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Incorporating both gain and loss into electromagnetic systems provides possibilities to engineer effects in unprecedented ways. Concerning electromagnetic effects in isotropic media that have concurrently electric and magnetic responses, there is in fact a degree of freedom to distribute the gain and loss in different effective material parameters. In this paper, we analytically scrutinize wave interactions with those media, and, most importantly, we contemplate the extreme scenario where such media are anti-Hermitian. Considering various conditions for excitation, polarization, and geometry, we uncover important effects and functionalities such as lasing into both surface waves and propagating waves, conversion of evanescent source fields to transmitted propagating waves, full absorption, and enhancing backward to forward scattering ratio. We hope that these findings explicitly show the potential of anti-Hermiticity to be used in optical physics as well as microwave engineering for creating and using unconventional wave phenomena.

In conventional inductive wireless power devices, the energy is transferred via only reactive near fields, which is equivalent to non-radiative F\"orster energy transfer in optics. Radiation from transmitting and receiving coils is usually considered as a parasitic effect that reduces the power transfer efficiency. As long as the distance between the two antennas is small as compared to the antenna size, conventional WPT devices offer rather high power transfer efficiency, of the order of 80-90\%. However, for larger distances, the transfer efficiency dramatically drops, making such devices not practical. In this paper, we develop a dynamic theory of wireless power transfer between two small loop antennas, clarify the role of far-field radiation, and find a possibility to realize efficient wireless power transfer at large distances utilizing the regime of radiation suppression due to optimized mutual dynamic interactions between the transmitting and receiving antennas. The analytical results have been validated by simulations and measurements, and they open a possibility to greatly expand the range of distances of compact wireless power transfer devices. The developed theory can be applied also to coupling between antennas of different types and to energy transfer between nano-objects.

In this work we report a theoretical study of the lateral resolution granted by a simple glass microcylinder. In this 2D study, we had in mind the 3D analogue -- a microsphere whose ability to form a deeply subwavelength and strongly magnified image of submicron objects has been known since 2011. Conventionally, the microscope in which such the image is observed is tuned so that to see the areas behind the microsphere. This corresponds to the location of the virtual source formed by the microsphere at a distance longer than the distance of the real source to the miscroscope. Recently, we theoretically found a new scenario of superresolution, when the virtual source is formed in the wave beam transmitted through the microsphere. However, in this work we concentrated on the case when the superresolution is achieved in the impractical imaging system, in which the microscope objective lens is replaced by a microlens located at a distance smaller than the Rayleigh range. The present paper theoretically answers an important question: which scenario of far-field nanoimaging by a microsphere grants the finest spatial at very large distances. We found that the novel scenario (corresponding to higher refractive indices) promises further enhancement of the resolution.

Recently a hypothesis explaining the non-resonant mechanism of subwavelength imaging granted by a dielectric microsphere has been suggested. In accordance to the hypothesis, the far-field image of a subwavelength scatterer strongly coupled to a microsphere by near fields is offered by the scatterer polarization normal to the sphere surface. The radiation of a closely located normally oriented dipole is shaped by the microsphere so that the transmitted wave beam has a practically flat phase front. Then this beam turns out to be imaging -- keeping the subwavelength information about the dipole location. However, this mechanism of subwavelength imaging was only supposed in our previous paper. In this paper, we present a theoretical study which confirms this hypothesis and better explains the underlying physics. In several scenarios of the imaging beam evolution either a flat or a slightly diverging phase front of the hollow wave beam formed by a microsphere enables the deeply subwavelength ($0.1-0.2\lambda$) resolution of two dipole sources. We numerically simulate one of these scenarios -- that one in which the focusing lens is located closer than the Rayleigh diffraction length to the beam-forming microsphere and represents a microsphere itself. In our simulations we replace a 3D microsphere by a 2D "sphere" (microcylinder) so that to use an available electromagnetic solver for dielectric microparticles of very large optical sizes. The physical mechanism of the imaging does not suffer of this replacement.

General concept of Fano resonance is considered so that to show the possibility of this resonance in space. Using a recently found solution for a Bessel wave beam impinging a dielectric sphere, we analyze the electromagnetic fields near a microsphere with different optical size and permittivity values. We theoretically reveal a spatial Fano resonance when a resonant mode of the sphere interferes with {an amount of } non-resonant modes. This resonance results in a giant jump of the electric field behind the sphere impinged by the first-order Bessel beam. The local minimum of the electromagnetic field turns out to be noticeably distanced from the rear edge of the microsphere. However, this is a near-field effect and we prove it. We also show that this effect can be utilized for engineering a submicron optical trap with unusual and useful properties.

Generation of a photonic nanojet by a linearly polarized wave beam or a plane wave impinging a dielectric microcylinder implies partial conversion of the propagating waves into the evanescent ones. This conversion is manifested in nanojet waist of subwavelength effective width. However, this known near-field effect is relatively weak. We theoretically show that the incidence of a wave beam formed by two plane waves enables much stronger near-field effects: a deeply subwavelength focusing of the incident beam and a strong enhancement of the electric field on the whole cylinder surface and near it. The domination of the evanescent waves in the vicinity of the cylinder results from the destructive interference of the propagating spatial harmonics of the scattered field dictated by the incident wave beam.

Role of the normal polarization in the far-field subwavelength imaging granted by a dielectric microsphere or microcylinder is discussed and the hypotheses explaining this experimental fact are suggested. One of these hypotheses is confirmed by exact numerical simulations. This mechanism of the magnifying superlens operation is based on the excitation of creeping waves at a curved dielectric interface by a normally polarized dipole. The set of creeping waves after their ejection from the surface creates an imaging beam which may mimic either a Bessel beam or a Mathieu beam depending on the microparticle radius. This mechanism corresponds to the asymmetric coherent illumination.

We generalize the concept of parity-time symmetric structures with the goal to create meta-atoms exhibiting extraordinary abilities to overcome the presumed limitations in the scattering of overall lossless particles, such as non-zero forward scattering and the equality of scattering and extinction powers for all lossless particles. Although the forward scattering amplitude and the extinction cross section of our proposed meta-atoms vanish, they scatter incident energy into other directions, with controllable directionality. These meta-atoms possess extreme electromagnetic properties not achievable for passive scatterers. As an example, we study meta-atoms consisting of two or three small dipole scatters. We consider possible microwave realizations in the form of short dipole antennas loaded by lumped elements. The proposed meta-atom empowers extraordinary response of a shadow-free scatterer and theoretically enables most unusual material properties when used as a building block of an artificial medium., Comment: 14 pages, 9 Figures

Conventional wireless power transfer systems consist of a microwave power generator and a microwave power receiver separated by some distance. To realize efficient power transfer, the system is typically brought to resonance, and the coupled-antenna mode is optimized to reduce radiation into the surrounding space. In this scheme, any modification of the receiver position or of its electromagnetic properties results in the necessity of dynamically tuning the whole system to restore the resonant matching condition. It implies poor robustness to the receiver location and load impedance, as well as additional energy consumption in the control network. In this study, we introduce a new paradigm for wireless power delivery based on which the whole system, including transmitter and receiver and the space in between, forms a unified microwave power generator. In our proposed scenario the load itself becomes part of the generator. Microwave oscillations are created directly at the receiver location, eliminating the need for dynamical tuning of the system within the range of the self-oscillation regime. The proposed concept has relevant connections with the recent interest in parity-time symmetric systems, in which balanced loss and gain distributions enable unusual electromagnetic responses., Comment: 10 pages, 13 figures

We show that the axial component of the magnetic permeability tensor is resonant for a wire medium consisting of high-index epsilon-positive nanowires, and its real part changes the sign at a certain frequency. At this frequency the medium experiences the topological transition from the hyperbolic to the elliptic type of dispersion. We show that the transition regime is characterized by extremely strong dependence of the permeability on the wave vector. This implies very high density of electromagnetic states that results in the filamentary pattern and noticeable Purcell factor for a transversely oriented magnetic dipole.

We theoretically show that a thermophotovoltaic (TPV) system enhanced by a wire metamaterial opens the door to a prospective microgap thermophotovoltaics which will combine high electric output with relatively low temperatures of the emitter. The suggested system comprises an array of parallel metal nanowires grown on top of a photovoltaic semiconductor and standing free in the vacuum gap between the host dielectric layer and the emitter, so that their ends are sufficiently close to the emitting surface. Due to the resonant near-field coupling between this wire medium and the emitter and due to the optimized layered structure of the whole system, the strongly super-Planckian radiative heat flux of resonant nature is engineered.

We present a general methodology for electromagnetic homogenization and characterization of bianisotropic metasurfaces formed by regular or random arrangements of small arbitrary inclusions at the interface of two different isotropic media. These topologies can provide the most general electric and magnetic surface-bound dipolar responses to incident electromagnetic waves. The approach unites and generalizes the earlier theories developed independently by two research groups: the joint group of profs. Holloway and Kuester and the joint group of profs. Simovski and Tretyakov. We demonstrate the use of both formalisms in several example cases and discuss the differences between the two approaches. Furthermore, we generalize the known theories for the bianisotropic metasurfaces located at interfaces between two different media or on a substrate. The generalized framework allows characterization of both periodical or amorphous metasurfaces. The developed analytical theory can be used in the analysis and synthesis of metasurfaces for a wide variety of applications.

In this paper, we theoretically show that a broadband resonant enhancement of emission may occur for infrared sources located in a polaritonic wire medium. The reason of this enhancement is overlapping of two topological transitions of the wave dispersion in the medium. The first topological transition has been recently revealed as an effect inherent to polaritonic wire media. The second one was theoretically uncovered in another material. In this work we reveal it for wire media and prove the possibility to combine both these transitions with the purpose to obtain the broadband resonant Purcell factor. We compare the results obtained for two orientations of a subwavelength electric dipole embedded into wire medium -- that along the optical axis and that perpendicular to it -- and report on the resonant isotropic radiation enhancement. Also, we reveal the enhancement of radiation to free space from a finite sample of the wire medium.

Recently published paper [PRL 114, 147701 (2015)] contains several misleading statements and misinterpretations of known facts. The main massage of the paper [PRL 114, 147701 (2015)] is as follows: "We have shown that explicit symmetry breaking in the structural configuration of charges leads to symmetry breaking of the electric field which results in electromagnetic radiation due to non-conservative current within a localized region of space and time" seems to transcend mere empiricism, touching the theoretical foundations of electromagnetism. Moreover, basic mistakes are numerous in this article and its main claim is wrong. Below we prove it citing the paper and arguing against it.

In this paper, we study topological phase transition in a wire medium operating at infrared frequencies. This transition occurs in the reciprocal space between the indefinite (open-surface) regime of the metamaterial to its dielectric (closed-surface) regime. Due to the spatial dispersion inherent to wire medium, a hybrid regime turns out to be possible at the transition frequency. Both such surfaces exist at the same frequency and touch one another. At this frequency, all values of the axial wavevector correspond to propagating spatial harmonics. The implication of this regime is the overwhelming radiation enhancement. We numerically investigated the gain in radiated power for a sub-wavelength dipole source submerged into such the medium. In contrast to all previous works, this gain (called the Purcell factor) turns out to be higher for an axial dipole than for a transversal one.

In this paper we show that arrays of core-shell nanoparticles function as effective thin absorbers of light. In contrast to known metamaterial absorbers, the introduced absorbers are formed by single planar arrays of spherical inclusions and enable full absorption of light incident on either or both sides of the array. We demonstrate possibilities for realizing different kinds of symmetric absorbers, including resonant, ultra-broadband, angularly selective, and all-angle absorbers. The physical principle behind these designs is explained considering balanced electric and magnetic responses of unit cells. Photovoltaic devices and thermal emitters are the two most important potential applications of the proposed designs., Comment: (e.g.: 18 pages, 5 figures)

We reveal and study the topological transition in a metamaterial formed by parallel nanowires of polaritonic material. When the dispersion transits from the elliptic (epsilon-positive) to hyperbolic (epsilon-indefinite) regime, a very specific isofrequency surface arises which implies an extraordinary Purcell factor in spite of noticeable optical losses.

We suggest and theoretically explore a possibility to strongly enhance the steady thermal radiation of a small thermal emitter using an infrared hyperlens. The hyperbolic metamaterial of the hyperlens converts emitter's near fields into the propagating waves which are efficiently irradiated from the hyperlens surface. Thus, with the hyperlens, emitter's spectral radiance goes well beyond the black-body limit for the same emitter in free space. Although the hyperlens can be kept at a much lower temperature than the emitter, the whole structure may radiate, in principle, as efficiently as a black body with the same size as that of the hyperlens and the same temperature as that of the emitter. We believe that this study can lead to a breakthrough in radiative cooling at microscale, which is crucial for microlasers and microthermophotovoltaic systems., Comment: 5 pages, 3 figures

For two electrically small nonreciprocal scatterers an analytical electromagnetic model of polarizabilities is developed. Both particles are bianisotropic: the so-called Tellegen-omega particle and moving-chiral particle. Analytical results are compared to the full-wave numerical simulations. Both models satisfy to main physical restrictions and leave no doubts in the possibility to realize these particles experimentally. This paper is a necessary step towards applications of nonreciprocal bianisotropic particles such as perfect electromagnetic isolators, twist polarizers, thin-sheet phase shifters, and other devices., Comment: 10 pages, 9 figures

Volume or statistical averaging of the microscopic Maxwell equations (MEs), i.e. transition from microscopic MEs to their macroscopic counterparts, is one of the main steps in electrodynamics of materials. In spite of the fundamental importance of the averaging procedure, it is quite rarely properly discussed in university courses and respective books; up to now there is no established consensus about how the averaging procedure has to be performed. In this paper we show that there are some basic principles for the averaging procedure (irrespective to what type of material is studied) which have to be satisfied. Any homogenization model has to be consistent with the basic principles. In case of absence of this correlation of a particular model with the basic principles the model could not be accepted as a credible one. Another goal of this paper is to establish the averaging procedure for bulk MM, which is rather close to the case of compound materials but should include magnetic response of the inclusions and their clusters. In the vast majority of cases the consideration of bulk materials means that we consider propagation of an electromagnetic wave far from the interfaces, where the eigenwave in the medium has been already formed and stabilized. In other words, in this paper we consider the possible eigenmodes, which could exist in the equivalent homogenized media, and the necessary math apparatus for an adequate description of these waves. A discussion about boundary conditions and layered MM is a subject of separate publication and will be done elsewhere.

In this Letter we theoretically study the possibility of total power absorption of light in a planar grid modelled as an effective sheet with zero optical thickness. The key prerequisite of this effect is the simultaneous presence of both resonant electric and magnetic modes in the structure. We show that the needed level of the magnetic mode is achievable using the effect of substrate-induced bianisotropy which also allows the huge local field enhancement at the same wavelength where the maximal absorption holds., Comment: 4 pages, 4 figures

Volume or statistical averaging of the microscopic Maxwell equations (MEs), i.e. transition from microscopic MEs to their macroscopic counterparts, is one of the main steps in electrodynamics of materials. In spite of the fundamental importance of the averaging procedure, it is quite rarely properly discussed in university courses and respective books; up to now there is no established consensus about how the averaging procedure has to be performed. In this paper we show that there are some basic principles for the averaging procedure (irrespective to what type of material is studied) which have to be satisfied. Any homogenization model has to be consistent with the basic principles. In case of absence of this correlation of a particular model with the basic principles the model could not be accepted as a credible one. Another goal of this paper is to establish the averaging procedure for metamaterials, which is rather close to the case of compound materials but should include magnetic response of the inclusions and their clusters. We start from the consideration of bulk materials, which means in the vast majority of cases that we consider propagation of an electromagnetic wave far from the interfaces, where the eigenwave in the medium has been already formed and stabilized. A basic structure for discussion about boundary conditions and layered metamaterials is a subject of separate publication and will be done elsewhere., Comment: 15 pages, no figures

We suggest a novel multifrequency broadband plasmonic Yagi-Uda-type nanoantenna equipped with an array of tapered directors. Each director can be used for the excitation of the antenna by nanoemitters matched spectrally with the director resonant frequency and placed in the director near-field region. Multifrequency op- eration of nanoantennas provides tremendous opportunities for broadband emission enhancement, spectroscopy and sensing. By the principle of reciprocity, the same tapered nanoantenna architecture can be used both as a transmitter and/or as a receiver, thus being useful for creating a broadband wireless communication system.

In this paper we suggest and theoretically study a tapered plasmonic nanostructure which connects the incident wave beam with a subwavelength spatial region where the field is locally enhanced in a broad frequency range or for different operation frequencies. This spatial region has a frequency stable location near the contour of the tapered structure. This results from a special waveguide mode which can also exist in the tapered structure. We foresee many possible applications for our structure from prospective near-field scanning optical microscopes to interconnects between conventional optical waveguides and prospective optical nanocircuits., Comment: Submitted on 01Feb11 to PRB

Nanostructured metamaterials, especially arrays of metallic nanoparticles which sustain the excitation of localized plasmon polaritons, provide excellent opportunities to mold the flow of light in the linear regime. We suggest a metamaterial structure whose properties are determined not only by its inner geometry but also by its entire shape. We call this structure a \emph{metamaterial nanotip}. We evaluate the potential of this nanotip to control the size and the location of the field enhancement. Two-dimensional implementations of this metamaterial nanotip were comprehensively numerically simulated and confirm the expected, physically distinct regimes of operation., Comment: 4 pages, 4 figures

The idea of isotropic resonant magnetism in the visible range of frequencies known from precedent publications is developed having in mind achievements of the modern chemistry. Plasmonic colloidal nanoparticles covering a silica core form a cluster with resonant and isotropic magnetic response. Two approximate models giving the qualitative mutual agreement are used to evaluate the magnetic polarizability of the cluster. It is shown that the electrostatic interaction of nanocolloids decreases the resonant frequency of an individual complex magnetic scatterer (nanocluster) compared to the previously studied variant of a planar circular nanocluster with same size. This means the reduction of the optical size of nanoclusters that presumably allows one to avoid strong spatial dispersion within the frequency range of the negative permeability., Comment: 17 pages, 5 figs

A new type of high-impedance surfaces (HIS) has been introduced by C.R. Simovski et al. recently. In this paper, we propose to use such layers as artificial magnetic materials in the design of patch antennas. The new HIS is simulated and patch antennas partially filled by these composite layers are measured in order to test how much the antenna dimensions can be reduced. In order to experimentally investigate the frequency behavior of the material, different sizes of the patches are designed and tested with the same material layer. Also the height of the patch is changed in order to find the best possible position for minimizing the antenna size. This composite layer of an artificial magnetic material has made the antenna smaller while keeping the bandwidth characteristics of the antenna about the same. About 40% of size reduction has been achieved., Comment: 7 pages, 3 tables, 13 figures

The work is dedicated to the theoretic analysis of wire media, i.e. lattices of perfectly conducting wires comprised of two or three doubly periodic arrays of parallel wires which are orthogonal to one another. An analytical method based on local field approach is used. The explicit dispersion equations are presented and studied. A possibility to introduce a dielectric permittivity is discussed. The theory is validated by comparison with the numerical data available in the literature., Comment: 9 pages, 8 figures, submitted to PRE

In this work a possible realizations of a uniaxial variant of left-handed material (LHM) at microwaves is considered. The meta-material has some features of the known structure studied by the group of D. Smith in 2001, however in the present structure a lattice of parallel wires and a lattice of artificial magnetic resonators (MRs) are unified and the MRs are not split-ring resonators. The optimizing of MRs allows significantly decrease the magnetic losses and broaden the band within which the meta-material becomes a LHM.

The influence of material coverings on the antenna bandwidth is investigated for antennas formed by thin electric or magnetic line sources. It is shown that uniform thin layers of arbitrary passive materials (including Veselago, left-handed, or double-negative materials) cannot help to overcome the bandwidth limitations imposed by the amount of energy stored in the antenna reactive field. Alternative possibilities offered by complex composite materials in the antenna design are identified.

The properties of artificial isotropic microwave composites which would possess simultaneously negative permittivity and permeability are studied theoretically. Four kinds of composites are considered. Two of them concern media with split-ring resonator particles with different particle arrangements. The other two are realized with Omega particles. An analytical antenna model of the electromagnetic behavior of a split-ring resonator (SRR) is suggested and verified by numerical simulations. Next, material parameters of composite media made with SRR or Omega particles are calculated. It is shown that the Omega-composites are more prospective for obtaining negative real values of permittivity and permeability than the split-ring resonators.

A structure formed by combined lattices of infinitely long wires and split-ring resonators is studied. A dispersion equation is derived and then used to calculate the effective permittivity and permeability in the frequency band where the lattice can be homogenized. The backward wave region in which both the effective permittivity and permeability are negative is analyzed. Some open and controversial questions are discussed. It is shown that previous experimental results confirming the existence of backward waves in such a structure can be in deed explained in terms of negative material parameters. However, these parameters are not quasi-static and thus the known analytical formulas for the effective material parameters of this structure, which have been widely used and discussed in the literature, were not correct, and it was the reason of some objections to the authors of that experiment., Comment: MiKTex, 9 pages with 7 figures, submitted to IEEE Trans. Antennas Propagat

It is found that there exist composite media that exhibit strong spatial dispersion even in the very large wavelength limit. This follows from the study of lattices of ideally conducting parallel thin wires (wire media). In fact, our analysis reveals that the description of this medium by means of a local dispersive uniaxial dielectric tensor is not complete, leading to unphysical results for the propagation of electromagnetic waves at any frequencies. Since non--local constitutive relations have been usually considered in the past as a second order approximation, meaningful in the short wavelength limit, the aforementioned result presents a relevant theoretical interest. In addition, since such wire media have been recently used as a constituent of some discrete artificial media (or metamaterials), the reported results open the question of the relevance of the spatial dispersion in the characterization of these artificial media., Comment: MiKTex, 4 pages with 2 figures, submitted to PRL