Personal exposure to radiofrequency electromagnetic fields: A comparative analysis of international, national, and regional guidelines

A worldwide overview and analysis for the existing limits of human exposure to Radiofrequency Electromagnetic Fields (RF-EMF) is given in this paper. These reference levels have been established by different national and even regional governments, which can be based on the guidelines provided by the recommendations of the International Commission on Non-Ionizing Radiation Protection (ICNIRP), the International Committee on Electromagnetic Safety of the Institute of Electrical and Electronics Engineers (IEEE), and even in the United States of the Federal Communications Commission (FCC), as well as, are based on the so-called precautionary principle. Explicit reference is made to the exposure limits adopted in countries or regions, such as Canada, Italy, Poland, Switzerland, China, Russia, France, and regions of Belgium (Brussels, Flanders, Wallonia), where the limits are much lower than the international standards. The limits are compared to a selected set of in-situ measurements. This clearly shows that the measured values are typically very small compared to the international standards but could be somewhat higher compared to the reduced limits. Based on this observation and the reasonable assumption that the sensitivity of people to Electromagnetic Fields (EMF) is the same everywhere (whole-body), we propose the idea to establish a worldwide reference limit for the general public, thus applicable in all countries, if the ICNIRP considers it appropriate. Research must continue to generate measurement data that demonstrate the levels of exposure to which we are really exposed, and with this, provide arguments to the organizations that established the guidelines, especially the ICNIRP, to evaluate whether the current limits are too much. High and can be modified when considered pertinent. To the best of our knowledge, at no time has the reference level for the general public been exceeded.

Enhancing Circular Polarization Performance of Low-Profile Patch Antennas for Wearables Using Characteristic Mode Analysis

A wearable antenna functioning in the 2.4 GHz band for health monitoring and sensing is proposed. It is a circularly polarized (CP) patch antenna made from textiles. Despite its low profile (3.34 mm thickness, 0.027 λ₀), an enhanced 3-dB axial ratio (AR) bandwidth is achieved by introducing slit-loaded parasitic elements on top of analysis and observations within the framework of Characteristic Mode Analysis (CMA). In detail, the parasitic elements introduce higher-order modes at high frequencies that may contribute to the 3-dB AR bandwidth enhancement. More importantly, additional slit loading is investigated to preserve the higher-order modes while relaxing strong capacitive coupling invoked by the low-profile structure and the parasitic elements. As a result, unlike conventional multilayer designs, a simple single-substrate, low-profile, and low-cost structure is achieved. While compared to traditional low-profile antennas, a significantly widened CP bandwidth is realized. These merits are important for the future massive application. The realized CP bandwidth is 2.2-2.54 GHz (14.3%), which is 3-5 times that of traditional low-profile designs (thickness < 4 mm, 0.04 λ₀). A prototype was fabricated and measured with good results.

Measurement studies of personal exposure to radiofrequency electromagnetic fields: A systematic review

The last 25 years have seen an increase in the number of radiofrequency sources with the global adoption of smartphones as primary connectivity devices. The objective of this work was to review and evaluate the measured studies of personal exposure to Radiofrequency Electromagnetic Fields (RF-RMF) and meet the basic quality criteria eligible for inclusion in this Review, according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, following the eligibility criteria of the PECO (Population, Exposure, Comparator, and Outcome) methodology, and the instrument for critical reading Critical Appraisal Skills Programme Español (CASPe). We systematically reviewed the works published between January 1, 1998, and December 31, 2021, yielding 56 publications. Of the different types of studies in which personal exposure to RF-EMF has been measured with two measurement methodologies can be highlighted: Personal measurements with volunteers and Personal measurements with a trained researcher (touring a specific area, one or several microenvironments, an entire city, walking or in some means of transport). Personal exposimeters were used in 83% of the studies. The lowest mean was measured in Egypt with a value of 0.00100 μW/m² (1.00 nW/m²) in 2007 and the highest mean was measured in Belgium with a value of 285000 μW/m² (0.285 W/m²) in 2019. The results of our study confirm that RF-EMF exposure levels are well below the maximum levels established by the ICNIRP guidelines.

Nonlocal response of plasmonic core-shell nanotopologies excited by dipole emitters

In light of the emergence of nonclassical effects, a paradigm shift in the conventional macroscopic treatment is required to accurately describe the interaction between light and plasmonic structures with deep-nanometer features. Towards this end, several nonlocal response models, supplemented by additional boundary conditions, have been introduced, investigating the collective motion of the free electron gas in metals. The study of the dipole-excited core-shell nanoparticle has been performed, by employing the following models: the hard-wall hydrodynamic model; the quantum hydrodynamic model; and the generalized nonlocal optical response. The analysis is conducted by investigating the near and far field characteristics of the emitter-nanoparticle system, while considering the emitter outside and inside the studied topology. It is shown that the above models predict striking spectral features, strongly deviating from the results obtained via the classical approach, for both simple and noble constitutive metals.

Transmissive terahertz metasurfaces with vanadium dioxide split-rings and grids for switchable asymmetric polarization manipulation

Metasurfaces containing arrays of thermally tunable metal-free (double-)split-ring meta-atoms and metal-free grids made of vanadium dioxide (VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2), a phase-change material can deliver switching between (1) polarization manipulation in transmission mode as well as related asymmetric transmission and (2) other functionalities in the terahertz regime, especially when operation in the transmission mode is needed to be conserved for both phases of VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2. As the meta-atom arrays function as arrays of metallic subwavelength resonators for the metallic phase of VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2, but as transmissive phase screens for the insulator phase of VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2, numerical simulations of double- and triple-array metasurfaces strongly indicate extreme scenarios of functionality switching also when the resulting structure comprises only VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 meta-atoms and VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 grids. More switching scenarios are achievable when only one meta-atom array or one grid is made of VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 components. They are enabled by the efficient coupling of the geometrically identical resonator arrays/grids that are made of the materials that strongly differ in terms of conductivity, i.e. Cu and VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 in the metallic phase.

Detecting mid-infrared light by molecular frequency upconversion in dual-wavelength nanoantennas

[Figure: see text].

Medium-Sized Highly Coupled Planar Arrays with Maximum Aperture Efficiency

This paper presents a technique to design strongly coupled planar arrays with very high aperture efficiency. The key innovation is that, based on an irregular 2 × 1 array, very compact medium-sized arrays of size 2 × 2, 2 × 4, and 2 × 6 are constructed with very strong and constructive mutual coupling between the elements. In this way, a maximum aperture efficiency is reached for a given footprint of the array. The occupied space of the antenna in comparison with conventional linear patch arrays is studied. A prototype 2 × 4 array operating around 5.8 GHz is designed, fabricated, built, and measured. The results show a large bandwidth of 20% and a very high aperture efficiency of 100%, which is the largest found in the literature for similarly sized arrays. These results are important in view of the future Internet of Things, where small and medium-sized arrays are planned to be mounted on numerous devices where a very limited physical area is available.

A Wearable Button Antenna Sensor for Dual-Mode Wireless Information and Power Transfer

A novel wearable button antenna sensor is proposed for the concept of simultaneous wireless information and power transfer (SWIPT). This integrates two working modes for the transfer of power and information, respectively, and optimizes transfer efficiency. An omni-directional radiation pattern is achieved in the 3.5 GHz World Interoperability for Microwave Access (WiMAX) band to support on-body wireless communications, while a circularly polarized broadside radiation pattern is obtained in the 5 GHz wireless local area networks (WLAN) band to harvest power. The measured −10 dB return loss bandwidths are 4.0% (3.47–3.61 GHz) in the lower band, and 25.0% (4.51–5.80 GHz) in the higher band, respectively. An artificial magnetic conductor (AMC) structure with wideband characteristics is applied to obtain a low-profile design and to increase the stability of the antenna sensor. A high radiation efficiency of over 80% in the whole working band is observed. The specific absorption rate (SAR) of the proposed antenna sensor is below 0.509 W/kg at 3.55 GHz, and below 0.0532 W/kg at 5.5 GHz, respectively, which is much lower than the European standard threshold of 2 W/kg. All these characteristics make the designed antenna sensor suitable for on-body information transmission and off-body energy harvesting. The antenna sensor has been prototyped. Simulations and measurements agree well, proving the validity of the new concept.

Comparison of peak electromagnetic exposures from mobile phones operational in either data mode or voice mode

The aim of this study is to compare the typical peak ElectroMagnetic (EM) exposures from smart phones during data transmission and voice calls, respectively, in the typical Western-European city of Leuven, Belgium. Since transmission powers towards the outdoor network in an indoor environment are expected to be higher than in an outdoor environment, measurements were executed indoors. The influence of factors like network generation [2G, 3G, and 4G] and choice of mobile operator was also investigated. The most important conclusion of the study is that there is a huge difference between peak exposures generated by the 3 network generations currently active in Leuven. To the average, in many cases the peak exposure for 3G is more than a factor 20 lower than for 2G, and about a factor 5-10 lower than for 4G. These numbers are much higher than expected. There are also systematic differences between peak exposures for data mode and voice mode.

Bandwidth Optimization of a Textile PIFA with DGS Using Characteristic Mode Analysis

This work presents the design and optimization of an antenna with defected ground structure (DGS) using characteristic mode analysis (CMA) to enhance bandwidth. This DGS is integrated with a rectangular patch with circular meandered rings (RPCMR) in a wearable format fully using textiles for wireless body area network (WBAN) application. For this integration process, both CMA and the method of moments (MoM) were applied using the same electromagnetic simulation software. This work characterizes and estimates the final shape and dimensions of the DGS using the CMA method, aimed at enhancing antenna bandwidth. The optimization of the dimensions and shape of the DGS is simplified, as the influence of the substrates and excitation is first excluded. This optimizes the required time and resources in the design process, in contrast to the conventional optimization approaches made using full wave "trial and error" simulations on a complete antenna structure. To validate the performance of the antenna on the body, the specific absorption rate is studied. Simulated and measured results indicate that the proposed antenna meets the requirements of wideband on-body operation.

A Novel Design Approach for Compact Wearable Antennas Based on Metasurfaces

This paper presents a novel approach to design compact wearable antennas based on metasurfaces. The behavior of compact metasurfaces is modeled with a composite right-left handed transmission line (CRLH TL). By controlling the dispersion curve, the resonant modes of the compact metasurface can be tuned efficiently. A printed coplanar waveguide (CPW) monopole antenna is used as the feed structure to excite the compact metasurface, which will result in a low profile antenna with low backward radiation. Following this approach, two compact antennas are designed for wearable applications. The first antenna is designed to operate at its first negative mode (-1 mode), which can realize miniaturization, but maintain the broadside radiation as for a normal microstrip antenna. The proposed prototype resonates around 2.65 GHz, with a matching bandwidth of 300 MHz. The total dimensions of the antenna are 39.4 × 33.4 mm² (0.1 λ₀²), and its maximum gain is 2.99 dBi. The second antenna targets dual-band operation at 2.45 and 3.65 GHz. A pair of symmetric modes (±1 modes) are used to generate similar radiation patterns in these two bands. The size of the antenna is 55.79 × 52.25 mm² (0.2 λ₀²), and the maximum gains are 4.25 and 7.35 dBi in the two bands, respectively. Furthermore, the performance of the antennas is analyzed on the human body. The results show that the proposed antennas are promising candidates for Wireless Body Area Networks (WBAN).

Localized Nanoresonator Mode in Plasmonic Microcavities

Submicron-thick hexagonal boron nitride crystals embedded in noble metals form planar Fabry-Perot half-microcavities. Depositing Au nanoparticles on top of these microcavities forms previously unidentified angle- and polarization-sensitive nanoresonator modes that are tightly laterally confined by the nanoparticle. Comparing dark-field scattering with reflection spectroscopies shows plasmonic and Fabry-Perot-like enhancements magnify subtle interference contributions, which lead to unexpected redshifts in the dark-field spectra, explained by the presence of these new modes.

Dual-Band Dual-Polarized Wearable Button Array With Miniaturized Radiator

A dual-band dual-polarized wearable array is proposed, based on a miniaturized innovating button radiator topology. The diameter of the rigid button is only 19.5 mm (0.29 λ at 4.5 GHz), which optimizes the users' comfort, and makes it the smallest up to date in literature. The operational bands are 4.50-4.61 GHz and 5.04-5.50 GHz. The antenna thus covers the 4.5-4.6 unlicensed future 5th generation (5G) communication band for the internet of things (IoT), and the 5.1-5.5 GHz wireless local area network (WLAN) band, respectively. Two orthogonal linear polarizations are obtained in each band. A low mutual coupling between the button antenna elements (below -18 dB) and between the two ports within each element (below -20 dB) is achieved, guaranteeing a good diversity performance. The envelope correlation coefficient (ECC) and the specific absorption rate (SAR) performance are also analyzed. In order to demonstrate the robustness of the button antenna and to mimic realistic situations, a more complicated asymmetrical ground plane model of the button antenna is studied for the first time. A prototype of a two-element button array has been fabricated. The measurement results match well with the simulations. A 10-element button array is studied within the context of a 3-D channel model, taking into account the button element's radiation pattern. A high achievable spectral efficiency (SE) is obtained.

Light guiding, bending, and splitting via local modification of interfaces of a photonic waveguide

A general approach to the surface control of the localization, guiding, and redirecting of volume-mode light in photonic waveguides via tailoring their interfaces (surfaces) is proposed. The approach is demonstrated for dielectric rod-type photonic crystal slabs, whose regular and defect parts are distinguished by whether the nanocylinders are covered by metal caps. Thus, the rod-array part of the structure is not changed, while the local modifications are only applied to the interfaces. The basic functionalities, i.e., localized volume wave guiding, bending, and splitting are achievable. Selective dual-mode operation is possible due to the co-existence of a defect mode and a chainlike mode in one structure.

Study of the electromagnetic exposure from mobile phones in a city like environment: The case study of Leuven, Belgium

A measuring campaign for the assessment of electromagnetic exposure levels from mobile phones in the city center of Leuven, Belgium, has been carried out. The main objective of the assessment is to study the dependency of the exposure of the user by his own mobile phone in terms of location in the city (very close to base stations and at randomly selected locations). The measurements were performed in both public and private areas in 60 outdoor and 60 indoor locations in Leuven. The campaign was focused on GSM 900 mobile communications. The results show that the exposure is considerably higher for indoor environments compared to outdoor environments, and at the randomly chosen locations compared to locations very close to base stations. However, the most important observation is that the average outdoor exposure in Leuven of the user of a mobile phone is about 8 times higher than the average outdoor exposure by base stations. Indoors, this factor rises to about 30.

Study of the correlation between outdoor and indoor electromagnetic exposure near cellular base stations in Leuven, Belgium

A measuring campaign for the assessment of electromagnetic radiation near base stations in the city center of Leuven, Belgium, has been carried out. The main objective of this assessment is to study the correlation between the outdoor and the indoor exposure produced by cellular base stations and to investigate the changes of electromagnetic exposure within a typical day and over 1 month in the vicinity of these base stations. The study was also carried out as a function of location and time using highly precise measurement equipment. The measurements were performed in both public and private areas in sixty (30 indoor and 30 outdoor) different locations in Leuven. The measurement was focused on mobile communication networks: GSM (Global System for Mobile Communication, 900 MHz and 1800 MHz) and UMTS (Universal Mobile Telecommunications System, 2110 MHz) were the frequency bands of interest. The data at these frequencies were extracted from raw measurements in the 824-2170 MHz frequency band. The results show that all analyzed locations are in compliance with the exposure limits recommended by ICNIRP (International Commission on Non-Ionizing Radiation Protection) and that the (maximum) indoor exposure correlates to the outdoor exposure with a factor of about 0.5.

Design of Wideband Button Antenna Based on Characteristic Mode Theory

A wideband wearable button antenna working around 2.4 GHz is proposed in this paper. The function of the textile antenna ground is analyzed based on characteristic mode theory. By properly locating the button on the ground, the latter can be efficiently excited and operates as a radiator. This is shown to greatly increase the impedance bandwidth. The antenna is analyzed both in free space and on the human body. A prototype is fabricated, and the measured results agree satisfactorily with the simulations. In free space, the bandwidth, the realized gain, and radiation efficiency are 658 MHz, 1.8 dBi, and 97%, respectively. While on the human body, the values can reach 788 MHz, 5.1 dBi, and 71%, respectively. This wide band behavior provides robustness across different environments and to relatively large fabrication tolerances. The specific absorption rate is below 0.45 W/kg for an equivalent isotropically radiated power of 20 dBm.

Tailoring far-infrared surface plasmon polaritons of a single-layer graphene using plasmon-phonon hybridization in graphene-LiF heterostructures

Being one-atom thick and tunable simultaneously, graphene plays the revolutionizing role in many areas. The focus of this paper is to investigate the modal characteristics of surface waves in structures with graphene in the far-infrared (far-IR) region. We discuss the effects exerted by substrate permittivity on propagation and localization characteristics of surface-plasmon-polaritons (SPPs) in single-layer graphene and theoretically investigate characteristics of the hybridized surface-phonon-plasmon-polaritons (SPPPs) in graphene/LiF/glass heterostructures. First, it is shown how high permittivity of substrate may improve characteristics of graphene SPPs. Next, the possibility of optimization for surface-phonon-polaritons (SPhPs) in waveguides based on LiF, a polar dielectric with a wide polaritonic gap (Reststrahlen band) and a wide range of permittivity variation, is demonstrated. Combining graphene and LiF in one heterostructure allows to keep the advantages of both, yielding tunable hybridized SPPPs which can be either forwardly or backwardly propagating. Owing to high permittivity of LiF below the gap, an almost 3.2-fold enhancement in the figure of merit (FoM), ratio of normalized propagation length to localization length of the modes, can be obtained for SPPPs at 5-9 THz, as compared with SPPs of graphene on conventional glass substrate. The enhancement is efficiently tunable by varying the chemical potential of graphene. SPPPs with characteristics which strongly differ inside and around the polaritonic gap are found.

Dielectric Properties of Ex Vivo Porcine Liver Tissue Characterized at Frequencies Between 5 and 500 kHz When Heated at Different Rates

OBJECTIVE

The released energy during radio frequency thermal ablation therapy changes the dielectric properties of biological tissues. Understanding changes of dielectric properties of biological tissues during heating is fundamental to suitably model the medical procedure. The aim of this work is to obtain the thermal dependences of conductivity and permittivity of ex vivo porcine liver tissue at six frequencies from 5 to 500 kHz, during heating from 37 °C to 100 °C at three heating rates of approximately 0.1, 3, and 10 °C/min.

METHODS

Two experimental setups using different heating sources and a four-needle electrode connected to an impedance analyzer were developed to evaluate the thermal dependencies.

RESULTS

The results at a body temperature of 37 °C show a good agreement with the data reported in the literature. The conductivity initially shows an increase followed by a decrease, whereas the permittivity increases before a subsequent sharp decrease. Above 60 °C, different trends are observed for the three heating rates studied.

CONCLUSION

The electric conductivity and permittivity show a similar behavior at all evaluated frequencies and heating rates. The observed abrupt change of the slope near 45 °C at a slow heating rate may be used to identify the region of reversible changes in the tissue.

SIGNIFICANCE

These results confirm the connection among tissue dielectric properties, working frequency, and exposure time with thermal damage during heating.

Near-Field Mapping of Optical Fabry-Perot Modes in All-Dielectric Nanoantennas

Subwavelength optical resonators and scatterers are dramatically expanding the toolset of the optical sciences and photonics engineering. By offering the opportunity to control and shape light waves in nanoscale volumes, recent developments using high-refractive-index dielectric scatterers gave rise to efficient flat-optical components such as lenses, polarizers, phase plates, color routers, and nonlinear elements with a subwavelength thickness. In this work, we take a deeper look into the unique interaction of light with rod-shaped amorphous silicon scatterers by tapping into their resonant modes with a localized subwavelength light source-an aperture scanning near-field probe. Our experimental configuration essentially constitutes a dielectric antenna that is locally driven by the aperture probe. We show how leaky transverse electric and magnetic modes can selectively be excited and form specific near-field distribution depending on wavelength and antenna dimensions. The probe's transmittance is furthermore enhanced upon coupling to the Fabry-Perot cavity modes, revealing all-dielectric nanorods as efficient transmitter antennas for the radiation of subwavelength emitters, in addition to constituting an elementary building block for all-dielectric metasurfaces and flat optics.

Utilization of Stainless-steel Furnace Dust as an Admixture for Synthesis of Cement-based Electromagnetic Interference Shielding Composites

Electromagnetic interference (EMI) shielding receives attention due to the increasing abundance of electronics. The Cement based material can obtain EMI shielding properties through the use of appropriate “fillers” such as carbon, metal, and ferrite. As the most important by-product of stainless steelmaking operations, through the metal droplets and ferrite that it contains, stainless-steel dust can be considered as a potential filler for EMI shielding applications. We have therefore utilized stainless-steel dust as an admixture for the synthesis of cement-based EMI shielding composites and show that it raises the EMI shielding effectiveness. In particular, a 45 mass pct of stainless-steel dust mixture of 5 mm thickness results in the enhancement of EMI shielding effectiveness to 6–9 dB as tested in the frequency range of 500 MHz–1.5 GHz.

Benchmarking of software tools for the characterization of nanoparticles

Although many commercially available electromagnetic tools are conveniently used in RF and microwave applications, only a few of them provide the capability to analyze the optical response of nanometric radiators and scatterers. The assessment of their performance in the visible to near ultraviolet part of the electromagnetic (EM) spectrum becomes more and more important, considering the exponential rise of nanoscale systems. Since the accuracy of these numerical tools has not been fully investigated in literature, in this paper we essentially demonstrate a comparative study of the most widely used EM field solvers in the area of nano-plasmonics: COMSOL, CST and Lumerical. This is done through the investigation of the near and far field characteristics of basic canonical nanoparticles such as spheres, shells, cubes and cuboids, varying their sizes and constituting materials. The benchmarking results clearly show that at this moment not all EM field solvers offer the same accuracy.

Dendritic optical antennas: scattering properties and fluorescence enhancement

With the development of nanotechnologies, researchers have brought the concept of antenna to the optical regime for manipulation of nano-scaled light matter interactions. Most optical nanoantennas optimize optical function, but are not electrically connected. In order to realize functions that require electrical addressing, optical nanoantennas that are electrically continuous are desirable. In this article, we study the optical response of a type of electrically connected nanoantennas, which we propose to call “dendritic” antennas. While they are connected, they follow similar antenna hybridization trends to unconnected plasmon phased array antennas. The optical resonances supported by this type of nanoantennas are mapped both experimentally and theoretically to unravel their optical response. Photoluminescence measurements indicate a potential Purcell enhancement of more than a factor of 58.

High performance analysis of layered nanolithography masks by a surface impedance generating operator

A fast computational algorithm is presented for the analysis of multilayered nanolithography masks. The technique used is an exact field-theoretical approach which can model the diffraction effects in subwavelength propagation regimes. The field scattered by the mask pattern is obtained in two steps. First, a surface impedance generating operator (SIGO) that relates the tangential electric field on the boundary of each etched area to its equivalent surface electric current is computed. Second, the exterior problem is formulated based on the equivalence theorem in electromagnetics and is combined with the SIGO model. These two steps may be executed in parallel, making the lithography simulation fast and numerically efficient. For an arbitrary 2D mask illuminated by a TMy-polarized incident wave, the required Green's functions are obtained. The Green's function of the interior problem is calculated directly in the spatial domain while the complex images method is used for computing the Green's functions of the exterior multilayer problem. Based on this forward modeling procedure, a parameter sweep is performed and a binary mask pattern under normal incident coherent illumination is analyzed.

Compact circularly polarized truncated square ring slot antenna with suppressed higher resonances

This paper presents a compact circularly polarized (CP) antenna with an integrated higher order harmonic rejection filter. The proposed design operates within the ISM band of 2.32 GHz– 2.63 GHz and is suitable for example for wireless power transfer applications. Asymmetrical truncated edges on a square ring create a defected ground structure to excite the CP property, simultaneously realizing compactness. It offers a 50.5% reduced patch area compared to a conventional design. Novel stubs and slot shapes are integrated in the transmission line to reduce higher (up to the third) order harmonics. The proposed prototype yields a -10 dB reflection coefficient (S₁₁) impedance bandwidth of 12.53%, a 3 dB axial ratio bandwidth of 3.27%, and a gain of 5.64 dBi. Measurements also show good agreement with simulations.

Revealing Nanostructures through Plasmon Polarimetry

Polarized optical dark-field spectroscopy is shown to be a versatile noninvasive probe of plasmonic structures that trap light to the nanoscale. Clear spectral polarization splittings are found to be directly related to the asymmetric morphology of nanocavities formed between faceted gold nanoparticles and an underlying gold substrate. Both experiment and simulation show the influence of geometry on the coupled system, with spectral shifts Δλ = 3 nm from single atoms. Analytical models allow us to identify the split resonances as transverse cavity modes, tightly confined to the nanogap. The direct correlation of resonance splitting with atomistic morphology allows mapping of subnanometre structures, which is crucial for progress in extreme nano-optics involving chemistry, nanophotonics, and quantum devices.

Stored electromagnetic energy and quality factor of radiating structures

This paper deals with the old yet unsolved problem of defining and evaluating the stored electromagnetic energy-a quantity essential for calculating the quality factor, which reflects the intrinsic bandwidth of the considered electromagnetic system. A novel paradigm is proposed to determine the stored energy in the time domain leading to the method, which exhibits positive semi-definiteness and coordinate independence, i.e. two key properties actually not met by the contemporary approaches. The proposed technique is compared with an up-to-date frequency domain method that is extensively used in practice. Both concepts are discussed and compared on the basis of examples of varying complexity.

Two-Stage Design Method for Enhanced Inductive Energy Transmission with Q-Constrained Planar Square Loops

Q-factor constraints are usually imposed on conductor loops employed as proximity range High Frequency Radio Frequency Identification (HF-RFID) reader antennas to ensure adequate data bandwidth. However, pairing such low Q-factor loops in inductive energy transmission links restricts the link transmission performance. The contribution of this paper is to assess the improvement that is reached with a two-stage design method, concerning the transmission performance of a planar square loop relative to an initial design, without compromise to a Q-factor constraint. The first stage of the synthesis flow is analytical in approach, and determines the number and spacing of turns by which coupling between similar paired square loops can be enhanced with low deviation from the Q-factor limit presented by an initial design. The second stage applies full-wave electromagnetic simulations to determine more appropriate turn spacing and widths to match the Q-factor constraint, and achieve improved coupling relative to the initial design. Evaluating the design method in a test scenario yielded a more than 5% increase in link transmission efficiency, as well as an improvement in the link fractional bandwidth by more than 3%, without violating the loop Q-factor limit. These transmission performance enhancements are indicative of a potential for modifying proximity HF-RFID reader antennas for efficient inductive energy transfer and data telemetry links.

Integral equations formulation of plasmonic transmission lines

In this paper, a comprehensive integral equation formulation of plasmonic transmission lines is presented for the first time. Such lines are made up of a number of metallic strips with arbitrary shapes and dimensions immersed within a stack of planar dielectric or metallic layers. These lines support a number of propagating modes. Each mode has its own phase constant, attenuation constant, and field distribution. The presented integral equation formulation is solved using the Method of Moments (MoM). It provides all the propagation characteristics of the modes. The new formulation is applied to a number of plasmonic transmission lines, such as: single rectangular strip, horizontally coupled strips, vertically coupled strips, triangular strip, and circular strip. The numerical study is performed in the frequency (wavelength) range of 150-450 THz (0.66-2.0 μm). The results of the proposed technique are compared with those obtained using Lumerical mode solution, and CST. Very good agreement has been observed. The main advantage of the MoM is its intrinsic speed for this type of problem compared to general purpose solvers.

Directional fluorescence emission by individual V-antennas explained by mode expansion

Specially designed plasmonic antennas can, by far-field interference of different antenna elements or a combination of multipolar antenna modes, scatter light unidirectionally, allowing for directional light control at the nanoscale. One of the most basic and compact geometries for such antennas is a nanorod with broken rotational symmetry, in the shape of the letter V. In this article, we show that these V-antennas unidirectionally scatter the emission of a local dipole source in a direction opposite the undirectional side scattering of a plane wave. Moreover, we observe high directivity, up to 6 dB, only for certain well-defined positions of the emitter relative to the antenna. By employing a rigorous eigenmode expansion analysis of the V-antenna, we fully elucidate the fundamental origin of its directional behavior. All findings are experimentally verified by measuring the radiation patterns of a scattered plane wave and the emission pattern of fluorescently doped PMMA positioned in different regions around the antenna. The fundamental interference effects revealed in the eigenmode expansion can serve as guidelines in the understanding and further development of nanoscale directional scatterers.

Nonlinear superchiral meta-surfaces: tuning chirality and disentangling non-reciprocity at the nanoscale

Circularly polarized light is incident on a nanostructured chiral meta-surface. In the nanostructured unit cells whose chirality matches that of light, superchiral light is forming and strong optical second harmonic generation can be observed.

Interacting plasmonic nanostructures beyond the quasi-static limit: a "circuit" model

The interaction between individual plasmonic nanoparticles plays a crucial role in tuning and shaping the surface plasmon resonances of a composite structure. Here, we demonstrate that the detailed character of the coupling between plasmonic structures can be captured by a modified "circuit" model. This approach is generally applicable and, as an example here, is applied to a dolmen-like nanostructure consisting of a vertically placed gold monomer slab and two horizontally placed dimer slabs. By utilizing the full-wave eigenmode expansion method (EEM), we extract the eigenmodes and eigenvalues for these constituting elements and reduce their electromagnetic interaction to the structures' mode interactions. Using the reaction concept, we further summarize the mode interactions within a "coupling" matrix. When the driving voltage source imposed by the incident light is identified, an equivalent circuit model can be constructed. Within this model, hybridization of the plasmonic modes in the constituting nanostructure elements is discussed. The proposed circuit model allows the reuse of powerful circuit analysis techniques in the context of plasmonic structures. As an example, we derive an equivalent of Thévenin's theorem in circuit theory for nanostructures. Applying the equivalent Thévenin's theorem, the well-known Fano resonance is easily explained.

Rendering dark modes bright by using asymmetric split ring resonators

We have studied both theoretically and experimentally symmetric and asymmetric planar metallic Split Ring Resonators. We demonstrate that introducing structural asymmetry makes it possible to excite several higher order modes of both even (l = 2) and odd (l = 3, 5) order, which are otherwise inaccessible for a normally incident plane wave in symmetric structures. Experimentally we observe that the even mode resonances of asymmetric resonators have a quality factor 5.8 times higher than the higher order odd resonances.

Nanostripe length dependence of plasmon-induced material deformations

Following the impact of a single femtosecond light pulse on nickel nanostripes, material deformations-or "nanobumps"-are created. We have studied the dependence of these nanobumps on the length of nanostripes and verified the link with plasmons. More specifically, local electric currents can melt the nanostructures in the hotspots, where hydrodynamic processes give rise to nanobumps. This process is further confirmed by independently simulating local magnetic fields, since these are produced by the same local electric currents.

Plasmon-enhanced sub-wavelength laser ablation: plasmonic nanojets

In response to the incident light's electric field, the electron density oscillates in the plasmonic hotspots producing an electric current. Associated Ohmic losses raise the temperature of the material within the plasmonic hotspot above the melting point. A nanojet and nanosphere ejection can then be observed precisely from the plasmonic hotspots.

The role of chiral local field enhancements below the resolution limit of Second Harmonic Generation microscopy

While it has been demonstrated that, above its resolution limit, Second Harmonic Generation (SHG) microscopy can map chiral local field enhancements, below that limit, structural defects were found to play a major role. Here we show that, even below the resolution limit, the contributions from chiral local field enhancements to the SHG signal can dominate over those by structural defects. We report highly homogeneous SHG micrographs of star-shaped gold nanostructures, where the SHG circular dichroism effect is clearly visible from virtually every single nanostructure. Most likely, size and geometry determine the dominant contributions to the SHG signal in nanostructured systems.

Dark and bright localized surface plasmons in nanocrosses

A metallic nanocross geometry sustaining broad dipole and sharp higher order localized surface plasmon resonances is investigated. Spectral tunability is achieved by changing the cross arm length and the angle between the arms. The degree of rotational symmetry of the nanocross is varied by adding extra arms, changing the arm angle and shifting the arm intersection point. The particle's symmetry is shown to have a crucial influence on the plasmon coupling to incident radiation. Pronounced dipole, quadrupole, octupole and Fano resonances are observed in individual cross structures. Furthermore, the nanocross geometry proves to be a useful building block for coherently coupled plasmonic dimers and trimers where the reduced symmetry results in hybridized subradiant and superradiant modes and multiple Fano interferences. Finite difference time domain calculations of absorption and scattering cross-sections as well as charge density profiles are used to reveal the nature of the different plasmon modes. Experimental spectra for the discussed geometries support the calculations.

Hotspot decorations map plasmonic patterns with the resolution of scanning probe techniques

In high definition mapping of the plasmonic patterns on the surfaces of nanostructures, the diffraction limit of light remains an important obstacle. Here we demonstrate that this diffraction limit can be completely circumvented. We show that upon illuminating nanostructures made of nickel and palladium, the resulting surface-plasmon pattern is imprinted on the structures themselves; the hotspots (regions of local field enhancement) are decorated with overgrowths, allowing for their subsequent imaging with scanning-probe techniques. The resulting resolution of plasmon pattern imaging is correspondingly improved.

Plasmon line shaping using nanocrosses for high sensitivity localized surface plasmon resonance sensing

The detection of small changes in the wavelength position of localized surface plasmon resonances in metal nanostructures has been used successfully in applications such as label-free detection of biomarkers. Practical implementations, however, often suffer from the large spectral width of the plasmon resonances induced by large radiative damping in the metal nanocavities. By means of a tailored design and using a reproducible nanofabrication process, high quality planar gold plasmonic nanocavities are fabricated with strongly reduced radiative damping. Moreover, additional substrate etching results in a large enhancement of the sensing volume and a subsequent increase of the sensitivity. Coherent coupling of bright and dark plasmon modes in a nanocross and nanobar is used to generate high quality factor subradiant Fano resonances. Experimental sensitivities for these modes exceeding 1000 nm/RIU with a Figure of Merit reaching 5 are demonstrated in microfluidic ensemble spectroscopy.

Asymmetric optical second-harmonic generation from chiral G-shaped gold nanostructures

We present a new electromagnetic phenomenon-the asymmetric second-harmonic generation from planar chiral structures. The effect consists in distinguishing the handedness of a chiral material by rotating the sample in an experiment involving solely linearly polarized light. This phenomenon originates in the surface plasmon resonance of chiral gold nanostructures, where homodyne interference of anisotropic and chiral electric and/or magnetic multipoles appears to play an important role.