arXiv subjects

M. S. Mirmoosa

Publications and source records attributed to M. S. Mirmoosa.

31 records · Page 2Linked to original sources

arXiv2019

Instantaneous Radiation from Time-Varying Electric and Magnetic Dipoles

Radiation from magnetic and electric dipole moments is a key subject in theory of electrodynamics. Although people treat the problem thoroughly in the context of frequency domain, the problem is still not well understood in the context of time domain, especially if dipole moments arbitrarily vary in time under action of external forces. Here, we scrutinize the instantaneous power radiated by magnetic and electric dipole moments, and report findings that are different from the conventional understanding of their instantaneous radiation found in textbooks. In contrast to the traditional far-field approach based on the Poynting vector, our analysis employs a near-field method based on the induced electromotive force, leading to corrective terms that are found to be consistent with time-domain numerical simulations, unlike previously reported expressions. Beyond its theoretical value, this work may also have significant impact in the field of time-varying metamaterials, especially in the study of radiation from subwavelength meta-atoms, scatterers and emitters that are temporally modulated.

physics.class-ph↗

arXiv2019

Complementary metasurfaces for guiding electromagnetic waves

Waveguides are critically important components in microwave, THz, and optical technologies. Due to recent progress in two-dimensional materials, metasurfaces can be efficiently used to design novel waveguide structures which confine the electromagnetic energy while the structure is open. Here, we introduce a special type of such structures formed by two penetrable metasurfaces which have complementary isotropic surface impedances. We theoretically study guided modes supported by the proposed structure and discuss the corresponding dispersion properties. Furthermore, we show the results for different scenarios in which the surface impedances possess non-resonant or resonant characteristics, and the distance between the metasurfaces changes from large values to the extreme limit of zero. As an implication of this work, we demonstrate that there is a possibility to excite two modes with orthogonal polarizations having the same phase velocity within a broad frequency range. This property is promising for applications in leaky-wave antennas and field focusing.

physics.app-ph↗

arXiv2019

Parallel-Plate Waveguides Formed by Penetrable Metasurfaces

In this paper, we introduce and study parallel-plate waveguides formed by two penetrable metasurfaces having arbitrary isotropic sheet impedances. We investigate guided modes of this structure and derive the corresponding dispersion relations. The conditions under which transverse magnetic and transverse electric modes can exist are discussed, and different scenarios including lossless (reactive) metasurfaces, gain-and-loss sheets and extreme cases are under general consideration. Resonant and non-resonant dispersive sheets and corresponding extreme cases are investigated. Our theoretical results are confirmed with full-wave simulated results considering practical realizations of the proposed parallel-plate waveguide which exploit two frequency-selective surfaces. Finally, the obtained theoretical formulas are applied to study the dispersion diagrams for waves along resonant sheets at different distances between the two identical or different metasurfaces. We hope that this study is useful for future applications at both microwave and optical regimes.

physics.app-ph↗

arXiv2019

Non-scattering Metasurface-bound Cavities for Field Localization, Enhancement, and Suppression

We propose and analyse metasurface-bound invisible (non-scattering) partially open cavities where the inside field distribution can be engineered. It is demonstrated both theoretically and experimentally that the cavities exhibit unidirectional invisibility at the operating frequency with enhanced or suppressed field at different positions inside the cavity volume. Several examples of applications of the designed cavities are proposed and analyzed, in particular, cloaking sensors and obstacles, enhancement of emission, and "invisible waveguides". The non-scattering mode excited in the proposed cavity is driven by the incident wave and resembles an ideal bound state in the continuum of electromagnetic frequency spectrum. In contrast to known bound states in the continuum, the mode can stay localized in the cavity infinitely long, provided that the incident wave illuminates the cavity.

physics.app-ph↗

arXiv2019

Non-Scattering Multi-Mirror Systems for Field Localization

Resonators composed of two mirrors, such as Fabry-Perot cavities, provide a simple but effective approach to achieve wave transmission with high finesse. Further increase of the quality factor requires reflectors with higher conductivity or multi-mirror solutions. However, the analytical complexity of resonators with more than two mirrors prevents the design of optimal structures without recurring to numerical methods. This work considers an alternative approach to this problem by using cavities which do not produce any electromagnetic scattering. We demonstrate that these invisible cavities can be placed inside one another, resulting in a `matryoshka-doll'-like resonator. The standing-wave distribution of the inner resonator can be controlled without disturbing that of the outer one, whereas the whole system remains non-scattering.

physics.app-ph↗

arXiv2019

Parallel-Plate Waveguides Formed by Arbitrary Impedance Sheets

In this work, we introduce and study parallel-plate-waveguide structures that are formed by two penetrable metasurfaces having arbitrary sheet impedances. We investigate the guided modes which can propagate in such structures and derive the corresponding dispersion relations. Different scenarios including series- and parallel-resonant impedance sheets are considered. The obtained theoretical formulas are applied to predict the dispersion properties for different separations between the two symmetric or asymmetric metasurfaces.

physics.app-ph↗

arXiv2018

Unlimited Accumulation of Electromagnetic Energy Using Time-Varying Reactive Elements

Accumulation of energy by reactive elements is limited by the amplitude of time-harmonic external sources. In the steady-state regime, all incident power is fully reflected back to the source, and the stored energy does not increase in time, although the external source continuously supplies energy. Here, we show that this claim is not true if the reactive element is time-varying, and time-varying lossless loads of a transmission line or lossless metasurfaces can accumulate electromagnetic energy supplied by a time-harmonic source continuously in time without any theoretical limit. We analytically derive the required time dependence of the load reactance and show that it can be in principle realized as a series connection of mixers and filters. Furthermore, we prove that properly designing time-varying LC circuits one can arbitrarily engineer the time dependence of the current in the circuit fed by a given time-harmonic source. As an example, we theoretically demonstrate a circuit with a linearly increasing current through the inductor. Such LC circuits can accumulate huge energy from both the time-harmonic external source and the pump which works on varying the circuit elements in time. Finally, we discuss how this stored energy can be released in form of a time-compressed pulse.

physics.app-ph↗

arXiv2016

Magnetic hyperbolic metamaterial of polaritonic nanowires

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.

cond-mat.mes-hall↗

arXiv2015

Microgap thermophotovoltaic systems with low emission temperature and high electric output

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.

cond-mat.mes-hall↗

arXiv2015

Double resonant wideband Purcell effect in a polaritonic wire medium

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.

cond-mat.mes-hall↗

arXiv2015

Topological phase transition in wire medium enables high Purcell factor at infrared frequencies

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.

cond-mat.mes-hall↗

arXiv2015

Topological transition in a nanowire medium and its radiative implication

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.

cond-mat.mes-hall↗

arXiv2014

Polarizabilities of nonreciprocal bianisotropic particles

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.

physics.optics↗