Incident Electromagnetic Field Research Articles

On Electromagnetic Scattering from a Penetrable Corner

This article is concerned with the time-harmonic electromagnetic (EM) scattering from a generic inhomogeneous medium. It is shown that if there is a right corner on the support of the medium, then it scatters every pair of incident EM fields, excluding a possible class of EM fields which are of very particular forms. That is, for every pair of admissible incident EM fields, the corresponding scattered wave fields associated to the medium scatterer cannot be identically vanishing outside the support of the medium. Indeed, we achieve the corner scattering result by establishing a stronger result, that shows the failure of the analytic extension across the corner of certain EM fields satisfying the so-called interior transmission eigenvalue problem. This extends the relevant study in [3] for the acoustic scattering governed by the Helmholtz equation to the electromagnetic case governed by the Maxwell system. Substantial new challenges arise from the corresponding extension from the scalar PDE to the system of PDEs. Our mathematical arguments combine the analysis for interior transmission eigenvalue problems associated to the Maxwell system; the derivation of novel orthogonality relation for the solutions of Maxwell systems; the construction of complex-geometrical-optics (CGO) solutions for the Maxwell system with new Lp-estimates (p > 6) on the remainder terms; and the proof of the non-vanishing property for the Laplace transform of vectorial homogeneous harmonic polynomials.

Femtosecond time-domain observation of atmospheric absorption in the near-infrared spectrum

As light propagates through a medium, absorption caused by electronic or rovibrational transitions is evident in the transmitted spectrum. The incident electromagnetic field polarizes the medium and the absorption is due to the imaginary part of the linear susceptibility. In the time domain, the field establishes a coherence in the medium that radiates out of phase with the initial field. This coherence can persist for tens of picoseconds in atmospheric molecules such as ${\mathrm{H}}_{2}\mathrm{O}$. We propagate a few-cycle laser pulse centered at $1.8\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ through the atmosphere and measure the long-lasting molecular coherence in the time domain by high-order harmonic cross correlation. The measured optical free-induction decay of the pulse is compared with a calculation based on the calculated rovibrational spectrum of ${\mathrm{H}}_{2}\mathrm{O}$ absorption.

Integral equation method in problems of electromagnetic-wave reflection from inhomogeneous interfaces between media

Problems on reflection of a plane electromagnetic wave from various irregular interfaces between media are studied by the integral equation method in the cases of two- and three-dimensional incident electromagnetic field. The reflecting surfaces are meant as periodic transparent interfaces between two media and plane boundaries with locally inhomogeneous and transparent sections. The boundary value problems for the system of Maxwell’s equations in an infinite domain with an irregular boundary are reduced to Fredholm or singular integral equations, depending on the problem considered. Numerical algorithms for solving such integral equations are developed. Results of calculation of currents induced on inhomogeneities and characteristics of the electric field in the far zone are presented.Problems on reflection of a plane electromagnetic wave from various irregular interfaces between media are studied by the integral equation method in the cases of two- and three-dimensional incident electromagnetic field. The reflecting surfaces are meant as periodic transparent interfaces between two media and plane boundaries with locally inhomogeneous and transparent sections. The boundary value problems for the system of Maxwell’s equations in an infinite domain with an irregular boundary are reduced to Fredholm or singular integral equations, depending on the problem considered. Numerical algorithms for solving such integral equations are developed. Results of calculation of currents induced on inhomogeneities and characteristics of the electric field in the far zone are presented.

A facile fabrication of Ag-Au-Ag nanostructures with nanogaps for intensified surface-enhanced Raman scattering

Nanogap between two metallic nanostructures has been demonstrated to be able to efficiently concentrate an incident electromagnetic field into a small space. As a result, the formed strong field localization could extraordinarily enhance the surface-enhanced Raman scattering (SERS). In this study, controllable plasmonic nanogaps are formed by separating two layers of plasmonic Ag nanoparticles (50–100nm) with small Au nanoparticles (2.5–6nm). The size of the nanogaps can be readily tuned by altering the size of the Au nanoparticles. Utilizing an SERS substrate with such nanogaps, the SERS performance can be significantly improved. Such improvement could be attributed to the strongly enhanced electric field within the nanogaps, which is demonstrated by the Finite-difference time-domain simulations. The present work provides a facile strategy to rationally fabricate SERS substrates with controllable nanogaps and intensified SERS signals.

On‐body calibration and measurements using personal radiofrequency exposimeters in indoor diffuse and specular environments

For the first time, response of personal exposimeters (PEMs) is studied under diffuse field exposure in indoor environments. To this aim, both numerical simulations, using finite-difference time-domain method, and calibration measurements were performed in the range of 880-5875 MHz covering 10 frequency bands in Belgium. Two PEMs were mounted on the body of a human male subject and calibrated on-body in an anechoic chamber (non-diffuse) and a reverberation chamber (RC) (diffuse fields). This was motivated by the fact that electromagnetic waves in indoor environments have both specular and diffuse components. Both calibrations show that PEMs underestimate actual incident electromagnetic fields. This can be compensated by using an on-body response. Moreover, it is shown that these responses are different in anechoic chamber and RC. Therefore, it is advised to use an on-body calibration in an RC in future indoor PEM measurements where diffuse fields are present. Using the response averaged over two PEMs reduced measurement uncertainty compared to single PEMs. Following the calibration, measurements in a realistic indoor environment were done for wireless fidelity (WiFi-5G) band. Measured power density values are maximally 8.9 mW/m(2) and 165.8 μW/m(2) on average. These satisfy reference levels issued by the International Commission on Non-Ionizing Radiation Protection in 1998. Power density values obtained by applying on-body calibration in RC are higher than values obtained from no body calibration (only PEMs) and on-body calibration in anechoic room, by factors of 7.55 and 2.21, respectively. Bioelectromagnetics. 37:298-309, 2016. © 2016 Wiley Periodicals, Inc.

Extraordinary Light-Induced Local Angular Momentum near Metallic Nanoparticles.

The intense local field induced near metallic nanostructures provides strong enhancements for surface-enhanced spectroscopies, a major focus of plasmonics research over the past decade. Here we consider that plasmonic nanoparticles can also induce remarkably large electromagnetic field gradients near their surfaces. Sizeable field gradients can excite dipole-forbidden transitions in nearby atoms or molecules and provide unique spectroscopic fingerprinting for chemical and bimolecular sensing. Specifically, we investigate how the local field gradients near metallic nanostructures depend on geometry, polarization, and wavelength. We introduce the concept of the local angular momentum (LAM) vector as a useful figure of merit for the design of nanostructures that provide large field gradients. This quantity, based on integrated fields rather than field gradients, is particularly well-suited for optimization using numerical grid-based full wave electromagnetic simulations. The LAM vector has a more compact structure than the gradient matrix and can be straightforwardly associated with the angular momentum of the electromagnetic field incident on the plasmonic structures.

Two-Photon Excitation of a Plasmonic Nanoswitch Monitored by Single-Molecule Fluorescence Microscopy.

Visible-light excitation of the surface plasmon band of silver nanoplates can effectively localize and concentrate the incident electromagnetic field enhancing the photochemical performance of organic molecules. Herein, the first single-molecule study of the plasmon-assisted isomerization of a photochrome-fluorophore dyad, designed to switch between a nonfluorescent and a fluorescent state in response to the photochromic transformation, is reported. The photochemistry of the switchable assembly, consisting of a photochromic benzooxazine chemically conjugated to a coumarin moiety, is examined in real time with total internal reflection fluorescence microscopy in the presence of silver nanoplates excited with a 633 nm laser. The metallic nanostructures significantly enhance the visible light-induced performance of the photoconversion, which normally requires ultraviolet excitation. The resulting ring-open isomer is strongly fluorescent and can also be excited at 633 nm. These stochastic emission events are used to monitor photochromic activation and show quadratic dependence on incident power. The utilization of a single laser wavelength for both photochromic activation and excitation effectively mimics a pseudo two-colours system.

3D‐FDTD‐based hybridisation technique for wave interaction with scatterers in layered media

A simple method for the computation of 3D scatterers remotely located from the source with layered media is presented. This domain decomposition method is based on the total-field/scattered-field formulation of the 3D finite-difference in time-domain method. The electromagnetic field along the propagation path is not directly involved; it focuses only on the electromagnetic field in the surroundings of the scatterer. The incident electromagnetic field radiated from the source is computed separately with an adaptive algorithm and then applied on the Huygens’ surface, leading to the proposed hybridisation technique.

Design and Analysis of Disordered Optical Nanoantenna Structures

Nanoantennas are optical wave radiating structures operating based on the same physical principles as those underlying their radio-frequency counterparts. However, the physical properties of materials in the optical range of frequency cause different response to the incident electromagnetic field leading to novel applications in optics. The conventional methods for the analysis and design of antennas need to be modified for these structures. In this paper, an integral equation formulation using the complex images Green's function technique is employed to model the optical wave interactions with nanoantennas embedded in multilayer media. The developed method is combined with the simulated annealing algorithm to design and optimize structures composed of strongly coupled nanoscale scatterers. It is shown that not only can they lead to improved efficiency in optical devices, but also orthogonal transmission channels are attainable when they are excited properly. The latter provides a new scheme for encoding and multiplexing signals in optical information systems.

A frequency reconfigurable cylindrically shaped surface with cloaking-like properties

A cylindrically shaped surface with electromagnetic (EM) cloaking-like properties is presented in this communication. The surface comprises of a number of 2-port frequency reconfigurable microstrip (2-PFRM) elements printed on individual substrates, and to enclose a particular region, several 2-PFRM elements are interconnected in a cylindrical shape. Each 2-PFRM element has the capability of coupling an incident EM field on the surface to the adjacent interconnected elements to be re-radiated in a direction away from the transmitter. Moreover, to demonstrate this concept, a frequency reconfigurable surface operating at 3.68 GHz and 3.89 GHz is simulated in HFSS, manufactured and measured in a full anechoic chamber. The prototype in this paper illustrates that this surface has the additional benefit of overcoming many of the manufacturing difficulties of traditional cloaks because microstrip structures are used. Simulation and measurement results are in good agreement and validate the proposed EM cloak-like surface for applications that may not require precise phase-control such as antenna array shielding to reduce mutual coupling, radar cross section reduction and low-bandwidth communications in complex EM environments. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1323–1329, 2016

Bilayer metamaterial design for switchable electromagnetically-induced transparency-like response

We demonstrated experimentally that a controllable electromagnetically-induced transparency (EIT)-like spectral response at microwave frequencies can be obtained in metamaterials. The present design consists of two snake-shape resonators (SRs) with one and two bars. At the same time, each resonator is constructed at the top and the bottom layers. In our scheme, phase coupling between two SRs (assigned as the bright mode), which is activated by incident electromagnetic field, can lead to the EIT-like effect. Furthermore, we confirmed that the EIT-like feature could be controlled by simply varying the bar length of two different SRs. Our work proposes the alternative way to acquire the tunable EIT-like effect and these results reflect the possibility for achieving in slow-light devices and novel sensors.

Supramolecular Organic Nanowires as Plasmonic Interconnects

Metallic nanostructures are able to interact with an incident electromagnetic field at subwavelength scales by plasmon resonance which involves the collective oscillation of conduction electrons localized at their surfaces. Among several possible applications of this phenomenon, the theoretical prediction is that optical circuits connecting multiple plasmonic elements will surpass classical electronic circuits at nanoscale because of their much faster light-based information processing. However, the placement and coupling of metallic elements smaller than optical wavelengths currently remain a formidable challenge by top-down manipulations. Here, we show that organic supramolecular triarylamine nanowires of ≈1 nm in diameter are able to act as plasmonic waveguides. Their self-assembly into plasmonic interconnects between arrays of gold nanoparticles leads to the bottom-up construction of basic optical nanocircuits. When the resonance modes of these metallic nanoparticles are coupled through the organic nanowires, the optical conductivity of the plasmonic layer dramatically increases from 259 to 4271 Ω(-1)·cm(-1). We explain this effect by the coupling of a hot electron/hole pair in the nanoparticle antenna with the half-filled polaronic band of the organic nanowire. We also demonstrate that the whole hybrid system can be described by using the abstraction of the lumped circuit theory, with a far field optical response which depends on the number of interconnects. Overall, our supramolecular bottom-up approach opens the possibility to implement processable, soft, and low cost organic plasmonic interconnects into a large number of applications going from sensing to metamaterials and information technologies.

Excitation of graphene plasmons as an analogy with the two-level system

The excitation of graphene plasmons (GPs) is presented as an interaction between the GPs and the incident electromagnetic field. In this Letter, the excitation of GPs in a plasmonic system is interpreted as an analogy with the two-level system by taking the two-coupled graphene-covered gratings as an example. Based on the equivalent circuit theory, the excitation of GPs in the graphene-covered grating is equivalent to the resonance of an oscillator. Thus, according to the governing equation, the electric currents at the resonant frequencies for two-coupled graphene-covered gratings correspond to the energy states in a two-level system. In addition, the excitation of GPs in different two-coupled graphene-covered gratings is numerically studied to validate our theoretical model. Our work provides an intuitive understanding of the excitation of GPs using an analogy with the two-level system.

Tip-Enhanced Upconversion Luminescence in Yb3+–Er3+ Codoped NaYF4 Nanocrystals

Lanthanide doped upconversion nanoparticles reveal enormous potential for biomedical applications. However, they are limited by low upconversion efficiency. In this paper, we demonstrate tip-enhanced upconversion luminescence (UCL) from a single Yb3+–Er3+-codoped NaYF4 nanoparticle with a maximum enhancement factor of 11 by the plasmonic effect, which is a solution for improving the conversion efficiency and will become a potential technique for the applications in sensitive imaging and detection via speeding up both the absorption and the emission processes of lanthanide doped upconversion nanoparticles. By investigating the optical properties of the tip-enhanced UCL at 550 and 660 nm separately in Yb3+–Er3+-codoped NaYF4 nanoparticles, we observe that the gold-coated tip influences the upconversion process in Yb3+–Er3+-codoped NaYF4 particles by improving the reception and the transmission of incident electromagnetic fields.

Beam width estimation of microwave antennas using lock-in infrared thermography

The beam width of microwave antennas can be easily determined using infrared thermography. A thin absorption screen made up of some carbon loaded polymer is placed in front of a microwave antenna. The electromagnetic waves impinging the screen are partially absorbed by it, resulting in temperature rise of this screen. This temperature rise is monitored by an infrared camera. These infra red images of the temperature distribution are identical to the power distribution of the incident electromagnetic field at the screen location. The distance between half power points on the main lobe of the radiation pattern at the screen location in two orthogonal directions (E- and H-planes) can be easily found. From these distances the half power beam widths in both E- and H-planes can be calculated. Further from these beam widths, directivity is also calculated.The experiments were carried with a patch antenna radiating at 2.45GHz. In order to show the repeatability of the results, experiments were carried for different distances between patch antenna and absorption screen. A great agreement is seen between simulation, measurement and experimental (thermographic) results.The microwave source is modulated at a low frequency so as to avoid the lateral spread of heat on the absorption screen and to permit lock-in detection of temperature distribution.

Near-field radiofrequency electromagnetic exposure assessment

Personal wireless telecommunication devices, such as radiofrequency (RF) electromagnetic field (EMF) sources operated in vicinity of human body, have possible adverse health effects. Therefore, the correct EMF assessment is necessary in their near field. According to international near-field measurement criteria, the specific absorption rate (SAR) is used for absorbed energy distribution assessment in tissue simulating liquid phantoms. The aim of this investigation is to validate the relationship between the H-field of incident EMF and absorbed energy in phantoms. Three typical wireless telecommunication system frequencies are considered (900, 1800 and 2450 MHz). The EMF source at each frequency is an appropriate half-wave dipole antenna and the absorbing medium is a flat phantom filled with the suitable tissue simulating liquid. Two methods for SAR estimation have been used: standard procedure based on E-field measured in tissue simulating medium and a proposed evaluation by measuring the incident H-field. Compared SAR estimations were performed for various distances between sources and phantom. Also, these research data were compared with simulation results, obtained by using finite-difference time-domain method. The acquired data help to determine the source near-field space characterized by the smallest deviation between SAR estimation methods. So, this region near the RF source is suitable for correct RF energy absorption assessment using the magnetic component of the RF fields.

Impressive Transmission Mode Electromagnetic Interference Shielding Parameters of Graphene-like Nanocarbon/Polyurethane Nanocomposites for Short Range Tracking Countermeasures.

Shielding the microwave signature (8.2-12.4 GHz, X-band) of a locked in target is a tactically important electronic countermeasure. Herein, we report on mitigation in X-band transmission mode shielding parameters for polyurethane (PU), after incorporating graphene-like nanocarbon sheets (GNCs). Initially, PU and variable weight percent (1-25) GNCs/PU paste samples were subjected to Fourier transform infrared, Raman spectroscopy, and scanning electron microscopy. These samples were molded into toroidal shaped specimens by adiabatic hot-pressing technique, for microwave scattering measurements. Parameters, such as complex permittivity (ϵ'-jϵ″), alternating current (ac) conductivity, skin thickness, transmission (S21), effective transmission loss (SET), and shielding effectiveness were determined, in addition to direct current (dc) conductivity. The real and imaginary components are increased, respectively, by ∼5 and ∼30 times, whereas loss tangent, by ∼7, at 25 wt % loading of GNCs. The ac conductivity is enhanced from 0.248 (PU) to 7.288 S/m (25 wt %) by maximizing transmission loss to -26.45 dB (99.9%) and minimizing thickness to 1-2 mm. The dc percolation threshold is found to be low at ∼5 wt %, indicating superior dispersibility of GNCs, thereafter. In analysis, the atomic polarization (at ∼10 GHz) associated with the aromatic urethane amide rings acts as a backbone to engage incident electromagnetic field wiggles. The coupling occurs via charge transfer polarization currents at doubly bonded nitrogen, oxygen, and hydrosorpted sp3 carbon sites in GNCs. The field-matter interaction is dominant at the microvoid skin interface between GNCs and PU. Possible polarization mechanism is explained. The GNCs/PU nanocomposites are realized as an effective electromagnetic interference shielding block in the tracking band. The details are presented.

Plasmonic enhancement of blue fluorescence in ZnO nanoparticles

Silver nanoparticles (Ag NP) can behave like optical nano antennas by confining and enhancing the incident electromagnetic field (EM) around them due to localized surface Plasmon resonance (LSPR). Such augmented near field of plasmonic nano antennas has been harnessed to enhance blue fluorescence emission from ZnO nanoparticles optimally conjugated with Ag NPs. Overlapping of LSPR band with excitation range of ZnO and near field calculated through Finite difference time domain simulations of exact Ag NP correlates with fluorescence enhancement results. Plasmon coupled visible fluorescence enhancement from semiconductor emitters opens up fresh possibilities in light harvesting.

Effect of spacer layer on enhancement and quenching of photoluminescence by surface plasmon

Metal nanoparticles have the ability to localize and strongly enhance the incident electromagnetic field due to the surface plasmon polariton. The coupling between AuNPs and ZnO thin films affects their emission properties. In this study, we prepared the ZnO / Al 2 O 3/ AuNPs multilayer films, and investigated the effect of spacer layer on enhancement and quenching of photoluminescence by surface plasmon. The emission intensities of the thin films vary when Al 2 O 3 space layer with different thickness was embedded in the ZnO / Au films, consistent with theoretical predictions. The photoluminescence modulation is also made via controlling the polarization of the excitation source. The photoluminescence anisotropy of these systems shows that enhanced photoluminescence can be achieved through coupling of the emission from ZnO with the surface plasmon resonance of AuNPs .

Optical-phonon-mediated photocurrent in terahertz quantum-well photodetectors

Strong and sharp photocurrent peak at longitudinal optical (LO) phonon frequency (8.87 THz) is found in GaAs/(Al,Ga)As terahertz quantum-well photodetectors (QWPs). Two mesa-structure terahertz QWPs with and without one-dimensional metal grating are fabricated to investigate the behavior of such photoresponse peak. The experimental and simulation results indicate that the photocurrent peak originates from a two-step process. First, at the LO phonon frequency, a large number of non-equilibrium LO phonons are excited by the incident electromagnetic field, and the electromagnetic energy is localized and enhanced in the thin multi-quantum-well layer. Second, through the Frohlich interaction, the localized electrons are excited to continuum states by absorbing the non-equilibrium LO phonons, which leads to the strong photoresponse peak. This finding is useful for exploring strong light-matter interaction and realizing high sensitive terahertz photodetectors.