arXiv subjects

Constantin Simovski

Publications and source records attributed to Constantin Simovski.

At least 37 records · Page 2Linked to original sources

arXiv2018

Passive Decoupling of Two Closely Located Dipole Antennas

In this paper, we prove that two parallel dipole antennas can be decoupled by a similar but passive dipole located in the middle between them. The decoupling is proved for whatever excitation of these antennas and for ultimately small distances between them. Our theoretical model based on the method of induced electromotive forces is validated by numerical simulations and measurements. A good agreement between theory, simulation and measurement proves the veracity of our approach.

physics.app-ph↗

arXiv2017

Giant magneto-electric field separation via near-field interference on anapole-like states

Quality of spatial separation between electric and magnetic fields in an electromagnetic wave is fundamentally constrained by nonlocal nature of Maxwell equations. While electric and magnetic energy densities in a wave, propagating in vacuum, are equal at each point in space, carefully designed photonic structures can enable surpassing this limit. Here, a set of high index dielectric tubes was for the first time proposed and theoretically and experimentally demonstrated to deliver a record high spatial separation, overcoming the free space scenario by more than three orders of magnitude with simultaneous enhancement of the magnetic field. Separation effect in the proposed structure is enabled by the near-field interference on anapole-like states, designed by tuning geometrical parameters of coupled dielectric tubes. The void layout of the structure enables the direct observation of the effect with near-field probes and could be further employed for relevant applications. Novel devices, providing tunable high quality separation between electric and magnetic fields, are extremely important for metrology, spectroscopy, spintronics, and opto-electronic applications.

physics.optics↗

arXiv2016

Demonstration of the enhanced Purcell factor in all-dielectric structures

The Purcell effect is usually described as a modification of the spontaneous decay rate in the presence of a resonator. In plasmonics, this effect is commonly associated with a large local-field enhancement in "hot spots" due to the excitation of surface plasmons. However, high-index dielectric nanostructures, which become the basis of all-dielectric nanophotonics, can not provide high values of the local-field enhancement due to larger radiation losses. Here, we demonstrate how to achieve a strong Purcell effect in all-dielectric nanostructures, and show theoretically that the Purcell factor can be increased by two orders of magnitude in a finite chain of silicon nanoparticles. Using the eigenmode analysis for an infinite chain, we demonstrate that the high Purcell factor regime is associated with a Van Hove singularity. We perform a proof-of-concept experiment for microwave frequencies and observe the 65-fold enhancement of the Purcell factor in a chain of 10 dielectric particles.

physics.optics↗

arXiv2016

Enhancement of perovskite solar cells by plasmonic nanoparticles

Synthetic perovskites with photovoltaic properties open a new era in solar photovoltaics. Due to high optical absorption perovskite-based thin-film solar cells are usually considered as fully absorbing solar radiation on condition of ideal blooming. However, is it really so? The analysis of the literature data has shown that the absorbance of all photovoltaic pervoskites has the spectral hole at infrared frequencies where the solar radiation spectrum has a small local peak. This absorption dip results in the decrease of the optical efficiency of thin-film pervoskite solar cells by nearly 3% and close the ways of utilise them at this range for any other applications. In our work we show that to cure this shortage is possible complementing the basic structure by an inexpensive plasmonic array.

physics.optics↗

arXiv2015

Polarizability Tensor Calculation: Induced Local Charge and Current Distributions

We develop a semi-analytical approach to calculate the polarizability tensors of an arbitrary individual scatterer. The approach is based on the calculation of induced electric and/or magnetic dipole moments on the scatterer. By taking the advantages of the present approach, we calculate the individual polarizability tensors of an isolated scatterer in a homogeneous isotropic medium. Moreover, we obtain the polarizability tensors of scatterers located between two different isotropic media. Furthermore, due to the nature of the proposed method, we may determine the effective polarizability of a scatterer in a periodic array. To this end and for comparison reasons, we investigate two other methods for calculating effective polarizability tensors of scatterers in a two-dimensional periodic array. The proposed approach, in comparison to other reported approaches, is simpler, easily implemented, and does not require spherical harmonic expansion or complicated far-field calculations. We examine the validity of the proposed approach using several examples and compare the results with other approaches.

physics.class-ph↗

arXiv2015

Large Purcell enhancement without strong field localization

The Purcell effect is defined as the modification of spontaneous decay in the presence of a resonator, and in plasmonics it is usually associated with the large local-field enhancement in "hot spots" due to surface plasmon polaritons. Here we propose a novel strategy for enhancing the Purcell effect through engineering the radiation directivity without a strict requirement of the local field enhancement. Employing this approach, we demonstrate how to enhance the Purcell effect by two orders of magnitude in all-dielectric nanostructures recently suggested as building blocks of low-loss nanophotonics and metamaterials. We support our concept by proof-of-principle microwave experiments with arrays of high-index dielectric resonators.

physics.optics↗

arXiv2015

Optimization of optical absorption in thin layers of amorphous silicon enhanced by silver nanospheres

We study a highly controllable perfect plasmonic absorber -- a thin metamaterial layer which possess balanced electric and magnetic responses in some frequency range. We show that this regime is compatible with both metal-backed variant of the structure or its semitransparent variant. This regime can be implemented in a prospective thin-film photovoltaic cell with negligible parasitic losses.

physics.optics↗

arXiv2014

A System Analysis of Micron-Gap Thermophotovoltaic Systems Enhanced by Nanowires

We introduce new micron-gap thermophotovoltaic systems enhanced by tungsten nanowires. We theoretically show that these systems allow the frequency-selective super-Planckian spectrum of radiative heat transfer that promises a very efficient generation of electricity. Our system analysis covers practical aspects such as output power per unit area and efficiency of the tap water cooling.

cond-mat.mes-hall↗

arXiv2014

A non-resonant dielectric metamaterial for enhancement of thin-film solar cells

Recently, we have suggested dielectric metamaterial composed as an array of submicron dielectric spheres located on top of an amorphous thin-film solar cell. We have theoretically shown that this metamaterial can decrease the reflection and simultaneously can suppress the transmission through the photovoltaic layer because it transforms the incident plane wave into a set of focused light beams. This theoretical concept has been strongly developed and experimentally confirmed in the present paper. Here we consider the metamaterial for oblique angle illumination, redesign the solar cell and present a detailed experimental study of the whole structure. In contrast to our precedent theoretical study we show that our omnidirectional light-trapping structure may operate better than the optimized flat coating obtained by plasma-enhanced chemical vapor deposition.

physics.optics↗

arXiv2013

Superdirective dielectric nanoantennas with effect of light steering

We introduce a novel concept of superdirective antennas based on the generation of higher-order optically-induced magnetic multipole modes. All-dielectric nanoantenna can be realized as an optically small spherical dielectric nanoparticle with a notch excited by a point source (e.g. a quantum dot) located in the notch. The superdirectivity effect is not associated with high dissipative losses. For these dielectric nanoantennas we predict the effect of the beam steering at the nanoscale characterized by a subwavelength sensitivity of the beam radiation direction to the source position. We confirm the predicted effects experimentally through the scaling to the microwave frequency range.

physics.optics↗

arXiv2011

An arrayed nanoantenna for broadband light emission and detection

We suggest a broadband optical unidirectional arrayed nanoantenna consisting of equally spaced nanorods of gradually varying length. Each nanorod can be driven by near-field quantum emitters radiating at different frequencies or, according to the reciprocity principle, by an incident light at the same frequency. Broadband unidirectional emission and reception characteristics of the nano-antenna open up novel opportunities for subwavelength light manipulation and quantum communication, as well as for enhancing the performance of photoactive devices such as photovoltaic detectors, light-emitting diodes, and solar cells.

physics.optics↗

arXiv2011

Giant radiation heat transfer through the micron gaps

Near-field heat transfer between two closely spaced radiating media can exceed in orders radiation through the interface of a single black body. This effect is caused by exponentially decaying (evanescent) waves which form the photon tunnel between two transparent boundaries. However, in the mid-infrared range it holds when the gap between two media is as small as few tens of nanometers. We propose a new paradigm of the radiation heat transfer which makes possible the strong photon tunneling for micron thick gaps. For it the air gap between two media should be modified, so that evanescent waves are transformed inside it into propagating ones. This modification is achievable using a metamaterial so that the direct thermal conductance through the metamaterial is practically absent and the photovoltaic conversion of the transferred heat is not altered by the metamaterial.

physics.optics↗

arXiv2011

An ultra-broadband electromagnetically indefinite medium formed by aligned carbon nanotubes

Anisotropic materials with different signs of components of the permittivity tensor are called indefinite materials. Known realizations of indefinite media suffer of high absorption losses. We show that periodic arrays of parallel carbon nanotubes (CNTs) can behave as a low-loss indefinite medium in the infrared range. We show that a finite-thickness slab of CNTs supports the propagation of backward waves with small attenuation in an ultra-broad frequency band. In prospective, CNT arrays can be used for subwavelength focusing and detection, enhancing the radiation efficiency of small sources.

physics.optics↗

arXiv2010

On effective electromagnetic parameters of artificial nanostructured magnetic materials

In this paper we discuss effective material parameter description of new nanostructures designed to perform as artificial magnetic materials for vis- ible light. Among these structures there are various split-ring resonators, dual-bar structures, fishnet layers and other geometries. Artificial magnetic response in these structures appears due to weak spatial dispersion effects, and it is important to study the conditions under which the magnetic re- sponse can be adequately measured with effective permeability tensor. On the examples of dual bars and split rings we show that this is possible only under some quite restrictive conditions. In the general case, more compli- cated constitutive relations with more effective material parameters need to be developed.

physics.optics↗

arXiv2010

A multi-frequency local field enhancement by a metamaterial nanopyramid

We suggest and theoretically study the local field enhancement in a metamaterial sample shaped as a pyramid and formed by plasmonic nanoplates alternating with dielectric ones in parallel to the pyramid base. Due to very small thickness of metal nanoplates and different transversal sizes of them the structure not only offers the efficient conversion of the light wave field impinging the pyramid base into hot spots near the pyramid apex, but also a large number of plasmonic resonances at which the field enhancement holds. These resonances cover the whole visible range.

physics.optics↗

arXiv2009

Subwavelength resolution for horizontal and vertical polarization by coupled arrays of oblate nanoellipsoids

A structure comprising a coupled pair of two-dimensional arrays of oblate plasmonic nanoellipsoids in a dielectric host medium is proposed as a superlens in the optical domain for both horizontal and vertical polarizations. By means of simulations it is demonstrated that a structure formed by silver nanoellipsoids is capable of restoring subwavelength features of the object for both polarizations at distances larger than half-wavelength. The bandwidth of subwavelength resolution is in all cases very large (above 13%).

physics.optics↗

arXiv2008

Simple and accurate analytical model of planar grids and high-impedance surfaces comprising metal strips or patches

This paper introduces simple analytical formulas for the grid impedance of electrically dense arrays of square patches and for the surface impedance of high-impedance surfaces based on the dense arrays of metal strips or square patches over ground planes. Emphasis is on the oblique-incidence excitation. The approach is based on the known analytical models for strip grids combined with the approximate Babinet principle for planar grids located at a dielectric interface. Analytical expressions for the surface impedance and reflection coefficient resulting from our analysis are thoroughly verified by full-wave simulations and compared with available data in open literature for particular cases. The results can be used in the design of various antennas and microwave or millimeter wave devices which use artificial impedance surfaces and artificial magnetic conductors (reflect-array antennas, tunable phase shifters, etc.), as well as for the derivation of accurate higher-order impedance boundary conditions for artificial (high-) impedance surfaces. As an example, the propagation properties of surface waves along the high-impedance surfaces are studied.

physics.class-ph↗

arXiv2007

Mesoscopic effective material parameters for thin layers modeled as single and double grids of interacting loaded wires

As an example of thin composite layers we consider single and double grids of periodically arranged interacting wires loaded with a certain distributed reactive impedance. Currents induced to the wires by a normally incident plane wave are rigorously calculated and the corresponding dipole moment densities are determined. Using this data and the averaged fields we assign mesoscopic material parameters for the proposed grid structures. These parameters depend on the number of grids, and measure the averaged induced polarizations. It is demonstrated that properly loaded double grids possess polarization response that over some frequency range can be described by assigning negative values for the mesoscopic parameters. Discussion is conducted on the physical meaningfulness to assign such material parameters for thin composite slabs. The results predicted by the proposed method for the double-grid structures are compared with the results obtained using the commonly adopted S-parameter retrieval procedure.

physics.class-ph↗