentally demonstrate a non-reciprocal acoustic system composed of three compact cavities interlinked with both dynamic and static couplings, in which phase-correlated modulations induce a synthetic magnetic flux that breaks time-reversal symmetry. Within the rotating wave approximation, the transport properties of the system are controlled to efficiently realize large non-reciprocal acoustic transport. By optimizing the coupling strengths and modulation phases, we achieve frequency-preserved unidirectional transport with 45-dB isolation ratio and 0.85 forward transmission. Our results open to the realization of acoustic non-reciprocal technologies with high efficiency and large isolation, and offer a route towards Floquet topological insulators for sound. [ABSTRACT FROM AUTHOR]

Asymmetric structures support different field distributions and electromagnetic responses when excited from different directions. Here we show that time-reversal symmetry imposes fundamental constraints on their overall response, beyond those dictated by reciprocity. For two-port devices, the asymmetry in field distribution for opposite excitations is shown to be fundamentally bounded by the reflection at the ports, and the fields are identical everywhere in space in the case of full transmission. In multiport and open scenarios, these bounds have implications on radiation and scattering at different ports and towards different directions. Beyond their theoretical significance, these results provide relevant insights into the operation of nonlinear isolators, metasurfaces, and other nanophotonic devices. [ABSTRACT FROM AUTHOR]

Incandescent sources that produce light from electrically heated filaments or films tend to feature low efficiencies and offer poor spectral and angular control. We demonstrate that a judicious nanostructuring of a SiC surface can focus thermal emission of a preselected spectral range to a well-defined height above the surface. SiC is known to support electromagnetic surface waves that afford the required thermal emission control. Here, we provide general design rules for this type of focusing element that can be extended to other material systems, such as metals supporting surface plasmon-polariton waves. These rules are verified using full-wave calculations of the spatial variation of thermal emission. The obtained results establish a foundation for developing more complex algorithms for the design of complex thermal lenses. [ABSTRACT FROM AUTHOR]

Isolators, devices with unidirectional wave transmission, are integral components in computing networks, enabling a one-way division of a large system into independent subunits. Isolators are created by breaking the inversion symmetry between a source and a receiver, known as reciprocity. In acoustics, a steady flow of the background medium in which sound travels can break reciprocity, but significant isolation is typically achieved only for large, often impractical speeds. This article proposes acoustic isolator designs enabled by duct flow that do not require large flow velocities. A basic isolator design is simulated based on the acoustic analogue of a Mach-Zehnder interferometer, with monomodal entry and exit ports. The simulated device footprint is then reduced by using bimodal ports. Further, a nonuniform velocity profile combined with a grating to induce phononic transitions is considered, which, combined with filters, can provide significant isolation. By coupling a waveguide with flow to free space through an array of small apertures, largely nonreciprocal leaky-wave radiation is demonstrated, breaking the symmetry between reception and transmission patterns of an acoustic linear aperture array. These investigations open interesting pathways towards efficient acoustic isolation, which may be translated into integrated acoustic and surface acoustic waves, as well as phononic technology. [ABSTRACT FROM AUTHOR]

The topological properties of an object, associated with an integer called the topological invariant, are global features that cannot change continuously but only through abrupt variations, hence granting them intrinsic robustness. Engineered metamaterials (MMs) can be tailored to support highly nontrivial topological properties of their band structure, relative to their electronic, electromagnetic, acoustic and mechanical response, representing one of the major breakthroughs in physics over the past decade. Here, we review the foundations and the latest advances of topological photonic and phononic MMs, whose nontrivial wave interactions have become of great interest to a broad range of science disciplines, such as classical and quantum chemistry. We first introduce the basic concepts, including the notion of topological charge and geometric phase. We then discuss the topology of natural electronic materials, before reviewing their photonic/phononic topological MM analogues, including 2D topological MMs with and without time-reversal symmetry, Floquet topological insulators, 3D, higher-order, non-Hermitian and nonlinear topological MMs. We also discuss the topological aspects of scattering anomalies, chemical reactions and polaritons. This work aims at connecting the recent advances of topological concepts throughout a broad range of scientific areas and it highlights opportunities offered by topological MMs for the chemistry community and beyond. [ABSTRACT FROM AUTHOR]

Frequency conversion processes, such as second- and third-harmonic generation, are commonly realized in nonlinear optics, offering opportunities for applications in photonics, chemistry, material science and biosensing. Given the inherently weak nonlinear response of natural materials, optically large samples and complex phase-matching techniques are typically required to realize significant nonlinear responses. To produce similar effects in much smaller volumes, current research has been devoted to the quest of synthesizing novel materials with enhanced optical nonlinearities at moderate input intensities. In particular, several approaches to engineer the nonlinear properties of artificial materials, metamaterials and metasurfaces have been introduced. Here, we review the current state of the art in the field of small-scale nonlinear optics, with special emphasis on high-harmonic generation from ultrathin metasurfaces based on plasmonic and high-index dielectric resonators, as well as semiconductor-loaded plasmonic metasurfaces. In this context, we also discuss recent advances in controlling the optical wavefront of generated nonlinear waves using metasurfaces. Finally, we compare viable approaches to enhance nonlinearities in ultrathin metasurfaces, and we offer an outlook on the future development of this exciting field of research. [ABSTRACT FROM AUTHOR]

Surface-enhanced Raman spectroscopy (SERS) is a promising optical method for analyzing molecular samples of various nature. Most SERS studies are of an applied nature, indicating a serious potential for their application in analytical practice. Dendritelike nanostructures have great potential for SERS, but the lack of a method for their predictable production significantly limits their implementation. In this paper, a method for controllably obtaining spatially separated, self-organized, and highly-branched silver dendrites via template synthesis in pores of SiO2/Si is proposed. The dendritic branches have nanoscale roughness, creating many plasmon-active "hotspots" required for SERS. The first held 3D modeling of the external electromagnetic wave interaction with such a dendrite, as well as experimental data, confirms this theory. Using the example of a reference biological analyte, which is usually used as a label for other biological molecules, the dendrites' SERS-sensitivity up to 10−15M was demonstrated with an enhancement factor of 108. The comparison of simulation results with SERS experiments allows distinguishing the presence of electromagnetic and chemical contributions, which have a different effect at various analyte concentrations. [ABSTRACT FROM AUTHOR]

The article discusses a study on photonics conducted by a group of researchers and published in the journal "Science Advance." The study explores the use of Dirac-like dispersion in photonic systems to create spin-degenerate artificial optical resonators that are compatible with spin-full topological photonic structures, potentially opening up new avenues for designing nanophotonic devices, spin-full resonators, and topological light sources.

Materials that require coupling between the stress– strain and momentum–velocity constitutive relations were first proposed byWillis (Willis 1981 Wave Motion 3, 1–11. (doi:10.1016/0165-2125(81)90008-1)) and are now known as elastic materials of the Willis type, or simply Willis materials. As coupling between these two constitutive equations is a generalization of standard elastodynamic theory, restrictions on the physically admissible material properties for Willis materials should be similarly generalized. This paper derives restrictions imposed on the material properties of Willis materials when they are assumed to be reciprocal, passive and causal. Considerations of causality and low-order dispersion suggest an alternative formulation of the standard Willis equations. The alternative formulation provides improved insight into the subwavelength physical behaviour leading to Willis material properties and is amenable to time-domain analyses. Finally, the results initially obtained for a generally elastic material are specialized to the acoustic limit. [ABSTRACT FROM AUTHOR]

Graphene plasmonic structures can support enhanced and localized light–mater interactions within extremely small mode volumes. However, the external quantum efficiency of the resulting devices is fundamentally limited by material scattering and radiation loss. Here, such radiation loss channels are suppressed by tailoring the structure to support a symmetry‐protected bound‐state‐in‐the‐continuum (BIC) system. With practical loss rates and doping level in graphene, over 90% absorption near critical coupling is expected from numerical simulation. Experimentally measured peak absorption of 68% is achieved in such a tailored graphene photonic–plasmonic system, with maximum 50% contrast to the control sample without graphene. Significant reduction of the plasmon absorption for a different spacer thickness verifies the sensitivity of the system to the quasi‐BIC condition. [ABSTRACT FROM AUTHOR]

Since the late 19th century, enormous endeavors have been made in extending the scope and capability of optical interferometers. Recently, plasmonic vortices that strongly confine the orbital angular momentum to surface have attracted considerable attention. However, current research interests in this area have focused on the mechanisms and dynamics of polarization‐dependent single plasmonic vortex generation and evolution, while the interference between different plasmonic vortices for practical applications has been unexplored. Here, a method for flexible on‐chip spin‐to‐orbital angular momentum conversion is introduced, resulting in exotic interferograms. Based on this method, a new form of interferometers that is realized by the interference between customized plasmonic vortices is demonstrated. Within wavelength‐scale dimension, the proposed plasmonic vortex interferometers exhibit superior performance to directly measure the polarization state, spin and orbital angular momentum of incident beams. The proposed interferometry is straightforward and robust, and can be expected to be applied to different scenarios, fueling fundamental advances and applications alike. [ABSTRACT FROM AUTHOR]

Two-dimensional magnetostatic cloaks, even when perfectly designed to mitigate the magnetic field disturbance of a scatterer, may be still detectable with Aharonov-Bohm (AB) measurements, and therefore may affect quantum interactions and experiments with elongated objects. We explore a multilayered cylindrical cloak whose permeability profile is tailored to nullify the magnetic-flux perturbation of the system, neutralizing its effect on AB measurements, and simultaneously optimally suppress the overall scattering. In this way, our improved magnetostatic cloak combines substantial mitigation of the magnetostatic scattering response with zero detectability by AB experiments. [ABSTRACT FROM AUTHOR]

Coupled resonances in non‐Hermitian systems can lead to exotic optical features, such as bound states in the continuum (BICs) and exceptional points (EPs), which have been recently emerged as powerful tools to control the propagation and scattering of light. Yet, similar tools to control diffraction and engineer spatial wavefronts have remained elusive. Here, it is shown that, by operating a metagrating around BICs and EPs, it is possible to achieve an extreme degree of control over coupling to different diffraction orders. Subwavelength metallic slit arrays stacked on a metal‐insulator‐metal waveguide, enabling a careful control of the coupling between localized and guided modes are explored. By tuning the coupling strength from weak to strong, the overall spectral response can be tailored and the emergence of singular features, like BICs and EPs can be enabled. Perfect unitary diffraction efficiency with large spectrum selectivity is achieved around these singular features, with promising applications for selective wavefront shaping, filtering, and sensing. [ABSTRACT FROM AUTHOR]

We investigate the use of a split-ring resonator (SRR) incorporated with an atomic-vapor cell to improve the sensitivity and the minimal detectable electric (E) field of Rydberg atom-based sensors. In this approach, a sub-wavelength SRR is placed around an atomic vapor-cell filled with cesium atoms for E-field measurements at 1.3 GHz. The SRR provides a factor of 100 in the enhancement of the E-field measurement sensitivity. Using electromagnetically induced transparency (EIT) with Aulter–Townes splitting, E-field measurements down to 5 mV/m are demonstrated with the SRR, while in the absence of the SRR, the minimal detectable field is 500 mV/m. We demonstrate that by combining EIT with a heterodyne Rydberg atom-based mixer approach, the SRR allows for a sensitivity of 5.5 μV/m Hz , which is two-orders of magnitude improvement in sensitivity than when the SRR is not used. [ABSTRACT FROM AUTHOR]

The twist angle between a pair of stacked 2D materials has been recently shown to control remarkable phenomena, including the emergence of flat‐band superconductivity in twisted graphene bilayers, of higher‐order topological phases in twisted moiré superlattices, and of topological polaritons in twisted hyperbolic metasurfaces. These discoveries, at the foundations of the emergent field of twistronics, have so far been mostly limited to explorations in atomically thin condensed matter and photonic systems, with limitations on the degree of control over geometry and twist angle, and inherent challenges in the fabrication of carefully engineered stacked multilayers. Here, this work extends twistronics to widely reconfigurable macroscopic elastic metasurfaces consisting of LEGO pillar resonators. This work demonstrates highly tailored anisotropy over a single‐layer metasurface driven by variations in the twist angle between a pair of interleaved spatially modulated pillar lattices. The resulting quasi‐periodic moiré patterns support topological transitions in the isofrequency contours, leading to strong tunability of highly directional waves. The findings illustrate how the rich phenomena enabled by twistronics and moiré physics can be translated over a single‐layer metasurface platform, introducing a practical route toward the observation of extreme phenomena in a variety of wave systems, potentially applicable to both quantum and classical settings without multilayered fabrication requirements. [ABSTRACT FROM AUTHOR]

We explore the unusual electromagnetic response of ultrathin anisotropic (7-near-zero uniaxial metasurfaces, demonstrating extreme topological transitions--from closed elliptical to open hyperbolic--for surface plasmon propagation, associated with a dramatic tailoring of the local density of states. The proposed metasurfaces may be implemented using nanostructured graphene monolayers and open unprecedented venues for extreme light confinement and unusual propagation and guidance, combined with large tunability via electric bias. [ABSTRACT FROM AUTHOR]

Enabling efficient nonreciprocal acoustic devices is challenging, yet very desirable for a variety of applications, including acoustic imaging, underwater communications, energy concentration and harvesting, signal processing, and noise control. We discuss the theory and design of a fully linear compact acoustic circulator based on spatiotemporal modulation of the effective acoustic index, providing a compact and practical way to realize large sound circulation at any desired frequency. Our proposal enables tunable isolation levels of over 40 dB, with insertion losses as low as 0.3 dB, in a noise-free, integrable, frequency scalable device whose total size does not exceed λ/6. [ABSTRACT FROM AUTHOR]

Based on Mie scattering theory, we derive the complete dynamic polarizability tensor for circular, azimuthally symmetric cylinders excited by an arbitrary field distribution, and provide compact expressions for all of its elements. Our results comprise fully dynamic cylinder polarizabilities, improving existing approximate models that use averaged electric or magnetic current lines to describe the scattering response of moderately thin cylinders. We show that the derived polarizability tensor satisfies reciprocity and passivity relations, and analyze its response under different conditions, varying the excitation angle, material properties, and cylinder radius. Interestingly, magnetoelectric effects are shown to arise at oblique incidence, even in the case of centrosymmetric achiral thin cylinders, associated with a weak form of spatial dispersion. This finding is particularly relevant for the proper modeling of individual cylinders and arrays of them, as in the case of metamaterials. We expect this work to find applications in antenna and metamaterial design, and to improve the physical understanding of the wave interaction and spatial dispersion in artificial materials composed of elongated inclusions such as wire media. [ABSTRACT FROM AUTHOR]

Twisted metamaterials, or arrays of identical planar metasurfaces stacked with a sequential rotation, have been recently introduced to realize broadband circular dichroism. Here we develop a generalized Floquet analysis to obtain the exact modal solutions for eigen waves supported by these structures. The dispersion relation and wave propagation in twisted metamaterials are discussed in detail. Our analysis shows how the modal dispersion in these metamaterials becomes inherently different from the one of conventional periodic structures and how the eigenmodes support specific circular polarization properties based on a lattice effect, even when achiral inclusions are considered. These wave properties are ideal to realize optical devices that manipulate the polarization state of light over broad bandwidths. By analyzing the physical nature of these modes, including complex modes, we also extend the application of twisted metamaterials to realize passband and stop-band nanophotonic structures with strong polarization manipulation properties. [ABSTRACT FROM AUTHOR]

Efficient second-harmonic generation may be induced based on the anomalous tunneling properties of double zero-permittivity narrow cross-slit nonlinear channels. Ideal phase matching conditions, large coherence length, combined with uniform field enhancement entail the ideal conditions to achieve giant second-harmonic conversion efficiencies for backward and forward signals. It is shown that these conditions are particularly well suited to enhance nonlinear effects over a wide range of frequencies, leading to efficient electromagnetic wave mixers and parametric amplifiers. [ABSTRACT FROM AUTHOR]

We derive and discuss general physical bounds on the electromagnetic scattering and absorption of passive structures. Our theory, based on passivity and power conservation, quantifies the minimum and maximum allowed scattering for an object that absorbs a given level of power. We show that there is a fundamental tradeoff between absorption and overall scattering suppression for each scattering harmonic, providing a tool to quantify the performance of furtive sensors, regardless of the applied principle for scattering suppression. We illustrate these fundamental limitations with examples of light scattering from absorbing plasmonic nanoparticles and loaded dipole antennas, envisioning applications to the design of cloaked sensors and absorbers with maximized absorption efficiency. [ABSTRACT FROM AUTHOR]

We introduce the concept and practical design of broadband, ultrathin cloaks based on non-Foster, negatively capacitive metasurfaces. By using properly tailored, active frequency-selective screens conformal to an object, within the realm of a practical realization, we show that it is possible to drastically reduce the scattering over a wide frequency range in the microwave regime, orders of magnitude broader than any available passive cloaking technology. The proposed active cloak may impact not only invisibility and camouflaging, but also practical antenna and sensing applications. [ABSTRACT FROM AUTHOR]

The article focuses on efficient nonreciprocal mode transitions in spatiotemporally modulated acoustic metamaterials. Topics include the mechanism relies on the coupling between an ultrathin membrane and external biasing electromagnetic fields, realizing programmable dynamic control of the acoustic impedance over a motionless and noiseless platform, and the fast and flexible impedance modulation of our metamaterial imparts an effective unidirectional momentum in space-time.

We discuss the potential of multilayered plasmonic particles to tailor the optical scattering response. The interplay of plasmons localized in thin stacked shells realizes peculiar degenerate cloaking and resonant states occurring at arbitrarily close frequencies. These concepts are applied to realize ultrasharp comblike scattering responses and synthesize staggered, ideally strong superscattering states closely coupled to invisible states. We demonstrate robustness to material losses and to variations in the background medium, properties that make these structures ideal for optical tagging. [ABSTRACT FROM AUTHOR]

Alternating isotropic layers have been widely used in the design of acoustic cloaks to achieve the necessary anisotropy required by coordinate-transformation techniques. In this paper, this concept is expanded to plasmonic-type acoustic cloaking using a bilaminate shell consisting of two isotropic layers with uniform thickness. Explicit analytic expressions based on thin-shell approximations for the necessary cloaking layer properties are developed, facilitating an examination of the fundamental physical behavior and dominant design parameters for a bilaminate plasmonic-type acoustic cloak. Based on this analysis, the performance of a bilaminate plasmonic-type acoustic cloak is examined, and practical means of achieving the desired cloaking layer properties are discussed. [ABSTRACT FROM AUTHOR]

We numerically study complex dual-interface grating systems to enhance absorption efficiency in thin-film silicon solar cells. We combine a plasmonic grating at the back side of the solar cell with a dielectric grating at the front side of the cell. We show a proof of principle, with one-dimensional gratings, that the distinctly different nature of the gratings can provide complementary enhancement mechanisms, which we further exploit by tailoring the specific periodicities, and by introducing blazing. Having different periods at specific interfaces allows for more efficient diffraction into both plasmonic and dielectric guided modes. In addition, grating specific blazing exposes extra modes to normal incident light through symmetry breaking. Multiple optimization routes are possible depending on the choice of photonic phenomena. [ABSTRACT FROM AUTHOR]

The integration of radiofrequency electronic methodologies on micro- as well as nanoscale platforms is crucial for information processing and data-storage technologies. In electronics, radiofrequency signals are controlled and manipulated by 'lumped' circuit elements, such as resistors, inductors and capacitors. In earlier work, we theoretically proposed that optical nanostructures, when properly designed and judiciously arranged, could behave as nanoscale lumped circuit elements-but at optical frequencies. Here, for the first time we experimentally demonstrate a two-dimensional optical nanocircuit at mid-infrared wavelengths. With the guidance of circuit theory, we design and fabricate arrays of Si3N4 nanorods with specific deep subwavelength cross-sections, quantitatively evaluate their equivalent impedance as lumped circuit elements in the mid-infrared regime, and by Fourier transform infrared spectroscopy show that these nanostructures can indeed function as two-dimensional optical lumped circuit elements. We further show that the connections among nanocircuit elements, in particular whether they are in series or in parallel combination, can be controlled by the polarization of impinging optical signals, realizing the notion of 'stereo-circuitry' in metatronics-metamaterials-inspired optical circuitry. [ABSTRACT FROM AUTHOR]

We show that the powerful framework of transformation optics may be exploited for engineering the nonlocal response of artificial electromagnetic materials. Relying on the form-invariant properties of coordinate-transformed Maxwell's equations in the spectral domain, we derive the general constitutive "blueprints" of transformation media yielding prescribed nonlocal field-manipulation effects and provide a physically incisive and powerful geometrical interpretation in terms of deformation of the equifrequency contours. In order to illustrate the potentials of our approach, we present an example of application to a wave-splitting refraction scenario, which may be implemented via a simple class of artificial materials. Our results provide a systematic and versatile framework which may open intriguing venues in dispersion engineering of artificial materials. [ABSTRACT FROM AUTHOR]

We discuss the well-known limitations in the retrieval of homogenized constitutive parameters based on reflection and transmission from metamaterial slabs arising near the Fabry-Perot resonances of the samples under analysis. The associated artifacts can significantly affect the retrieval results, causing Lorentzian-like nonphysical features. We discuss the nature of these artifacts and propose a general method to correct them based on a numerical regression procedure. These concepts are applied to a homogeneous slab and a periodic metamaterial array, showing that the proposed correction can provide an improved retrieval technique for metamaterial homogenization purposes. [ABSTRACT FROM AUTHOR]