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Constantin R. Simovski

Publications and source records attributed to Constantin R. Simovski.

At least 19 recordsLinked to original sources
arXiv2020

Ionically Gated Small Molecule OPV: Interfacial doping of Charge collector and Transport layer

We demonstrate an improvement in the performance of organic photovoltaic (OPV) systems based on small molecules by ionic gating via controlled reversible n-doping of multi-wall carbon nanotube (MWCNT) coated on fullerenes ETL: C60 and C70. Such electric double layer charging (EDLC) doping, achieved by ionic liquid (IL) charging, allows tuning the electronic concentration in MWCNT and in the fullerene planar acceptor layers, increasing it by orders of magnitude. This leads to decreasing the series and increasing the shunt resistances of OPV and allows to use of thick (up to 200 nm) ETLs, increasing the durability and stability of OPV. Two stages of OPV enhancement are described, upon the increase of gating bias Vg: at small (or even zero) Vg the extended interface of IL and porous transparent MWCNT is charged by gating, and the fullerene charge collector is significantly improved, becoming an ohmic contact. This changes the S-shaped I-V curve via improving the electron collection by n-doped MWCNT cathode with ohmic interfacial contact. The I-V curves further improve at higher gating bias Vg due to the raising of the Fermi level and lowering of MWCNT work function. At the next qualitative stage, the acceptor fullerene layer becomes n-doped by electron injection from MWCNT while ions of IL penetrate into fullerene. At this step the internal built-in field is created within OPV, which helps exciton dissociation and charge separation/transport, increasing further the Jsc and the FF (Fill factor). Overall power conversion efficiency (PCE) increases nearly 50 times in CuPc/fullerene OPV with MWCNT cathode. The concept of ionically gated MWCNT-ETL interface is numerically simulated by the drift-diffusion model which allows to fit the observed I-V curves.

physics.app-ph↗
arXiv2017

Self-Oscillating Wireless Power Transfer Systems

Conventional wireless power transfer systems consist of a microwave power generator and transmitter located at one place and a microwave power receiver located at a distance. Here we show that wireless power transfer can be realized as a single distributed microwave generator with an over-the-air feedback, so that the microwave power is generated directly at the place where the energy needs to be delivered. We demonstrate that the use of this paradigm increases efficiency and dramatically reduces sensitivity to misalignments, variations in load and power, and possible presence of obstacles between the source and receiver.

physics.app-ph↗
arXiv2015

Overcoming black body radiation limit in free space: metamaterial superemitter

Here, we demonstrate that the power spectral density of thermal radiation at a specific wavelength produced by a body of finite dimensions set up in free space under a fixed temperature could be made theoretically arbitrary high, if one could realize double negative metamaterials with arbitrary small loss and arbitrary high absolute values of permittivity and permeability (at a given frequency). This result refutes the widespread belief that Planck's law itself sets a hard upper limit on the spectral density of power emitted by a finite macroscopic body whose size is much greater that the wavelength. Here we propose a physical realization of a metamaterial emitter whose spectral emissivity can be greater than that of the ideal black body under the same conditions. Due to the reciprocity between the heat emission and absorption processes such cooled down superemitter also acts as an optimal sink for the thermal radiation --- the "thermal black hole" --- which outperforms Kirchhoff-Planck's black body which can absorb only the rays directly incident on its surface. The results may open a possibility to realize narrowband super-Planckian thermal radiators and absorbers for future thermo-photovoltaic systems and other devices.

physics.optics↗
arXiv2015

Electromagnetic Energy Sink

The ideal black body fully absorbs all incident rays, that is, all propagating waves created by arbitrary sources. The known idealized realization of a black body is the perfectly matched layer (PML), widely used in numerical electromagnetics. However, ideal black bodies and PMLs do not interact with evanescent fields existing near any finite-size source, and the energy stored in these fields cannot be harvested. Here we introduce the concept of the ideal conjugate matched layer (CML), which fully absorbs energy of both propagating and evanescent fields of sources acting as an ideal sink for electromagnetic energy. Conjugate matched absorbers have exciting application potentials, as resonant attractors of electromagnetic energy into the absorber volume. We derive the conditions on the constitutive parameters of media which can serve as CML materials, numerically study the performance of planar and cylindrical CML and discuss possible realizations of such materials as metal-dielectric composites.

physics.optics↗
arXiv2015

Antenna model of the Purcell effect

The Purcell effect - the modification of the spontaneous emission rate in presence of resonant cavities or other resonant objects - is a fundamental effect of quantum electrodynamics. However, a change of the emission rate caused by environment different from free space has a classical counterpart. Not only quantum emitters, but any small antenna tuned to the resonance is an oscillator with radiative losses, and the influence of the environment on its radiation can be understood and measured in terms of the antenna radiation resistance. We present a general approach which is applicable to measurements of the Purcell factor for radio antennas and to calculations of these factors for quantum emitters. Our methodology is suitable for calculation and measurement of both electric and magnetic Purcell factors, it is versatile and applies to various frequency ranges. The approach is illustrated by a general equivalent scheme and allows the Purcell factor to be expressed through the continious radiation of a small antenna in presence of the environment.

physics.optics↗
arXiv2014

Nanoantennas for enhanced light trapping in transparent organic solar cells

We propose a light-trapping structure offering a significant enhancement of photovoltaic absorption in transparent organic solar cells operating at infrared while the visible light transmission keeps sufficiently high. The main mechanism of light trapping is related with the excitation of collective oscillations of the metal nanoantenna arrays, characterized by advantageous field distribution in the volume of the solar cell. It allows more than triple increase of infrared photovoltaic absorption.

physics.optics↗
arXiv2014

Equivalent Circuit Analysis of Super-Planckian Emission into Far Field

Using our recently developed equivalent circuit model of radiative heat transfer we analyze the far-field thermal emission from bodies of constrained dimensions. We prove that the power radiated by a hypothetical metamaterial emitter which is conjugate matched with all the harmonics of the emitted field at a given wavelength can be infinitely higher than the power emitted by an ideal black body of the same radius. However, for optically large bodies, fulfilling the conjugate match conditions for higher-order harmonics is not a trivial task, which effectively limits the far-field radiated power by the well-known Planck law.

physics.optics↗
arXiv2013

Enhanced Efficiency of Light-Trapping Nanoantenna Arrays for Thin Film Solar Cells

We suggest a novel concept of efficient light-trapping structures for thin-film solar cells based on arrays of planar nanoantennas operating far from plasmonic resonances. The operation principle of our structures relies on the excitation of chessboard-like collective modes of the nanoantenna arrays with the field localized between the neighboring metal elements. We demonstrated theoretically substantial enhancement of solar-cell short-circuit current by the designed light-trapping structure in the whole spectrum range of the solar-cell operation compared to conventional structures employing anti-reflecting coating. Our approach provides a general background for a design of different types of efficient broadband light-trapping structures for thin-film solar-cell technologically compatible with large-area thin-film fabrication techniques.

cond-mat.mes-hall↗
arXiv2012

Equivalent circuit model of radiative heat transfer

Here, we develop a theory of radiative heat transfer based on an equivalent electrical network representation for the hot material slabs in an arbitrary multilayered environment with arbitrary distribution of temperatures and electromagnetic properties among the layers. Our approach is fully equivalent to the known theories operating with the fluctuating current density, while being significantly simpler in analysis and applications. A practical example of the near-infrared heat transfer through the micron gap filled with an indefinite metamaterial is considered using the suggested method. The giant enhancement of the transferred heat compared to the case of the empty gap is shown.

physics.optics↗
arXiv2012

Ultracompact all-dielectric superdirective antennas

We demonstrate a simple way to achieve superdirectivity of electrically small antennas based on a spherical dielectric particle with a notch. We predict this effect theoretically for nanoantennas excited by a point-like emitter located in the notch, and then confirm it experimentally at microwaves for a ceramic sphere excited by a small wire dipole. We explain the effect of superdirectivity by the resonant excitation of high-order multipole modes of electric and magnetic fields which are usually negligible for small perfect spherical particles.

physics.optics↗
arXiv2012

Equivalent circuit theory of radiative heat transfer on micro- and nanoscale

Here, we outline a theory of radiative heat transfer based on an equivalent electrical network representation for the hot material slabs in an arbitrary multilayered environment with arbitrary distribution of temperatures and electromagnetic properties among the layers. Our approach is fully equivalent to the known theories operating with the fluctuating current density, while being significantly simpler in analysis and applications.

physics.optics↗
arXiv2009

Experimental Studies of Antenna Miniaturization Using Magneto-Dielectric and Dielectric Materials

Measurement results for a meandered planar inverted-F antenna (PIFA) loaded with magneto-dielectric and dielectric materials are presented. Figures of merit and ways to compare antennas with different fillings materials are discussed. The used magneto-dielectric material is described, the radiation mechanism of the meandered PIFA is studied, and the proper position for dielectric and magneto-dielectric filling is discussed and identified. Identical-size antennas with dielectric and magneto-dielectric fillings are compared at the same resonance frequency using the radiation quality factor as the figure of merit. It is seen, that the benefit from the magneto-dielectric filling material is moderate and strongly dependent on the positioning of the filling.

physics.class-ph↗
arXiv2009

Grounded Uniaxial Material Slabs as Magnetic Conductors

The objective of this paper is all-angle artificial magnetic conductor, i.e. artificial magnetic conductor that has stable magnetic-wall effect with respect to the incidence angle. Furthermore, we seek for a design that would be easy for manufacturing. In order to achieve this we use grounded uniaxial material slabs and we do not constrict ourselves to naturally available materials. Instead, we assume that the desired parameters can be synthesized using the emerging artificial electromagnetic materials. It is found that it is possible to have an all-angle magnetic-wall effect for both TE and TM polarization. Especially for the TM fields the structure would be easily manufacturable. The proposed structure has similar appearance as more well-known artificial impedance surfaces, but the design parameters and the physical properties behind the magnetic wall effect are novel. The performance of the proposed artificial magnetic conductor is verified with numerical simulations. This paper introduces a new approach how to obtain a magnetic-wall effect. It is possible to use this this approach also together with other ways of obtaining the magnetic-wall effect for dual-band operation.

physics.class-ph↗
arXiv2009

Choosing Dielectric or Magnetic Material to Optimize the Bandwidth of Miniaturized Resonant Antennas

We address the question of the optimal choice of loading material for antenna miniaturization. A new approach to identify the optimal loading material, dielectric or magnetic, is presented for resonant antennas. Instead of equivalent resonance circuits or transmission-line models, we use the analysis of radiation to identify the fields contributing mostly to the stored energy. This helps to determine the beneficial material type. The formulated principle is qualitatively illustrated using three antenna types. Guidelines for different antenna types are presented.

physics.class-ph↗
arXiv2009

On cavity-and-surface enhanced Raman scattering from metamaterial shells

In this paper we theoretically show that the Raman scattering by a core-shell micron or submicron particle with epsilon-near-zero metamaterial shell and silica spherical or cylindrical core can combine useful features of cavity-enhanced and surface-enhanced Raman scattering. The cavity resonance together with the plasmon resonance lead to the giant enhancement of the field inside the metashell which is performed as a layer of silver or gold nanoparticles and is penetrable for molecules to be detected. This approach results in the significant increase of both effective volume in which molecules are affected by enhanced electric field and Raman gain averaged over this volume.

physics.optics↗
arXiv2008

Effects of Spatial Dispersion on Reflection from Mushroom-type Artificial Impedance Surfaces

Several recent works have emphasized the role of spatial dispersion in wire media, and demonstrated that arrays of parallel metallic wires may behave very differently from a uniaxial local material with negative permittivity. Here, we investigate using local and non-local homogenization methods the effect of spatial dispersion on reflection from the mushroom structure introduced by Sievenpiper. The objective of the paper is to clarify the role of spatial dispersion in the mushroom structure and demonstrate that under some conditions it is suppressed. The metamaterial substrate, or metasurface, is modeled as a wire medium covered with an impedance surface. Surprisingly, it is found that in such configuration the effects of spatial dispersion may be nearly suppressed when the slab is electrically thin, and that the wire medium can be modeled very accurately using a local model. This result paves the way for the design of artificial surfaces that exploit the plasmonic-type response of the wire medium slab.

physics.class-ph↗
arXiv2008

An Efficient and Simple Analytical Model for Analysis of Propagation Properties in Impedance Waveguides

In this paper propagation properties of a parallel-plate waveguide with tunable artificial impedance surfaces as sidewalls are studied both analytically and numerically. The impedance surfaces comprise an array of patches over a dielectric slab with embedded metallic vias. The tunability of surfaces is achieved with varactors. Simple design equations for tunable artificial impedance surfaces as well as dispersion equations for the TE and TM modes are presented. The propagation properties are studied in three different regimes: a multi-mode waveguide, a single-mode waveguide, and below-cutoff waveguide. The analytical results are verified with numerical simulations.

physics.class-ph↗